Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children

Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children Yesenia García-Alonso, Robinson Ramírez-Vélez, Gaizka Legarra-Gorgoñon, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4257280/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Numerous studies have documented disparities in Physical Fitness (PF) between boys and girls. This cross-sectional study investigates if echo-intensity (EI) explains the relationship between sex and PF among children. The study cohort consisted of 497 children aged 4.0–10.9, including 288 boys and 209 girls. Anthropometric measures, including height, weight, and waist circumference, were assessed. PF was evaluated using age-appropriate tests: the ALPHA Fitness battery for school children and the PREFIT battery for preschool children. Ultrasound (US) was employed to evaluate EI in the rectus femoris muscle. Our analysis revealed boys exhibited significantly higher PFcan components and echo-intensity levels compared to girls (all PF p < 0.001, EI p = 0.001). Mediation analysis revealed that the total effect of sex on overall fitness (z-score) was 0.51 (95% CI, 0.36 to 0.66), with an indirect effect mediated by EI of 0.07 (95% CI, 0.02 to 0.13), and a direct effect on overall fitness (z-score) of 0.44 (95% CI, 0.30, 0.58; 15.68% mediation). EI in the rectus femoris muscle serves as a factor elucidating how a child's sex influences their PF. These findings emphasize the importance of considering sex in exercise planning and interpreting the results of fitness assessments. These insights hold potential implications for public health strategies targeting children and adolescents. Physical fitness echo-intensity sex-differences and children Figures Figure 1 Figure 2 1. Introduction Sex differences can be attributed to specific aspects of human biology [ 1 ]. Significant sexual dimorphism in body composition concerning lean mass, fat mass, and bone mass becomes evident very early in life and undergoes the most pronounced changes during puberty [ 2 ]. Numerous studies have investigated disparities in Physical Fitness (PF) between boys and girls [ 3 ]. For instance, Latorre et al. reported that boys exhibited superior performance in CRF endurance, reaction time, strength, and running speed across various preschooler groups [ 4 ]. Consequently, reference values for PF have been established for each age group, differentiated by sex [ 5 ]. In contrast, limited research has explored gender-based disparities in muscle structure during early childhood. García-Alonso et al. are among the few who have established age-specific normal ranges for echo-intensity (EI) and developed reference values for different age groups and sexes [ 6 ]. Both PF and EI are potent indicators of children's health [ 7 ]. This association has also been observed in conditions such as type 2 diabetes and other cardