MRI assessment of leg muscle size and composition after a 7-day head-down bed rest

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Materials and Methods: In this prospective study, thigh and calf proton density fat fraction (PDFF) magnetic resonance imaging (MRI) was performed on 34 (thigh-measured) and 32 (calf-measured) healthy male participants. These participants were subjected to the HDBR trial between July 2024 and September 2024 at five time points: baseline (PRE), bed rest day 3 (BR3), day 7 (BR7), and recovery days 3 (R3) and 5 (R5). Muscle volume, subcutaneous fat volume, muscle PDFF, and subcutaneous fat PDFF were quantified. Blood and urine samples were collected to measure biomarkers. Analysis of dynamic changes in indicators of thigh and calf parameters and their correlation with biomarkers. Results: A total of 34 healthy male participants underwent HDBR. 34 (mean age, 28.03 ± 6.74 [SD]) had thigh measurements, and 32 (mean age, 27.47 ± 5.79 [SD]) had calf measurements. Bilateral thigh muscle cross-sectional area (MCSA) and bilateral medial head of gastrocnemius muscle cross-sectional area (MHMCSA) decreased consistently and significantly during HDBR (p<0.001). Thigh subcutaneous adipose tissue (SAT) and calf subcutaneous adipose tissue (CSAT) areas initially decreased, then gradually increased during HDBR, with inflection points at BR3. Left thigh subcutaneous adipose tissue fat fractions (SATL-FF) and left calf subcutaneous adipose tissue fat fractions (CSATL-FF) increased consistently and significantly during HDBR (p<0.001). The right medial head of the gastrocnemius muscle intramuscular fat fraction (MHIMFR-FF) increased significantly during HDBR (p=0.035). Significant correlations (p<0.05) were observed between urinary cortisol, cortisol, red blood cells, and calcium with muscle/fat parameters. Conclusion: Short-term HDBR induces lower limb muscle atrophy and subcutaneous fat redistribution, which are dynamically correlated with stress, metabolic, and bone metabolism-related biomarkers. Health sciences/Anatomy Health sciences/Biomarkers Health sciences/Health care Health sciences/Medical research Biological sciences/Physiology Microgravity Magnetic resonance imaging (MRI) Cross-sectional area (CSA) Proton density fat fraction (PDFF) Urinary cortisol (UCORT) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Spaceflight induces multisystem physiological deconditioning, with the musculoskeletal system being highly susceptible 1 , 2 . Unloading of weight-bearing muscles precipitates rapid and significant skeletal muscle atrophy, functional impairment, and compositional remodeling, posing a critical risk to astronaut health. Head-Down Bed Rest (HDBR) is a well-established terrestrial analog for simulating microgravity-induced adaptations, particularly muscle atrophy and functional decline 3 . Lower limb muscles are key to maintaining postural stability and movement under gravitational loading. During microgravity exposure, lower limb muscles decay at an accelerated rate due to interrupted weight-bearing cycles and reduced activity 4 . The HDBR model is particularly relevant for investigating the rapid deconditioning of lower-limb muscles. While HDBR has been widely used to study inactivity-induced adaptations, critical knowledge gaps persist. Specifically, the development of muscle fat infiltration secondary to gravitational unloading of postural muscles has received limited attention, despite its recognized role as a key indicator of muscle health 5 . Previous studies have reported that intramuscular lipid droplets in the − 6°head-down bed rest study had begun to accumulate by day 6 6 . In another 3-day dry immersion model, the muscle fiber cross-sectional area of intermuscular adipose tissue(IMAT) adipocytes tended to increase in comparison with the pre-dry immersion period 7 . The fat content of lumbar spine muscles increases in simulated microgravity conditions 8 . Elucidating the initial mechanisms of muscle fat infiltration is essential for developing effective intervention strategies. Existing studies have mostly focused on models of long-term disuse, while fat infiltration during the acute phase (< 7 days) remains uncharacterized. Understanding the onset of fatty infiltration is critical, as it may represent an early, potentially irreversible step in muscle degradation. Concurrently, emerging evidence indicates spaceflight-induced hematological alterations 9 , 10 , yet their association with skeletal muscle remodeling remains poorly understood. The ground-based studies have shown that indicators such as serum 25(OH)D and phosphate correlate with muscle status 11 , 12 . Elucidating such relationships in microgravity could reveal novel mechanistic insights to inform muscle preservation strategies. Magnetic Resonance Imaging (MRI) can accurately quantify changes in the lower muscles with advanced muscle structure assessment capabilities. Proton Density Fat Fraction (PDFF) enables objective assessment of intramuscular fat infiltration 13 . This technique offers superior reproducibility in tissue characterization compared to conventional methods 14 . Using PDFF, our study fills two key knowledge gaps: to describe and quantify acute changes in lower limb components (intramuscular/intermuscular fat, subcutaneous fat, bone marrow fat) and muscle strength during short-term HDBR; and to establish correlations between these muscular adaptations and biomarkers to improve understanding of the mechanisms responsible for the early changes in the components of the lower limb in the microgravity environment. 2. Mateirals and Methods 2.1Participants This prospective study was conducted from July 2024 to September 2024. The study protocol was approved by the Research Ethics Committee of Beijing Friendship Hospital, affiliated with Capital Medical University (2024-P2-069-04), and all participants signed an informed consent form (trial registration number: ChiCTR2500096128) before the experiment. 2.2Study Design and Subjects Thirty-four healthy male participants were recruited for this study. All of the participants had a secondary school education or higher, no history of psychiatric, genetic, or infectious diseases, no serious musculoskeletal or cardiovascular disorders, and no history of smoking or other unhealthy habits. The experimental procedure consisted of a 3-day adaptation and baseline measurement period (PRE), a 7-day head-down bed rest period (BR), and a 5-day recovery period (R). Throughout the procedure, intake of coffee, tea, alcohol, and any medication was prohibited, and smoking was strictly forbidden. To track muscle changes, MRI scans were performed at five time points (PRE, BR3, BR7, R3, R5) (Fig. 1 ). 2.3 Subjects diets The diets of the participants in our trial were based on the Dietary Guidelines for Chinese Residents (2022) and the Chinese Dietary Reference Intakes (DRIs) (2023 Revision). The dietary plan also incorporated the nutritional support program of the previous HDBR study 15 . The average daily calorie intake was approximately 2,800 kcal and varied by ≤ 10% over a 3-day menu cycle. 2.4.MRI Measurements MRI examinations were performed on a 3.0T scanner (Siemens Vida, Erlangen, Germany). Participants were placed in the supine position, and a combination of anterior and posterior coil arrays was used. Participants were scanned with MRI at five time points (PRE, BR3, BR7, R3, and R5). For each participant at each time point, 104 transverse thigh and calf images were acquired separately, (T1 weighted 2-point Dixon sequence, slice thickness = 2.0 mm; distance factor = 20%, TR = 4.34ms, TE1 = 1.34ms, TE2 = 2.76ms, flip angle = 9 deg; field of view = 400 mm × 324 mm at 1.3 mm × 1.3 mm pixel size). Total scan time was reduced to a minimum (total scan time of 5 to 7 min) and embedded in the patient’s regular clinical follow-up schedule. 2.5 Skeletal Muscle Segmentation Manual segmentation of fat and muscle tissues was conducted on merged water-separated images over the four stacks using ITK-SNAP 16 ( www.itksnap.org , accessed on 3 February 2020). To ensure reliability, the target area was precisely defined, and all segmentations were reviewed by a board-certified radiologist with 19 years of experience in whole-body imaging. Representative results are shown in Figs. 2 and 3 . Based on the extracted PDFF and cross-sectional area (CSA) values, the contractile mass index (CMI) was calculated as follows: \(\:CMI=CSA\:\times\:(1-PDFF)\) 2.6. Statistical Analysis Parameters are expressed as mean ± standard deviation. All parameters were subjected to the Shapiro-Wilk normality test. Based on the normality test results, comparisons between the five time points were performed using repeated measures ANOVA (consistent with normal distribution) or generalized estimating equations (not consistent with normal distribution), with post hoc tests using Bonferroni's method when significant main effects were present; comparisons between the right and left limbs at the same time point were selected using paired t-tests or Wilcoxon signed-rank tests based on the normality of the differences; and linear mixed model was used to analyses the association between blood indices and lower limb parameters at the five time points. Statistical significance was evaluated using a threshold of P < 0.05. Statistical analysis was performed by using IBM SPSS Statistics for Macintosh (Version 26.0, IBM Corp., Released 2019, Armonk, NY, USA) and R version 4.4.2. 3. Results 3.1. Participant Characteristics In this study, 44 participants underwent HDBR experiments. In thigh measurements, 10 participants were excluded because they dropped out of the experiment (n = 4) or had poor quality images (n = 6); in calf measurements, 12 participants were excluded because they dropped out of the experiment (n = 4) or had poor quality images (n = 8). The participant characteristics are provided in Table 1 . Table 1 Demographics of subjects in the PRE Thigh Calf Sex, N 34, male 32, male Age, years, mean ± SD 28.03 ± 6.74 27.47 ± 5.79 BMI, mean ± SD 23.17 ± 2.55 22.90 ± 2.34 3.2. Longitudinal Analysis Table 2 Comparison of thigh and calf parameters of subjects in different periods Parameter PRE BR3 BR7 R3 R5 P-value Thigh(n = 34) TMCSAR, mm 2 15,087.10 ± 1,991.38 14,453.24 ± 1,880.78a 14,149.14 ± 1,876.55ab 14,626.51 ± 1,992.24abc 14,598.19 ± 1,907.91ac < 0.001 TMCSAL, mm 2 14,839.04 ± 1,958.67 14,214.27 ± 1,897.19a 13,975.89 ± 1,860.76ab 14,369.13 ± 1,913.57abc 14,391.23 ± 1,824.96abc < 0.001 TSATR, mm 2 5,924.84 ± 2,002.31 5,607.39 ± 1,941.19a 5,642.26 ± 1,920.35a 5,820.70 ± 1,966.70 5,984.06 ± 1,956.22bc < 0.001 TSATL, mm 2 5,713.29 ± 1,976.22 5,283.91 ± 1,888.80a 5,393.34 ± 1,963.20a 5,556.45 ± 1,897.72 5,732.25 ± 1,959.96bc < 0.001 TTSMAR-FF, % 12.09 ± 2.61 11.66 ± 3.01 12.24 ± 3.12 12.11 ± 2.82 11.95 ± 2.87 0.259 TTSMAL-FF, % 11.87 ± 2.71 11.75 ± 2.80 12.65 ± 3.69 11.99 ± 2.91 11.98 ± 2.87 0.211 TSATR-FF, % 84.38 ± 2.02 84.29 ± 2.48 84.41 ± 2.31 84.44 ± 2.52 84.40 ± 2.56 0.730 TSATL-FF, % 85.27 ± 2.29 86.02 ± 2.18a 86.12 ± 2.21a 85.45 ± 2.53bc 85.85 ± 2.52 < 0.001 TCMIR, mm 2 13,276.72 ± 1,902.74 12,775.37 ± 1,790.03a 12,417.49 ± 1,712.57ab 12,870.73 ± 1,929.70ac 12,865.65 ± 1,831.62ac < 0.001 TCMIL, mm 2 13,088.86 ± 1,865.70 12,559.46 ± 1,848.74a 12,209.20 ± 1,732.71ab 12,660.55 ± 1,864.69abc 12,679.63 ± 1,769.43ac < 0.001 TBMATR-FF, % 82.95 ± 5.81 83.32 ± 4.77 83.29 ± 5.95 80.20 ± 6.31abc 79.50 ± 6.84abc < 0.001 TBMATL-FF, % 84.33 ± 4.83 85.48 ± 3.43 85.32 ± 3.99 83.66 ± 5.07bc 83.38 ± 4.86bc < 0.001 Calf(n = 32 ) MHMCSAR, mm 2 1,489.32 ± 336.88 1,401.64 ± 304.28a 1,349.34 ± 305.74ab 1,421.64 ± 316.91ac 1,392.86 ± 326.69ac < 0.001 MHMCSAL, mm 2 1,446.62 ± 339.28 1,380.53 ± 318.09a 1,339.81 ± 319.67a 1,404.01 ± 347.66c 1,373.78 ± 329.85a < 0.001 LHMCSAR, mm 2 897.62 ± 211.49 881.36 ± 209.38 842.20 ± 160.88 845.59 ± 178.86 840.67 ± 174.98 0.05 LHMCSAL, mm 2 885.66 ± 180.94 834.00 ± 158.75 837.01 ± 158.00 835.36 ± 168.32 838.45 ± 168.87 0.138 CSATR, mm 2 1,788.82 ± 589.39 1,699.57 ± 581.05a 1,703.99 ± 554.55a 1,754.99 ± 569.86b 1,735.82 ± 560.41abd < 0.001 CSATL, mm 2 1,806.15 ± 583.03 1,749.42 ± 573.50a 1,761.99 ± 577.11 1,802.62 ± 592.91bc 1,772.65 ± 580.66d < 0.001 MHIMFR-FF, % 8.57 ± 1.59 8.89 ± 2.47 8.92 ± 1.59 8.62 ± 1.66c 8.72 ± 1.68 0.035 MHIMFL-FF, % 8.84 ± 1.39 8.74 ± 2.47 8.86 ± 1.29 8.73 ± 1.48 8.92 ± 1.67 0.396 LHIMFR-FF, % 8.89 ± 1.28 8.44 ± 2.36 8.48 ± 1.42 8.80 ± 1.66 8.83 ± 1.66c 0.025 LHIMFL-FF, % 9.20 ± 1.64 9.96 ± 3.02 9.68 ± 1.72 8.96 ± 2.38 9.30 ± 1.80 0.068 CSATR-FF, % 80.27 ± 4.79 80.90 ± 4.95 80.67 ± 5.33 79.97 ± 4.92c 80.41 ± 5.05 0.002 CSATL-FF, % 80.73 ± 4.81 81.78 ± 4.37 81.67 ± 4.48a 77.81 ± 14.87 80.95 ± 4.28c < 0.001 CBMATR-FF, % 90.13 ± 1.33 87.28 ± 15.96 90.24 ± 0.91 90.29 ± 0.97 90.16 ± 0.84 0.718 CBMATL-FF, % 90.02 ± 1.06 86.88 ± 15.87 90.00 ± 0.89 89.87 ± 0.88 90.15 ± 0.79 0.205 MHCMIR, mm 2 1,362.40 ± 312.28 1,274.09 ± 284.26a 1,230.48 ± 287.55a 1,300.71 ± 298.58ac 1,272.56 ± 304.95a < 0.001 MHCMIL, mm 2 1,319.58 ± 314.06 1,257.88 ± 299.59a 1,222.15 ± 296.93a 1,282.70 ± 324.10c 1,253.16 ± 310.51a < 0.001 LHCMIR, mm 2 818.66 ± 197.69 805.30 ± 193.81 771.14 ± 149.72 772.18 ± 170.00 766.97 ± 162.82 0.076 LHCMIL, mm 2 804.57 ± 166.15 748.75 ± 147.31 756.24 ± 144.20 758.79 ± 156.54 760.82 ± 156.00 0.150 Note. T, Thigh; C, Calf; R, right side; L, left side; MH, the medial head of the gastrocnemius muscle; LH, the lateral head of the gastrocnemius muscle; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; TSMA, total skeletal muscle adiposity, which includes intermuscular fat and intramuscular fat; IMF, intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. a represents a statistically significant difference compared to PRE; b represents a statistically significant difference compared to BR3; c represents a statistically significant difference compared to BR7; d represents a statistically significant difference compared to R3. There was a significant decrease in bilateral thigh muscle cross-sectional area (TMCSA) during bed rest (p < 0.001). Thigh subcutaneous adipose tissue (TSAT) decreased at the beginning of bed rest, and then gradually increased. The inflection point of the TSAT change was on the 3rd day in bed, and the inflection point of the muscle area change was on the 7th day in bed. No statistically significant absolute change in PDFF (in %) for the thigh muscle was observed in this study. However, left thigh subcutaneous adipose tissue fat fractions (TSATL-FF) increased during bed rest(p < 0.001), but the right thigh did not change significantly. The thigh contractile mass index (TCMI) decreased during bed rest༈p < 0.001༉. There was no significant change in bilateral thigh bone marrow adipose tissue fat fraction (TBMAT-FF) during bed rest. However, TBMAT-FF was significantly reduced at the end of bed rest (p < 0.001) (Table 2 and Fig. 3 ). There was a significant decrease in the bilateral medial head of gastrocnemius muscle cross-sectional area (MHMCSA) during bed rest (p < 0.001). The calf subcutaneous adipose tissue area(CSAT) decreased at the beginning of bed rest, and then gradually increased. The inflection point of subcutaneous adipose tissue area change was on the 3rd day in bed, and the inflection point of muscle area change was on the 7th day in bed. The left calf subcutaneous adipose tissue fat fraction (CSATL-FF) increased during bed rest༈P < 0.001༉, but the right calf did not change significantly. The right medial head of the gastrocnemius muscle intramuscular fat fat fraction (MHIMFR-FF) continued to rise during bed rest and gradually recovered at the end of bed rest (p = 0.035). Medial head of gastrocnemius contractile mass index (MHCMI) decreased during bed rest༈P < 0.001༉. There was no significant change in calf tibia bone marrow adipose tissue fat fraction (CBMAT-FF) during bed rest (Table 2 and Fig. 4 ). 3.3. Comparison of left and right parameters By comparing the left and right thigh parameters of the participants at different time points, it was found that the parameters of the left and right thigh portions were different at the beginning and end of bed rest. TMCSAR was greater than that of the TMCSAL, TSATR was greater than that of the TSATL, TSTAL-FF was greater than that of the TSTAR-FF, TCMIR was greater than that of the TCMIL, and the TBMATL-FF was greater than that of the TBMATR-FF(Fig. 5 ). By comparing the left and right calf parameters of the participants at different time points, it was found that some of the parameters of the left and right calves were different during bed rest and the recovery period, and that the CSAT, the CSAT-FF, and the lateral head of the gastrocnemius muscle intramuscular fat fraction(LHIMF-FF) were greater on the left than on the right side at all time points, but the difference was significant only for some of the time points (P < 0.05). 3.4. Correlation analysis of blood and size parameters There were associations between some biomarkers and the MRI parameters, as shown in the following tables (Table 3 , Table 4 ). LMM found that urinary cortisol (UCORT), red blood cell (RBC), hemoglobin (Hb), calcium (Ca), phosphorus (P), direct renin (DR), adrenocorticotropic hormone (ACTH), calcitonin (CT), osteocalcin (OC), 25 hydroxyvitamin D (25 (OH) D) were significantly correlated with thigh parameters. UCORT was negatively correlated with TSATR, TSATL, TSATR-FF, and TCMIR (p < 0.05). There was a significant positive correlation between RBC and TBMATL-FF (P = 0.039). There was a significant negative correlation between Hb and TSATL-FF (P = 0.042). Ca was negatively correlated with the right of total skeletal muscle adiposity fat fraction (TTSMAR-FF)(P = 0.041), and positively correlated with TCMIR (P = 0.006); P was negatively correlated with the light of total skeletal muscle adiposity fat fraction(TTSMAL-FF) and CMIR (p < 0.05), and positively correlated with TBMATL-FF (p = 0.030); DR was negatively correlated with TMCSAL, TSATR, TSATL and TCMIR (p < 0.05), and positively correlated with TSATL-FF (p = 0.015); There was a significant positive correlation between ACTH and TSATR-FF (P = 0.009); CT was positively correlated with TTSMAR-FF and TTSMAL-FF (p < 0.05); There was a significant positive correlation between OC and TTSMAL-FF (P = 0.039); 25 (OH) d was positively correlated with TMCSAR, TMCSAL, TCMIR and TCMIL (p < 0.05), and negatively correlated with TTSMAR-FF and TTSMAL-FF (p < 0.05) (Table 3 ). Table 3 Significant Associations Between Thigh Parameters and Biomarkers From Linear Mixed Models.​ MRI parameter Biomarkers parameter β Estimate P value Effect size (95% CI) TMCSAR 25(OH)D 44.279 0.033 (3.695 to 84.863) TMCSAL DR -4.270 0.000 (-6.258 to -2.282) 25(OH)D 56.496 0.009 (14.668 to 98.324) TSATR UCORT -6.835 0.046 (-13.557 to -0.112) DR -3.704 0.008 (-6.426 to -0.982) TSATL UCORT -10.647 0.012 (-18.908 to -2.386) DR -4.350 0.015 (-7.844 to -0.857) TTSMAR-FF Ca -3.105 0.041 (-6.086 to -0.124) CT 0.126 0.029 (0.013 to 0.239) 25(OH)D -0.134 0.010 (-0.235 to -0.033) TTSMAL-FF P -1.733 0.031 (-3.307 to -0.158) CT 0.125 0.023 (0.018 to 0.232) OC 0.052 0.039 (0.003 to 0.102) 25(OH)D -0.111 0.032 (-0.213 to -0.010) TSATR-FF UCORT -0.017 0.024 (-0.033 to -0.002) ACTH 0.008 0.009 (0.002 to 0.015) TSATL-FF Hb 0.081 0.042 (0.003 to 0.160) DR 0.008 0.015 (0.002 to 0.015) TCMIR UCORT -4.857 0.031 (-9.251 to -0.462) Ca 1,595.958 0.006 (467.630 to 2,724.285) P -1,106.971 0.002 (-1,787.995 to -425.948) DR -6.099 0.000 (-8.785 to -3.413) 25(OH)D 84.635 0.002 (30.913 to 138.357) TCMIL 25(OH)D 71.463 0.001 (30.272 to 112.655) TBMATL-FF RBC 6.488 0.039 (0.331 to 12.644) P 4.783 0.030 (0.479 to 9.088) Note. T, Thigh; R, right side; L, left side; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; TSMA, total skeletal muscle adiposity, which includes intermuscular fat and intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. UCORT, urinary cortisol; CORT, cortisol; RBC, red blood cells; WBC, white blood cells; Hb, hemoglobin. Ca, calcium; P, phosphorus; DR, direct renin; ACTH, adrenocorticotropic hormone; IPTH, all-parathyroid hormone; CT, calcitonin; OC, osteocalcin; 25(OH)D, 25-hydroxyvitamin D LMM found that biomarkers significantly associated with calf parameters were UCORT, cortisol (CORT), RBC, Hb, Ca, P, DR, ACTH, all-parathyroid hormone (IPTH), OC, and 25-hydroxyvitamin D (25(OH)D). There was a negative correlation between UCORT and CSATR (P < 0.05), and a positive correlation with MHIMFR-FF, MHIMFL-FF, LHIMFL-FF, and CSTAR-FF (P < 0.05). There was a negative correlation between CORT and LHMCSAL (P = 0.029). There was a positive correlation between RBC and CSATL-FF (P = 0.002). There was a negative correlation between Hb and CSATR (P = 0.023). There was a positive correlation between Ca and LHIMFR-FF (P = 0.004). There was a positive correlation between P and MHMCSAR, MHMCSAL, LHMCSAR, ACTH was positively correlated with LHMCSAR, MHIMFL-FF (P < 0.05); IPTH was positively correlated with MHIMFR-FF, MHIMFL-FF, LHIMFR-FF (P < 0.05), and negatively correlated with CSATL-FF (P = 0.022); OC was positively correlated with MHIMFR-FF, MHIMFL-FF (P < 0.05), and CBMATL-FF (P < 0.05); 25(OH)D was positively correlated with LHMCSAR, LHCMIR (P < 0.05), and negatively correlated with LHIMFR-FF (P = 0.035) (Table 4 ). Table 4 Significant Associations Between Calf Parameters and Biomarkers From Linear Mixed Models.​ MRI parameter Biomarkers parameter β Estimate P value Effect size (95% CI) MHMCSAR P -158.273 0.011 (-279.118 to -37.429) DR -0.627 0.009 (-1.091 to -0.162) MHMCSAL P -207.734 0.000 (-321.078 to -94.391) LHMCSAR P -168.079 0.017 (-305.501 to -30.657) ACTH 0.548 0.042 (0.020 to 1.076) 25(OH)D 9.721 0.017 (1.779 to 17.663) LHMCSAL CORT -4.260 0.029 (-8.075 to -0.446) P -163.678 0.020 (-300.968 to -26.388) CSATR UCORT -1.168 0.021 (-2.158 to -0.178) Hb -2.092 0.023 (-3.897 to -0.287) P -162.330 0.006 (-276.177 to -48.483) MHIMFR-FF UCORT 0.015 0.003 (0.005 to 0.025) IPTH 0.014 0.034 (0.001 to 0.028) OC -0.037 0.007 (-0.064 to -0.010) MHIMFL-FF UCORT 0.015 0.014 (0.003 to 0.027) ACTH 0.006 0.001 (0.003 to 0.009) IPTH 0.034 0.000 (0.019 to 0.049) OC -0.031 0.038 (-0.060 to -0.002) LHIMFR-FF Ca 2.895 0.004 (0.964 to 4.827) IPTH 0.022 0.013 (0.005 to 0.039) 25(OH)D -0.064 0.038 (-0.125 to -0.004) LHIMFL-FF UCORT 0.027 0.016 (0.005 to 0.049) CSATR-FF UCORT 0.024 0.011 (0.006 to 0.043) DR 0.009 0.033 (0.001 to 0.018) CSATL-FF RBC 1.682 0.002 (0.621 to 2.742) DR 0.017 0.000 (0.009 to 0.024) IPTH -0.027 0.022 (-0.051 to -0.004) CBMATL-FF P 1.173 0.033 (0.096 to 2.249) OC -0.032 0.017 (-0.058 to -0.006) MHCMIR P -168.080 0.004 (-280.530 to -55.630) DR -0.641 0.006 (-1.093 to -0.190) MHCMIL P -192.697 0.001 (-301.772 to -83.622) DR -0.524 0.022 (-0.971 to -0.077) LHCMIR 25(OH)D 9.433 0.013 (1.994 to 16.872) Note. C, Calf; R, right side; L, left side; MH, the medial head of the gastrocnemius muscle; LH, the lateral head of the gastrocnemius muscle; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; IMF, intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. UCORT, urinary cortisol; CORT, cortisol; RBC, red blood cells; WBC, white blood cells; Hb, hemoglobin. Ca, calcium; P, phosphorus; DR, direct renin; ACTH, adrenocorticotropic hormone; IPTH, all-parathyroid hormone; CT, calcitonin; OC, osteocalcin; 25(OH)D, 25-hydroxyvitamin D 4. Discussion We did a study of acute changes in the components of the lower limbs in a microgravity environment. The relationship of these changes with biomarkers using a 7-day HDBR model and found that during HDBR, muscle volume of the lower limbs decreased, muscle strength of the lower limbs decreased, and subcutaneous fat decreased and then increased, and that there was a significant correlation between biomarkers such as UCORT, CORT, RBC, Ca, and P, and muscle/fat parameters, and that the lower limbs were bilaterally with a asymmetry. 4.1. Bilateral Asymmetry in Lower Limbs Our study identified significant left-right differences in thigh muscle cross-sectional area, subcutaneous adipose tissue fat fraction, contraction mass index, and bone marrow adipose tissue fat fraction. These findings align with previous observations of limb dominance effects 17 – 19 . Notably, left-sided elevations in SAT-FF and BMAT-FF may reflect reduced mechanical loading of the non-dominant limb 20 . This asymmetry underscores the importance of considering limb dominance in microgravity analog studies, as unilateral adaptations may complicate the interpretation of global muscle-fat dynamics. 4.2. Muscle atrophy: mechanisms and biomarker interactions The rapid atrophy of thigh and gastrocnemius muscles during HDBR confirms the susceptibility of weight-bearing muscles to unloading 21 , 22 . Our findings extend current knowledge by identifying integrated hormonal and metabolic pathways driving acute muscle loss. 25(OH)D levels are positively correlated with muscle cross-sectional area (TMCSA), and vitamin D plays a role in regulating myogenic cell growth, size, and gene expression 23 . VDR knockout mice exhibit reduced grip strength and fiber atrophy due to myostatin upregulation 24 . These data support vitamin D supplementation as a potential countermeasure, as 1,25(OH)₂D directly inhibits MuRF1 transcription via VDR binding In addition, changes in hormones such as growth hormone, cortisol, and growth factors, increased oxidative stress, and changes in neuromuscular function may also exacerbate the process of muscle atrophy 25 . DR correlated with reduced TMCSA, linking the Renin-angiotensin system (RAS) to local muscle pathology. Angiotensin II (Ang II) promotes oxidative stress via NAD(P)H oxidase 26 , 27 , triggering mitochondrial dysfunction and protein breakdown. PTH elevation, observed in our study, may exacerbate muscle loss by stimulating osteoclast-mediated calcium release, indirectly promoting proteolysis. Together, these factors contribute to the reduction in muscle mass during bedrest trials 25 . Our 7-day HDBR protocol captures early atrophy linked to concurrent declines in 25(OH)D and rises in DR/UCORT. This highlights the acute sensitivity of muscle to integrated stress-metabolic perturbations, underscoring the need for multi-targeted interventions. 4.3.Muscle Strength Decline The biphasic reduction in TCMI aligns with the "two-phase" model of strength loss. This suggests neuromuscular dysfunction dominates initial weakness at the early phase. Structural atrophy becomes prominent, with myofiber cross-sectional area reductions mirroring TCMI declines at the late phase 28 . Uros reported that 79% of the loss of muscle strength may be due to muscle atrophy, and the rest may be due to alterations in single-fiber mechanical properties, excitation-contraction coupling, muscle denervation, and neuromuscular junction damage 29 . 4.4. Fat Remodeling 4.4.1. Subcutaneous Fat (SAT) Biphasic Response The initial SAT reduction (BR3) followed by recovery contrasts with monotonic increases in long-term bed rest 30 . Urinary cortisol (UCORT) inversely correlated with TSAT area, consistent with glucocorticoid-mediated activation of hormone-sensitive lipase 31 . Altered cortisol levels in simulated microgravity may be psychologically stress-driven 32 . DR negatively correlated with TSAT, as Ang II suppresses preadipocyte differentiation via AT1R 33 . Our short-term data reveal a metabolic inflection point (BR3), where initial catabolism shifts to anabolism, likely driven by adaptive energy conservation. This challenges the assumption of unidirectional fat gain in microgravity analogs. 4.4.2. Intramuscular Fat (IMF) MHIMF-FF increased with BR time, and a 3-day dry immersion (DI) assay showed a trend towards enlarged IMAT adipocytes and a significant up-regulation of the expression of the fibro-adipogenic progenitor cells (FAPs) marker PDGFRα, suggesting that FAPs may be a key cell population mediating ectopic adipogenesis in human microgravity 7 . The differentiation of FAPs may be driven by PTH 34 .PTH up-regulates UCP1 in FAPs 34 , promoting beige adipocyte formation and lipid droplet deposition - consistent with our IPTH-MHIMF-FF correlation. Another study found that short-term bed rest led to the accumulation of glycogen in myocytes, triggering insulin resistance; also, medium-chain acylcarnitine was reduced, suggesting that fatty acid β-oxidation is inhibited in the early stages of muscle unloading, suggesting an early onset of lipid metabolism disorders. This suggests that bed rest induces a shift in skeletal muscle metabolic pathways to oxidize glucose more than fatty acids 6 . Both studies found lipid accumulation at an early stage. The DI experiments focused on IMAT, while the bed rest experiments focused on intracellular lipids in myocytes, revealing the complexity and multidimensionality of lipid metabolism disorders in the context of muscle inactivity. Our study has several limitations. The 7-day head-down bedrest only simulates short-term microgravity effects, and results may not apply to long-term spaceflight. The fixed caloric intake could have obscured dietary impacts. Additionally, the sample size was limited and lacked a parallel control group. Future research should consider larger samples, control groups, longer observation periods, and nutrient management. In conclusion, this study clarified the dynamic process of muscle-fat changes in short-term simulated microgravity, identified BR3 as a key intervention inflection point, and established the association between muscle atrophy and biomarkers such as vitamin D, RAS, and PTH, which will provide a new basis for astronauts' personalized healthcare countermeasures. Declarations Acknowledgements The authors are deeply grateful to all the individuals who participated as volunteers in this study. We extend our sincere appreciation for their time commitment, strict adherence to the study protocols, and invaluable contributions. Their dedication was essential to the successful completion of this research. Author contributions Y.S.: Original draft preparation, MRI Measurements, and multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), W.Y.: Data analysis, multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), and original draft preparation, Y.L.: Data analysis, Z.W.: Multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), Y.L., R.W., H.H., L.C., Y.H., and P.Q.: MRI Measurements, P.R.: Manuscript reviewing and modifications, Z.W.: Conceived and designed this study. Funding This study was supported by the Space Medical Experiment Project of China Manned Space Program (No. HYZHXMH01005), Beijing Hospitals Authority Innovation Studio of Young Staff Funding Support (No.202302), and Beijing Scholar 2015 (Zhenchang Wang). The funder played no role in study design, data collection, analysis and interpretation of data, or the writing of this manuscript. Data availability The data that support the findings of this study are available from the corresponding author upon request. Competing interests The authors declare no competing interests. References Grimm, D. et al. The impact of microgravity on bone in humans. Bone 87 , 44–56 (2016). Scott, J. M. et al. Effects of exercise countermeasures on multisystem function in long duration spaceflight astronauts. NPJ Microgravity 9 , 11 (2023). Gao, Y., Arfat, Y., Wang, H. & Goswami, N. Muscle atrophy induced by mechanical unloading: Mechanisms and potential countermeasures. Front Physiol 9 , 235 (2018). 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M. Dietary support of long-duration head-down bed rest. aviat space environ med 80 , A09-A14 (2009). Yushkevich, P. A. et al. User-guided 3D active contour segmentation of anatomical structures: Significantly improved efficiency and reliability. NeuroImage 31 , 1116–1128 (2006). Pang, J. et al. Age-related change in muscle strength, muscle mass, and fat mass between the dominant and non-dominant upper limbs. Front Public Health 11 , 1284959 (2023). Lanshammar, K. & Ribom, E. L. Differences in muscle strength in dominant and non-dominant leg in females aged 20–39 years – a population-based study. Physical Therapy in Sport 12 , 76–79 (2011). Belzunce, M. A., Henckel, J., Di Laura, A. & Hart, A. Intramuscular fat in gluteus maximus for different levels of physical activity. Sci Rep 11 , 21401 (2021). Wang, J. et al. Quantitative evaluation of bone marrow fat content and unsaturated fatty index in young male soccer players using proton magnetic resonance spectroscopy (1H-MRS): A preliminary study. Quant Imaging Med Surg 11 , 4275–4286 (2021). Trappe, T. A., Tesch, P., Alkner, B. & Trappe, S. Microgravity-induced skeletal muscle atrophy in women and men: Implications for long-duration spaceflights to the moon and mars. Journal of Applied Physiology 135 , 1115–1119 (2023). Williams, D., Kuipers, A., Mukai, C. & Thirsk, R. Acclimation during space flight: Effects on human physiology. CMAJ 180 , 1317–1323 (2009). Agoncillo, M., Yu, J. & Gunton, J. E. The role of vitamin D in skeletal muscle repair and regeneration in animal models and humans: A systematic review. Nutrients 15 , 4377 (2023). Gopinath, S. D. Inhibition of Stat3 signaling ameliorates atrophy of the soleus muscles in mice lacking the vitamin D receptor. Skeletal Muscle 7 , 2 (2017). Lee, P. H. U., Chung, M., Ren, Z., Mair, D. B. & Kim, D.-H. Factors mediating spaceflight-induced skeletal muscle atrophy. American Journal of Physiology-Cell Physiology 322 , C567–C580 (2022). Bendall, J. K., Cave, A. C., Heymes, C., Gall, N. & Shah, A. M. Pivotal role of a gp91 phox -containing NADPH oxidase in angiotensin II-induced cardiac hypertrophy in mice. Circulation 105 , 293–296 (2002). Miranda, E. R., Shahtout, J. L. & Funai, K. Chicken or egg? Mitochondrial phospholipids and oxidative stress in disuse-induced skeletal muscle atrophy. Antioxid. Redox Signaling ars.2022.0151 (2022) doi:10.1089/ars.2022.0151. Campbell, M. et al. Effect of immobilisation on neuromuscular function in vivo in humans: A systematic review. Sports Med 49 , 931–950 (2019). Marusic, U., Narici, M., Simunic, B., Pisot, R. & Ritzmann, R. Nonuniform loss of muscle strength and atrophy during bed rest: A systematic review. J Appl Physiol (1985) 131 , 194–206 (2021). Ogawa, M. et al. Effects of 8 weeks of bed rest with or without resistance exercise intervention on the volume of the muscle tissue and the adipose tissues of the thigh. Physiol. Rep. 8 , (2020). Fardet, L. & Fève, B. Systemic glucocorticoid therapy: A review of its metabolic and cardiovascular adverse events. Drugs 74 , 1731–1745 (2014). Choukèr, A. et al. Simulated microgravity, psychic stress, and immune cells in men: Observations during 120-day 6° HDT. J. Appl. Physiol. 90 , 1736–1743 (2001). Janke, J., Engeli, S., Gorzelniak, K., Luft, F. C. & Sharma, A. M. Mature adipocytes inhibit in vitro differentiation of human preadipocytes via angiotensin type 1 receptors. Diabetes 51 , 1699–1707 (2002). Iio, R. et al. Parathyroid hormone inhibits fatty infiltration and muscle atrophy after rotator cuff tear by browning of fibroadipogenic progenitors in a rodent model. Am J Sports Med 51 , 3251–3260 (2023). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 07 Mar, 2026 Reviews received at journal 23 Feb, 2026 Reviewers agreed at journal 03 Feb, 2026 Reviews received at journal 29 Jan, 2026 Reviewers agreed at journal 07 Jan, 2026 Reviewers invited by journal 10 Nov, 2025 Editor assigned by journal 04 Nov, 2025 Submission checks completed at journal 31 Oct, 2025 First submitted to journal 26 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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1","display":"","copyAsset":false,"role":"figure","size":217626,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental flow chart. MRI were taken at baseline, BR3, BR7, R3 and R5 (Day 3 and 7 head-down tilt bed rest, Day 3 and 5recovery ,respectively)\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/899fca2dc9a0f4bcdbb60a62.png"},{"id":96422473,"identity":"6440227d-493c-4f8a-8236-85a4a5b3f338","added_by":"auto","created_at":"2025-11-21 01:09:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":583538,"visible":true,"origin":"","legend":"\u003cp\u003eSegmentation of thighs and calves. \u003cstrong\u003e(A)\u003c/strong\u003e Segmentation of the right and left thigh muscle cross-sectional area (red and green), subcutaneous adipose tissue area (blue and yellow), and bone marrow adipose tissue fat fraction (light blue and purple). \u003cstrong\u003e(B)\u003c/strong\u003e Segmentation of the right and left calf medial head of the gastrocnemius muscle cross-sectional area (red and yellow), outer head of the gastrocnemius muscle cross-sectional area (green and light blue), subcutaneous adipose tissue area (blue and purple), bone marrow adipose tissue fat fraction (apricot and dark blue)\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/d795b7f5e535038a19602da9.png"},{"id":96422475,"identity":"8396b26e-4401-418a-963b-63902c4da807","added_by":"auto","created_at":"2025-11-21 01:09:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":263551,"visible":true,"origin":"","legend":"\u003cp\u003eTime trend plots for each parameter. \u003cstrong\u003e(A)\u003c/strong\u003e Right thigh muscle cross-sectional area (TMCSAR). \u003cstrong\u003e(B)\u003c/strong\u003e Left thigh muscle cross-sectional area (TMCSAL). \u003cstrong\u003e(C)\u003c/strong\u003eRight thigh subcutaneous adipose tissue area (TSATR). \u003cstrong\u003e(D)\u003c/strong\u003e Left thigh subcutaneous adipose tissue area (TSATL).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/bbd11abb81625b6d31b7eb31.png"},{"id":96422476,"identity":"b3bf6b5b-6167-4b18-8d7b-18378af0a30b","added_by":"auto","created_at":"2025-11-21 01:09:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":267028,"visible":true,"origin":"","legend":"\u003cp\u003eTime trend plots for each calf parameter. \u003cstrong\u003e(A)\u003c/strong\u003e Right medial head of gastrocnemius muscle cross-sectional area (MHMCSAR). \u003cstrong\u003e(B)\u003c/strong\u003e Left medial head of gastrocnemius muscle cross-sectional area (MHMCSAL). \u003cstrong\u003e(C)\u003c/strong\u003e Right calf subcutaneous adipose tissue areas (CSATR). \u003cstrong\u003e(D)\u003c/strong\u003e Left calf subcutaneous adipose tissue fat fraction (CSATL-FF).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/e389a6243ee825b892671a26.png"},{"id":96422472,"identity":"5579f895-e010-42fe-953a-af21ddbd6d50","added_by":"auto","created_at":"2025-11-21 01:09:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":256406,"visible":true,"origin":"","legend":"\u003cp\u003eThe left and right side comparison of each parameter. \u003cstrong\u003e(A)\u003c/strong\u003e Thigh muscle cross-sectional area (TMCSA). \u003cstrong\u003e(B)\u003c/strong\u003e Thigh subcutaneous adipose tissue areas (TSAT). \u003cstrong\u003e(C) \u003c/strong\u003eThigh subcutaneous adipose tissue fat fractions (TSAT-FF). \u003cstrong\u003e(D)\u003c/strong\u003e Thigh bone marrow adipose tissue fat fraction (TBMAT-FF).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/7a86e8e33a9df4804deef0c1.png"},{"id":96456931,"identity":"517f5310-e827-426a-a90a-d2e5710eac02","added_by":"auto","created_at":"2025-11-21 10:08:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2935054,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7960086/v1/1469841e-c8fe-4bac-aba2-acc8f1148943.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"MRI assessment of leg muscle size and composition after a 7-day head-down bed rest","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSpaceflight induces multisystem physiological deconditioning, with the musculoskeletal system being highly susceptible\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Unloading of weight-bearing muscles precipitates rapid and significant skeletal muscle atrophy, functional impairment, and compositional remodeling, posing a critical risk to astronaut health. Head-Down Bed Rest (HDBR) is a well-established terrestrial analog for simulating microgravity-induced adaptations, particularly muscle atrophy and functional decline\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Lower limb muscles are key to maintaining postural stability and movement under gravitational loading. During microgravity exposure, lower limb muscles decay at an accelerated rate due to interrupted weight-bearing cycles and reduced activity\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. The HDBR model is particularly relevant for investigating the rapid deconditioning of lower-limb muscles.\u003c/p\u003e\u003cp\u003eWhile HDBR has been widely used to study inactivity-induced adaptations, critical knowledge gaps persist. Specifically, the development of muscle fat infiltration secondary to gravitational unloading of postural muscles has received limited attention, despite its recognized role as a key indicator of muscle health\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Previous studies have reported that intramuscular lipid droplets in the \u0026minus;\u0026thinsp;6\u0026deg;head-down bed rest study had begun to accumulate by day 6\u003csup\u003e6\u003c/sup\u003e. In another 3-day dry immersion model, the muscle fiber cross-sectional area of intermuscular adipose tissue(IMAT) adipocytes tended to increase in comparison with the pre-dry immersion period\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. The fat content of lumbar spine muscles increases in simulated microgravity conditions\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Elucidating the initial mechanisms of muscle fat infiltration is essential for developing effective intervention strategies. Existing studies have mostly focused on models of long-term disuse, while fat infiltration during the acute phase (\u0026lt;\u0026thinsp;7 days) remains uncharacterized. Understanding the onset of fatty infiltration is critical, as it may represent an early, potentially irreversible step in muscle degradation. Concurrently, emerging evidence indicates spaceflight-induced hematological alterations\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, yet their association with skeletal muscle remodeling remains poorly understood. The ground-based studies have shown that indicators such as serum 25(OH)D and phosphate correlate with muscle status\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Elucidating such relationships in microgravity could reveal novel mechanistic insights to inform muscle preservation strategies.\u003c/p\u003e\u003cp\u003eMagnetic Resonance Imaging (MRI) can accurately quantify changes in the lower muscles with advanced muscle structure assessment capabilities. Proton Density Fat Fraction (PDFF) enables objective assessment of intramuscular fat infiltration\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. This technique offers superior reproducibility in tissue characterization compared to conventional methods\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Using PDFF, our study fills two key knowledge gaps: to describe and quantify acute changes in lower limb components (intramuscular/intermuscular fat, subcutaneous fat, bone marrow fat) and muscle strength during short-term HDBR; and to establish correlations between these muscular adaptations and biomarkers to improve understanding of the mechanisms responsible for the early changes in the components of the lower limb in the microgravity environment.\u003c/p\u003e"},{"header":"2. Mateirals and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1Participants\u003c/h2\u003e\n \u003cp\u003eThis prospective study was conducted from July 2024 to September 2024. The study protocol was approved by the Research Ethics Committee of Beijing Friendship Hospital, affiliated with Capital Medical University (2024-P2-069-04), and all participants signed an informed consent form (trial registration number: ChiCTR2500096128) before the experiment.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2Study Design and Subjects\u003c/h2\u003e\n \u003cp\u003eThirty-four healthy male participants were recruited for this study. All of the participants had a secondary school education or higher, no history of psychiatric, genetic, or infectious diseases, no serious musculoskeletal or cardiovascular disorders, and no history of smoking or other unhealthy habits. The experimental procedure consisted of a 3-day adaptation and baseline measurement period (PRE), a 7-day head-down bed rest period (BR), and a 5-day recovery period (R). Throughout the procedure, intake of coffee, tea, alcohol, and any medication was prohibited, and smoking was strictly forbidden. To track muscle changes, MRI scans were performed at five time points (PRE, BR3, BR7, R3, R5) (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3 Subjects diets\u003c/h2\u003e\n \u003cp\u003eThe diets of the participants in our trial were based on the Dietary Guidelines for Chinese Residents (2022) and the Chinese Dietary Reference Intakes (DRIs) (2023 Revision). The dietary plan also incorporated the nutritional support program of the previous HDBR study \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The average daily calorie intake was approximately 2,800 kcal and varied by \u0026le;\u0026thinsp;10% over a 3-day menu cycle.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4.MRI Measurements\u003c/h2\u003e\n \u003cp\u003eMRI examinations were performed on a 3.0T scanner (Siemens Vida, Erlangen, Germany). Participants were placed in the supine position, and a combination of anterior and posterior coil arrays was used. Participants were scanned with MRI at five time points (PRE, BR3, BR7, R3, and R5). For each participant at each time point, 104 transverse thigh and calf images were acquired separately, (T1 weighted 2-point Dixon sequence, slice thickness\u0026thinsp;=\u0026thinsp;2.0 mm; distance factor\u0026thinsp;=\u0026thinsp;20%, TR\u0026thinsp;=\u0026thinsp;4.34ms, TE1\u0026thinsp;=\u0026thinsp;1.34ms, TE2\u0026thinsp;=\u0026thinsp;2.76ms, flip angle\u0026thinsp;=\u0026thinsp;9 deg; field of view\u0026thinsp;=\u0026thinsp;400 mm \u0026times; 324 mm at 1.3 mm \u0026times; 1.3 mm pixel size). Total scan time was reduced to a minimum (total scan time of 5 to 7 min) and embedded in the patient\u0026rsquo;s regular clinical follow-up schedule.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5 Skeletal Muscle Segmentation\u003c/h2\u003e\n \u003cp\u003eManual segmentation of fat and muscle tissues was conducted on merged water-separated images over the four stacks using ITK-SNAP\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ewww.itksnap.org\u003c/span\u003e\u003c/span\u003e, accessed on 3 February 2020). To ensure reliability, the target area was precisely defined, and all segmentations were reviewed by a board-certified radiologist with 19 years of experience in whole-body imaging. Representative results are shown in Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eBased on the extracted PDFF and cross-sectional area (CSA) values, the contractile mass index (CMI) was calculated as follows: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:CMI=CSA\\:\\times\\:(1-PDFF)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e2.6. Statistical Analysis\u003c/h2\u003e\n \u003cp\u003eParameters are expressed as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation. All parameters were subjected to the Shapiro-Wilk normality test. Based on the normality test results, comparisons between the five time points were performed using repeated measures ANOVA (consistent with normal distribution) or generalized estimating equations (not consistent with normal distribution), with post hoc tests using Bonferroni\u0026apos;s method when significant main effects were present; comparisons between the right and left limbs at the same time point were selected using paired t-tests or Wilcoxon signed-rank tests based on the normality of the differences; and linear mixed model was used to analyses the association between blood indices and lower limb parameters at the five time points. Statistical significance was evaluated using a threshold of P\u0026thinsp;\u0026lt;\u0026thinsp;0.05. Statistical analysis was performed by using IBM SPSS Statistics for Macintosh (Version 26.0, IBM Corp., Released 2019, Armonk, NY, USA) and R version 4.4.2.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Participant Characteristics\u003c/h2\u003e\u003cp\u003eIn this study, 44 participants underwent HDBR experiments. In thigh measurements, 10 participants were excluded because they dropped out of the experiment (n\u0026thinsp;=\u0026thinsp;4) or had poor quality images (n\u0026thinsp;=\u0026thinsp;6); in calf measurements, 12 participants were excluded because they dropped out of the experiment (n\u0026thinsp;=\u0026thinsp;4) or had poor quality images (n\u0026thinsp;=\u0026thinsp;8). The participant characteristics are provided in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDemographics of subjects in the PRE\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eThigh\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCalf\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSex, N\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e34, male\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e32, male\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAge, years, mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e28.03\u0026thinsp;\u0026plusmn;\u0026thinsp;6.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e27.47\u0026thinsp;\u0026plusmn;\u0026thinsp;5.79\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBMI, mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e23.17\u0026thinsp;\u0026plusmn;\u0026thinsp;2.55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e22.90\u0026thinsp;\u0026plusmn;\u0026thinsp;2.34\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Longitudinal Analysis\u003c/h2\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eComparison of thigh and calf parameters of subjects in different periods\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePRE\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBR3\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eBR7\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eR3\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eR5\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eP-value\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e\u003cp\u003eThigh(n\u0026thinsp;=\u0026thinsp;34)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTMCSAR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15,087.10\u0026thinsp;\u0026plusmn;\u0026thinsp;1,991.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14,453.24\u0026thinsp;\u0026plusmn;\u0026thinsp;1,880.78a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e14,149.14\u0026thinsp;\u0026plusmn;\u0026thinsp;1,876.55ab\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e14,626.51\u0026thinsp;\u0026plusmn;\u0026thinsp;1,992.24abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e14,598.19\u0026thinsp;\u0026plusmn;\u0026thinsp;1,907.91ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTMCSAL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e14,839.04\u0026thinsp;\u0026plusmn;\u0026thinsp;1,958.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e14,214.27\u0026thinsp;\u0026plusmn;\u0026thinsp;1,897.19a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e13,975.89\u0026thinsp;\u0026plusmn;\u0026thinsp;1,860.76ab\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e14,369.13\u0026thinsp;\u0026plusmn;\u0026thinsp;1,913.57abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e14,391.23\u0026thinsp;\u0026plusmn;\u0026thinsp;1,824.96abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5,924.84\u0026thinsp;\u0026plusmn;\u0026thinsp;2,002.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5,607.39\u0026thinsp;\u0026plusmn;\u0026thinsp;1,941.19a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5,642.26\u0026thinsp;\u0026plusmn;\u0026thinsp;1,920.35a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5,820.70\u0026thinsp;\u0026plusmn;\u0026thinsp;1,966.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5,984.06\u0026thinsp;\u0026plusmn;\u0026thinsp;1,956.22bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5,713.29\u0026thinsp;\u0026plusmn;\u0026thinsp;1,976.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5,283.91\u0026thinsp;\u0026plusmn;\u0026thinsp;1,888.80a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5,393.34\u0026thinsp;\u0026plusmn;\u0026thinsp;1,963.20a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5,556.45\u0026thinsp;\u0026plusmn;\u0026thinsp;1,897.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5,732.25\u0026thinsp;\u0026plusmn;\u0026thinsp;1,959.96bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTTSMAR-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12.09\u0026thinsp;\u0026plusmn;\u0026thinsp;2.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e11.66\u0026thinsp;\u0026plusmn;\u0026thinsp;3.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12.24\u0026thinsp;\u0026plusmn;\u0026thinsp;3.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e12.11\u0026thinsp;\u0026plusmn;\u0026thinsp;2.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e11.95\u0026thinsp;\u0026plusmn;\u0026thinsp;2.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.259\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTTSMAL-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e11.87\u0026thinsp;\u0026plusmn;\u0026thinsp;2.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e11.75\u0026thinsp;\u0026plusmn;\u0026thinsp;2.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12.65\u0026thinsp;\u0026plusmn;\u0026thinsp;3.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e11.99\u0026thinsp;\u0026plusmn;\u0026thinsp;2.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e11.98\u0026thinsp;\u0026plusmn;\u0026thinsp;2.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.211\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATR-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e84.38\u0026thinsp;\u0026plusmn;\u0026thinsp;2.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e84.29\u0026thinsp;\u0026plusmn;\u0026thinsp;2.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e84.41\u0026thinsp;\u0026plusmn;\u0026thinsp;2.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e84.44\u0026thinsp;\u0026plusmn;\u0026thinsp;2.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e84.40\u0026thinsp;\u0026plusmn;\u0026thinsp;2.56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.730\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATL-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e85.27\u0026thinsp;\u0026plusmn;\u0026thinsp;2.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e86.02\u0026thinsp;\u0026plusmn;\u0026thinsp;2.18a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e86.12\u0026thinsp;\u0026plusmn;\u0026thinsp;2.21a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e85.45\u0026thinsp;\u0026plusmn;\u0026thinsp;2.53bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e85.85\u0026thinsp;\u0026plusmn;\u0026thinsp;2.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTCMIR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e13,276.72\u0026thinsp;\u0026plusmn;\u0026thinsp;1,902.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e12,775.37\u0026thinsp;\u0026plusmn;\u0026thinsp;1,790.03a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12,417.49\u0026thinsp;\u0026plusmn;\u0026thinsp;1,712.57ab\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e12,870.73\u0026thinsp;\u0026plusmn;\u0026thinsp;1,929.70ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e12,865.65\u0026thinsp;\u0026plusmn;\u0026thinsp;1,831.62ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTCMIL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e13,088.86\u0026thinsp;\u0026plusmn;\u0026thinsp;1,865.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e12,559.46\u0026thinsp;\u0026plusmn;\u0026thinsp;1,848.74a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12,209.20\u0026thinsp;\u0026plusmn;\u0026thinsp;1,732.71ab\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e12,660.55\u0026thinsp;\u0026plusmn;\u0026thinsp;1,864.69abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e12,679.63\u0026thinsp;\u0026plusmn;\u0026thinsp;1,769.43ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTBMATR-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e82.95\u0026thinsp;\u0026plusmn;\u0026thinsp;5.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e83.32\u0026thinsp;\u0026plusmn;\u0026thinsp;4.77\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e83.29\u0026thinsp;\u0026plusmn;\u0026thinsp;5.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e80.20\u0026thinsp;\u0026plusmn;\u0026thinsp;6.31abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e79.50\u0026thinsp;\u0026plusmn;\u0026thinsp;6.84abc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTBMATL-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e84.33\u0026thinsp;\u0026plusmn;\u0026thinsp;4.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e85.48\u0026thinsp;\u0026plusmn;\u0026thinsp;3.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e85.32\u0026thinsp;\u0026plusmn;\u0026thinsp;3.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e83.66\u0026thinsp;\u0026plusmn;\u0026thinsp;5.07bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e83.38\u0026thinsp;\u0026plusmn;\u0026thinsp;4.86bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e\u003cp\u003e\u003cb\u003eCalf(n\u0026thinsp;=\u0026thinsp;32\u003c/b\u003e)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHMCSAR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,489.32\u0026thinsp;\u0026plusmn;\u0026thinsp;336.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,401.64\u0026thinsp;\u0026plusmn;\u0026thinsp;304.28a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,349.34\u0026thinsp;\u0026plusmn;\u0026thinsp;305.74ab\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,421.64\u0026thinsp;\u0026plusmn;\u0026thinsp;316.91ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,392.86\u0026thinsp;\u0026plusmn;\u0026thinsp;326.69ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHMCSAL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,446.62\u0026thinsp;\u0026plusmn;\u0026thinsp;339.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,380.53\u0026thinsp;\u0026plusmn;\u0026thinsp;318.09a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,339.81\u0026thinsp;\u0026plusmn;\u0026thinsp;319.67a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,404.01\u0026thinsp;\u0026plusmn;\u0026thinsp;347.66c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,373.78\u0026thinsp;\u0026plusmn;\u0026thinsp;329.85a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHMCSAR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e897.62\u0026thinsp;\u0026plusmn;\u0026thinsp;211.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e881.36\u0026thinsp;\u0026plusmn;\u0026thinsp;209.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" 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colname=\"c4\"\u003e\u003cp\u003e837.01\u0026thinsp;\u0026plusmn;\u0026thinsp;158.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e835.36\u0026thinsp;\u0026plusmn;\u0026thinsp;168.32\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e838.45\u0026thinsp;\u0026plusmn;\u0026thinsp;168.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.138\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCSATR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,788.82\u0026thinsp;\u0026plusmn;\u0026thinsp;589.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,699.57\u0026thinsp;\u0026plusmn;\u0026thinsp;581.05a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,703.99\u0026thinsp;\u0026plusmn;\u0026thinsp;554.55a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,754.99\u0026thinsp;\u0026plusmn;\u0026thinsp;569.86b\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,735.82\u0026thinsp;\u0026plusmn;\u0026thinsp;560.41abd\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCSATL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,806.15\u0026thinsp;\u0026plusmn;\u0026thinsp;583.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,749.42\u0026thinsp;\u0026plusmn;\u0026thinsp;573.50a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,761.99\u0026thinsp;\u0026plusmn;\u0026thinsp;577.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,802.62\u0026thinsp;\u0026plusmn;\u0026thinsp;592.91bc\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,772.65\u0026thinsp;\u0026plusmn;\u0026thinsp;580.66d\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHIMFR-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.57\u0026thinsp;\u0026plusmn;\u0026thinsp;1.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.89\u0026thinsp;\u0026plusmn;\u0026thinsp;2.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.92\u0026thinsp;\u0026plusmn;\u0026thinsp;1.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.62\u0026thinsp;\u0026plusmn;\u0026thinsp;1.66c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.72\u0026thinsp;\u0026plusmn;\u0026thinsp;1.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.035\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHIMFL-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.84\u0026thinsp;\u0026plusmn;\u0026thinsp;1.39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.74\u0026thinsp;\u0026plusmn;\u0026thinsp;2.47\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.86\u0026thinsp;\u0026plusmn;\u0026thinsp;1.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.73\u0026thinsp;\u0026plusmn;\u0026thinsp;1.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.92\u0026thinsp;\u0026plusmn;\u0026thinsp;1.67\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.396\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHIMFR-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.89\u0026thinsp;\u0026plusmn;\u0026thinsp;1.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.44\u0026thinsp;\u0026plusmn;\u0026thinsp;2.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e8.48\u0026thinsp;\u0026plusmn;\u0026thinsp;1.42\u003c/p\u003e\u003c/td\u003e\u003ctd 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colname=\"c5\"\u003e\u003cp\u003e90.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.97\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e90.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.718\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCBMATL-FF, %\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e90.02\u0026thinsp;\u0026plusmn;\u0026thinsp;1.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e86.88\u0026thinsp;\u0026plusmn;\u0026thinsp;15.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e90.00\u0026thinsp;\u0026plusmn;\u0026thinsp;0.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e89.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e90.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.79\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.205\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHCMIR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,362.40\u0026thinsp;\u0026plusmn;\u0026thinsp;312.28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,274.09\u0026thinsp;\u0026plusmn;\u0026thinsp;284.26a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,230.48\u0026thinsp;\u0026plusmn;\u0026thinsp;287.55a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,300.71\u0026thinsp;\u0026plusmn;\u0026thinsp;298.58ac\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,272.56\u0026thinsp;\u0026plusmn;\u0026thinsp;304.95a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHCMIL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1,319.58\u0026thinsp;\u0026plusmn;\u0026thinsp;314.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1,257.88\u0026thinsp;\u0026plusmn;\u0026thinsp;299.59a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1,222.15\u0026thinsp;\u0026plusmn;\u0026thinsp;296.93a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1,282.70\u0026thinsp;\u0026plusmn;\u0026thinsp;324.10c\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1,253.16\u0026thinsp;\u0026plusmn;\u0026thinsp;310.51a\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHCMIR, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e818.66\u0026thinsp;\u0026plusmn;\u0026thinsp;197.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e805.30\u0026thinsp;\u0026plusmn;\u0026thinsp;193.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e771.14\u0026thinsp;\u0026plusmn;\u0026thinsp;149.72\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e772.18\u0026thinsp;\u0026plusmn;\u0026thinsp;170.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e766.97\u0026thinsp;\u0026plusmn;\u0026thinsp;162.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.076\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHCMIL, mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e804.57\u0026thinsp;\u0026plusmn;\u0026thinsp;166.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e748.75\u0026thinsp;\u0026plusmn;\u0026thinsp;147.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e756.24\u0026thinsp;\u0026plusmn;\u0026thinsp;144.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e758.79\u0026thinsp;\u0026plusmn;\u0026thinsp;156.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e760.82\u0026thinsp;\u0026plusmn;\u0026thinsp;156.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.150\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"7\"\u003eNote. T, Thigh; C, Calf; R, right side; L, left side; MH, the medial head of the gastrocnemius muscle; LH, the lateral head of the gastrocnemius muscle; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; TSMA, total skeletal muscle adiposity, which includes intermuscular fat and intramuscular fat; IMF, intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. a represents a statistically significant difference compared to PRE; b represents a statistically significant difference compared to BR3; c represents a statistically significant difference compared to BR7; d represents a statistically significant difference compared to R3.\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThere was a significant decrease in bilateral thigh muscle cross-sectional area (TMCSA) during bed rest (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Thigh subcutaneous adipose tissue (TSAT) decreased at the beginning of bed rest, and then gradually increased. The inflection point of the TSAT change was on the 3rd day in bed, and the inflection point of the muscle area change was on the 7th day in bed. No statistically significant absolute change in PDFF (in %) for the thigh muscle was observed in this study. However, left thigh subcutaneous adipose tissue fat fractions (TSATL-FF) increased during bed rest(p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), but the right thigh did not change significantly. The thigh contractile mass index (TCMI) decreased during bed rest༈p\u0026thinsp;\u0026lt;\u0026thinsp;0.001༉. There was no significant change in bilateral thigh bone marrow adipose tissue fat fraction (TBMAT-FF) during bed rest. However, TBMAT-FF was significantly reduced at the end of bed rest (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThere was a significant decrease in the bilateral medial head of gastrocnemius muscle cross-sectional area (MHMCSA) during bed rest (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). The calf subcutaneous adipose tissue area(CSAT) decreased at the beginning of bed rest, and then gradually increased. The inflection point of subcutaneous adipose tissue area change was on the 3rd day in bed, and the inflection point of muscle area change was on the 7th day in bed. The left calf subcutaneous adipose tissue fat fraction (CSATL-FF) increased during bed rest༈P\u0026thinsp;\u0026lt;\u0026thinsp;0.001༉, but the right calf did not change significantly. The right medial head of the gastrocnemius muscle intramuscular fat fat fraction (MHIMFR-FF) continued to rise during bed rest and gradually recovered at the end of bed rest (p\u0026thinsp;=\u0026thinsp;0.035). Medial head of gastrocnemius contractile mass index (MHCMI) decreased during bed rest༈P\u0026thinsp;\u0026lt;\u0026thinsp;0.001༉. There was no significant change in calf tibia bone marrow adipose tissue fat fraction (CBMAT-FF) during bed rest (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Comparison of left and right parameters\u003c/h2\u003e\u003cp\u003eBy comparing the left and right thigh parameters of the participants at different time points, it was found that the parameters of the left and right thigh portions were different at the beginning and end of bed rest. TMCSAR was greater than that of the TMCSAL, TSATR was greater than that of the TSATL, TSTAL-FF was greater than that of the TSTAR-FF, TCMIR was greater than that of the TCMIL, and the TBMATL-FF was greater than that of the TBMATR-FF(Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eBy comparing the left and right calf parameters of the participants at different time points, it was found that some of the parameters of the left and right calves were different during bed rest and the recovery period, and that the CSAT, the CSAT-FF, and the lateral head of the gastrocnemius muscle intramuscular fat fraction(LHIMF-FF) were greater on the left than on the right side at all time points, but the difference was significant only for some of the time points (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Correlation analysis of blood and size parameters\u003c/h2\u003e\u003cp\u003eThere were associations between some biomarkers and the MRI parameters, as shown in the following tables (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eLMM found that urinary cortisol (UCORT), red blood cell (RBC), hemoglobin (Hb), calcium (Ca), phosphorus (P), direct renin (DR), adrenocorticotropic hormone (ACTH), calcitonin (CT), osteocalcin (OC), 25 hydroxyvitamin D (25 (OH) D) were significantly correlated with thigh parameters. UCORT was negatively correlated with TSATR, TSATL, TSATR-FF, and TCMIR (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). There was a significant positive correlation between RBC and TBMATL-FF (P\u0026thinsp;=\u0026thinsp;0.039). There was a significant negative correlation between Hb and TSATL-FF (P\u0026thinsp;=\u0026thinsp;0.042). Ca was negatively correlated with the right of total skeletal muscle adiposity fat fraction (TTSMAR-FF)(P\u0026thinsp;=\u0026thinsp;0.041), and positively correlated with TCMIR (P\u0026thinsp;=\u0026thinsp;0.006); P was negatively correlated with the light of total skeletal muscle adiposity fat fraction(TTSMAL-FF) and CMIR (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and positively correlated with TBMATL-FF (p\u0026thinsp;=\u0026thinsp;0.030); DR was negatively correlated with TMCSAL, TSATR, TSATL and TCMIR (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and positively correlated with TSATL-FF (p\u0026thinsp;=\u0026thinsp;0.015); There was a significant positive correlation between ACTH and TSATR-FF (P\u0026thinsp;=\u0026thinsp;0.009); CT was positively correlated with TTSMAR-FF and TTSMAL-FF (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05); There was a significant positive correlation between OC and TTSMAL-FF (P\u0026thinsp;=\u0026thinsp;0.039); 25 (OH) d was positively correlated with TMCSAR, TMCSAL, TCMIR and TCMIL (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and negatively correlated with TTSMAR-FF and TTSMAL-FF (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSignificant Associations Between Thigh Parameters and Biomarkers From Linear Mixed Models.​\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMRI parameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomarkers parameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eβ Estimate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eP value\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eEffect size (95% CI)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTMCSAR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e44.279\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.033\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(3.695 to 84.863)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTMCSAL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-4.270\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-6.258 to -2.282)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e56.496\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.009\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(14.668 to 98.324)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-6.835\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.046\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-13.557 to -0.112)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-3.704\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.008\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-6.426 to -0.982)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-10.647\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.012\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-18.908 to -2.386)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-4.350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-7.844 to -0.857)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTTSMAR-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCa\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-3.105\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.041\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-6.086 to -0.124)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.126\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.029\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.013 to 0.239)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.134\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.010\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.235 to -0.033)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTTSMAL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-1.733\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.031\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-3.307 to -0.158)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.125\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.023\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.018 to 0.232)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.052\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.039\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.003 to 0.102)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.111\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.032\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.213 to -0.010)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATR-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.017\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.033 to -0.002)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eACTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.008\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.009\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.002 to 0.015)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTSATL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.081\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.042\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.003 to 0.160)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.008\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.002 to 0.015)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTCMIR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-4.857\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.031\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-9.251 to -0.462)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCa\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1,595.958\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.006\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(467.630 to 2,724.285)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-1,106.971\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-1,787.995 to -425.948)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-6.099\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-8.785 to -3.413)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e84.635\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(30.913 to 138.357)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTCMIL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e71.463\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(30.272 to 112.655)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTBMATL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRBC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e6.488\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.039\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.331 to 12.644)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e4.783\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.030\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.479 to 9.088)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"5\"\u003eNote. T, Thigh; R, right side; L, left side; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; TSMA, total skeletal muscle adiposity, which includes intermuscular fat and intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. UCORT, urinary cortisol; CORT, cortisol; RBC, red blood cells; WBC, white blood cells; Hb, hemoglobin. Ca, calcium; P, phosphorus; DR, direct renin; ACTH, adrenocorticotropic hormone; IPTH, all-parathyroid hormone; CT, calcitonin; OC, osteocalcin; 25(OH)D, 25-hydroxyvitamin D\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eLMM found that biomarkers significantly associated with calf parameters were UCORT, cortisol (CORT), RBC, Hb, Ca, P, DR, ACTH, all-parathyroid hormone (IPTH), OC, and 25-hydroxyvitamin D (25(OH)D). There was a negative correlation between UCORT and CSATR (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and a positive correlation with MHIMFR-FF, MHIMFL-FF, LHIMFL-FF, and CSTAR-FF (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05). There was a negative correlation between CORT and LHMCSAL (P\u0026thinsp;=\u0026thinsp;0.029). There was a positive correlation between RBC and CSATL-FF (P\u0026thinsp;=\u0026thinsp;0.002). There was a negative correlation between Hb and CSATR (P\u0026thinsp;=\u0026thinsp;0.023). There was a positive correlation between Ca and LHIMFR-FF (P\u0026thinsp;=\u0026thinsp;0.004). There was a positive correlation between P and MHMCSAR, MHMCSAL, LHMCSAR, ACTH was positively correlated with LHMCSAR, MHIMFL-FF (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05); IPTH was positively correlated with MHIMFR-FF, MHIMFL-FF, LHIMFR-FF (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and negatively correlated with CSATL-FF (P\u0026thinsp;=\u0026thinsp;0.022); OC was positively correlated with MHIMFR-FF, MHIMFL-FF (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and CBMATL-FF (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05); 25(OH)D was positively correlated with LHMCSAR, LHCMIR (P\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and negatively correlated with LHIMFR-FF (P\u0026thinsp;=\u0026thinsp;0.035) (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSignificant Associations Between Calf Parameters and Biomarkers From Linear Mixed Models.​\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMRI parameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBiomarkers parameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eβ Estimate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eP value\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eEffect size (95% CI)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHMCSAR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-158.273\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.011\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-279.118 to -37.429)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.627\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.009\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-1.091 to -0.162)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHMCSAL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-207.734\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-321.078 to -94.391)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHMCSAR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-168.079\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.017\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-305.501 to -30.657)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eACTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.548\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.042\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.020 to 1.076)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.721\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.017\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(1.779 to 17.663)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHMCSAL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-4.260\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.029\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-8.075 to -0.446)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-163.678\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.020\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-300.968 to -26.388)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCSATR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-1.168\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.021\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-2.158 to -0.178)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHb\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-2.092\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.023\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-3.897 to -0.287)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-162.330\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.006\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-276.177 to -48.483)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHIMFR-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.003\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.005 to 0.025)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIPTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.014\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.034\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.001 to 0.028)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.037\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.007\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.064 to -0.010)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHIMFL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.015\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.014\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.003 to 0.027)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eACTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.006\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.003 to 0.009)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIPTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.034\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.019 to 0.049)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.031\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.038\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.060 to -0.002)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHIMFR-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCa\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2.895\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.004\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.964 to 4.827)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIPTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.013\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.005 to 0.039)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.064\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.038\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.125 to -0.004)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHIMFL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.027\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.016\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.005 to 0.049)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCSATR-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUCORT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.024\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.011\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.006 to 0.043)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.009\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.033\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.001 to 0.018)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCSATL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRBC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.682\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.621 to 2.742)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.017\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.009 to 0.024)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIPTH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.027\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.051 to -0.004)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCBMATL-FF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.173\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.033\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(0.096 to 2.249)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.032\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.017\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.058 to -0.006)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHCMIR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-168.080\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.004\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-280.530 to -55.630)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.641\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.006\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-1.093 to -0.190)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMHCMIL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-192.697\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-301.772 to -83.622)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.524\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.022\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(-0.971 to -0.077)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLHCMIR\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25(OH)D\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.433\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.013\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e(1.994 to 16.872)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"5\"\u003eNote. C, Calf; R, right side; L, left side; MH, the medial head of the gastrocnemius muscle; LH, the lateral head of the gastrocnemius muscle; FF, fat fraction; MCSA, muscle cross-sectional area; SAT, subcutaneous adipose tissue; IMF, intramuscular fat; CMI, contractile mass index; BMAT, bone marrow adipose tissue. UCORT, urinary cortisol; CORT, cortisol; RBC, red blood cells; WBC, white blood cells; Hb, hemoglobin. Ca, calcium; P, phosphorus; DR, direct renin; ACTH, adrenocorticotropic hormone; IPTH, all-parathyroid hormone; CT, calcitonin; OC, osteocalcin; 25(OH)D, 25-hydroxyvitamin D\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eWe did a study of acute changes in the components of the lower limbs in a microgravity environment. The relationship of these changes with biomarkers using a 7-day HDBR model and found that during HDBR, muscle volume of the lower limbs decreased, muscle strength of the lower limbs decreased, and subcutaneous fat decreased and then increased, and that there was a significant correlation between biomarkers such as UCORT, CORT, RBC, Ca, and P, and muscle/fat parameters, and that the lower limbs were bilaterally with a asymmetry.\u003c/p\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.1. Bilateral Asymmetry in Lower Limbs\u003c/h2\u003e\u003cp\u003eOur study identified significant left-right differences in thigh muscle cross-sectional area, subcutaneous adipose tissue fat fraction, contraction mass index, and bone marrow adipose tissue fat fraction. These findings align with previous observations of limb dominance effects\u003csup\u003e\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Notably, left-sided elevations in SAT-FF and BMAT-FF may reflect reduced mechanical loading of the non-dominant limb \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. This asymmetry underscores the importance of considering limb dominance in microgravity analog studies, as unilateral adaptations may complicate the interpretation of global muscle-fat dynamics.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.2. Muscle atrophy: mechanisms and biomarker interactions\u003c/h2\u003e\u003cp\u003eThe rapid atrophy of thigh and gastrocnemius muscles during HDBR confirms the susceptibility of weight-bearing muscles to unloading \u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Our findings extend current knowledge by identifying integrated hormonal and metabolic pathways driving acute muscle loss.\u003c/p\u003e\u003cp\u003e25(OH)D levels are positively correlated with muscle cross-sectional area (TMCSA), and vitamin D plays a role in regulating myogenic cell growth, size, and gene expression\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. VDR knockout mice exhibit reduced grip strength and fiber atrophy due to myostatin upregulation\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. These data support vitamin D supplementation as a potential countermeasure, as 1,25(OH)₂D directly inhibits MuRF1 transcription via VDR binding\u003c/p\u003e\u003cp\u003eIn addition, changes in hormones such as growth hormone, cortisol, and growth factors, increased oxidative stress, and changes in neuromuscular function may also exacerbate the process of muscle atrophy\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. DR correlated with reduced TMCSA, linking the Renin-angiotensin system (RAS) to local muscle pathology. Angiotensin II (Ang II) promotes oxidative stress via NAD(P)H oxidase\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, triggering mitochondrial dysfunction and protein breakdown. PTH elevation, observed in our study, may exacerbate muscle loss by stimulating osteoclast-mediated calcium release, indirectly promoting proteolysis. Together, these factors contribute to the reduction in muscle mass during bedrest trials\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Our 7-day HDBR protocol captures early atrophy linked to concurrent declines in 25(OH)D and rises in DR/UCORT. This highlights the acute sensitivity of muscle to integrated stress-metabolic perturbations, underscoring the need for multi-targeted interventions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e4.3.Muscle Strength Decline\u003c/h2\u003e\u003cp\u003eThe biphasic reduction in TCMI aligns with the \"two-phase\" model of strength loss. This suggests neuromuscular dysfunction dominates initial weakness at the early phase. Structural atrophy becomes prominent, with myofiber cross-sectional area reductions mirroring TCMI declines at the late phase \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Uros reported that 79% of the loss of muscle strength may be due to muscle atrophy, and the rest may be due to alterations in single-fiber mechanical properties, excitation-contraction coupling, muscle denervation, and neuromuscular junction damage \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e4.4. Fat Remodeling\u003c/h2\u003e\u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\u003ch2\u003e4.4.1. Subcutaneous Fat (SAT) Biphasic Response\u003c/h2\u003e\u003cp\u003eThe initial SAT reduction (BR3) followed by recovery contrasts with monotonic increases in long-term bed rest\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Urinary cortisol (UCORT) inversely correlated with TSAT area, consistent with glucocorticoid-mediated activation of hormone-sensitive lipase\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Altered cortisol levels in simulated microgravity may be psychologically stress-driven \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. DR negatively correlated with TSAT, as Ang II suppresses preadipocyte differentiation via AT1R\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Our short-term data reveal a metabolic inflection point (BR3), where initial catabolism shifts to anabolism, likely driven by adaptive energy conservation. This challenges the assumption of unidirectional fat gain in microgravity analogs.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section3\"\u003e\u003ch2\u003e4.4.2. Intramuscular Fat (IMF)\u003c/h2\u003e\u003cp\u003eMHIMF-FF increased with BR time, and a 3-day dry immersion (DI) assay showed a trend towards enlarged IMAT adipocytes and a significant up-regulation of the expression of the fibro-adipogenic progenitor cells (FAPs) marker PDGFRα, suggesting that FAPs may be a key cell population mediating ectopic adipogenesis in human microgravity\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. The differentiation of FAPs may be driven by PTH\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.PTH up-regulates UCP1 in FAPs \u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, promoting beige adipocyte formation and lipid droplet deposition - consistent with our IPTH-MHIMF-FF correlation. Another study found that short-term bed rest led to the accumulation of glycogen in myocytes, triggering insulin resistance; also, medium-chain acylcarnitine was reduced, suggesting that fatty acid β-oxidation is inhibited in the early stages of muscle unloading, suggesting an early onset of lipid metabolism disorders. This suggests that bed rest induces a shift in skeletal muscle metabolic pathways to oxidize glucose more than fatty acids \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Both studies found lipid accumulation at an early stage. The DI experiments focused on IMAT, while the bed rest experiments focused on intracellular lipids in myocytes, revealing the complexity and multidimensionality of lipid metabolism disorders in the context of muscle inactivity.\u003c/p\u003e\u003cp\u003eOur study has several limitations. The 7-day head-down bedrest only simulates short-term microgravity effects, and results may not apply to long-term spaceflight. The fixed caloric intake could have obscured dietary impacts. Additionally, the sample size was limited and lacked a parallel control group. Future research should consider larger samples, control groups, longer observation periods, and nutrient management.\u003c/p\u003e\u003cp\u003eIn conclusion, this study clarified the dynamic process of muscle-fat changes in short-term simulated microgravity, identified BR3 as a key intervention inflection point, and established the association between muscle atrophy and biomarkers such as vitamin D, RAS, and PTH, which will provide a new basis for astronauts' personalized healthcare countermeasures.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are deeply grateful to all the individuals who participated as volunteers in this study. We extend our sincere appreciation for their time commitment, strict adherence to the study protocols, and invaluable contributions. Their dedication was essential to the successful completion of this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eY.S.: Original draft preparation, MRI Measurements, and multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), W.Y.: Data analysis, multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), and original draft preparation, Y.L.: Data analysis, Z.W.: Multi-tissue segmentation (including skeletal muscle, subcutaneous adipose tissue, and bone marrow), Y.L., R.W., H.H., L.C., Y.H., and P.Q.: MRI Measurements, P.R.: Manuscript reviewing and modifications, Z.W.: Conceived and designed this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by the Space Medical Experiment Project of China Manned Space Program (No. HYZHXMH01005), Beijing Hospitals Authority Innovation Studio of Young Staff Funding Support (No.202302), and Beijing Scholar 2015 (Zhenchang Wang). The funder played no role in study design, data collection, analysis and interpretation of data, or the writing of this manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGrimm, D. \u003cem\u003eet al.\u003c/em\u003e The impact of microgravity on bone in humans. \u003cem\u003eBone\u003c/em\u003e \u003cstrong\u003e87\u003c/strong\u003e, 44\u0026ndash;56 (2016).\u003c/li\u003e\n\u003cli\u003eScott, J. 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Mature adipocytes inhibit in vitro differentiation of human preadipocytes via angiotensin type 1 receptors. \u003cem\u003eDiabetes\u003c/em\u003e \u003cstrong\u003e51\u003c/strong\u003e, 1699\u0026ndash;1707 (2002).\u003c/li\u003e\n\u003cli\u003eIio, R. \u003cem\u003eet al.\u003c/em\u003e Parathyroid hormone inhibits fatty infiltration and muscle atrophy after rotator cuff tear by browning of fibroadipogenic progenitors in a rodent model. \u003cem\u003eAm J Sports Med\u003c/em\u003e \u003cstrong\u003e51\u003c/strong\u003e, 3251\u0026ndash;3260 (2023).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-microgravity","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmgrav","sideBox":"Learn more about [npj Microgravity](http://www.nature.com/npjmgrav/)","snPcode":"41526","submissionUrl":"https://submission.springernature.com/new-submission/41526/3","title":"npj Microgravity","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Microgravity, Magnetic resonance imaging (MRI), Cross-sectional area (CSA), Proton density fat fraction (PDFF), Urinary cortisol (UCORT)","lastPublishedDoi":"10.21203/rs.3.rs-7960086/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7960086/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eObjective: \u003c/strong\u003eTo investigate the effects of 7-day −6° head-down bed rest (HDBR) on lower limb muscle volume, fat, and muscle strength in healthy males.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaterials and Methods:\u003c/strong\u003e In this prospective study, thigh and calf proton density fat fraction (PDFF) magnetic resonance imaging (MRI) was performed on 34 (thigh-measured) and 32 (calf-measured) healthy male participants. These participants were subjected to the HDBR trial between July 2024 and September 2024 at five time points: baseline (PRE), bed rest day 3 (BR3), day 7 (BR7), and recovery days 3 (R3) and 5 (R5). Muscle volume, subcutaneous fat volume, muscle PDFF, and subcutaneous fat PDFF were quantified. Blood and urine samples were collected to measure biomarkers. Analysis of dynamic changes in indicators of thigh and calf parameters and their correlation with biomarkers.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults:\u003c/strong\u003e A total of 34 healthy male participants underwent HDBR. 34 (mean age, 28.03 ± 6.74 [SD]) had thigh measurements, and 32 (mean age, 27.47 ± 5.79 [SD]) had calf measurements. Bilateral thigh muscle cross-sectional area (MCSA) and bilateral medial head of gastrocnemius muscle cross-sectional area (MHMCSA) decreased consistently and significantly during HDBR (p\u0026lt;0.001). Thigh subcutaneous adipose tissue (SAT) and calf subcutaneous adipose tissue (CSAT) areas initially decreased, then gradually increased during HDBR, with inflection points at BR3. Left thigh subcutaneous adipose tissue fat fractions (SATL-FF) and left calf subcutaneous adipose tissue fat fractions (CSATL-FF) increased consistently and significantly during HDBR (p\u0026lt;0.001). The right medial head of the gastrocnemius muscle intramuscular fat fraction (MHIMFR-FF) increased significantly during HDBR (p=0.035). Significant correlations (p\u0026lt;0.05) were observed between urinary cortisol, cortisol, red blood cells, and calcium with muscle/fat parameters.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion: \u003c/strong\u003eShort-term HDBR induces lower limb muscle atrophy and subcutaneous fat redistribution, which are dynamically correlated with stress, metabolic, and bone metabolism-related biomarkers.\u003c/p\u003e","manuscriptTitle":"MRI assessment of leg muscle size and composition after a 7-day head-down bed rest","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-21 01:09:51","doi":"10.21203/rs.3.rs-7960086/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-07T22:02:54+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-24T01:00:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"308862877360148375328343193506650887055","date":"2026-02-03T17:53:34+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-29T15:42:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"238161873671270370482636388859914464545","date":"2026-01-08T04:40:16+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-10T20:08:30+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-04T18:54:54+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-31T16:41:49+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Microgravity","date":"2025-10-27T02:03:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-microgravity","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmgrav","sideBox":"Learn more about [npj Microgravity](http://www.nature.com/npjmgrav/)","snPcode":"41526","submissionUrl":"https://submission.springernature.com/new-submission/41526/3","title":"npj Microgravity","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1bcd9dd1-7d5c-4d53-bff4-04d28c982caa","owner":[],"postedDate":"November 21st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[{"id":57898623,"name":"Health sciences/Anatomy"},{"id":57898624,"name":"Health sciences/Biomarkers"},{"id":57898625,"name":"Health sciences/Health care"},{"id":57898626,"name":"Health sciences/Medical research"},{"id":57898627,"name":"Biological sciences/Physiology"}],"tags":[],"updatedAt":"2026-05-14T22:38:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-21 01:09:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7960086","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7960086","identity":"rs-7960086","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}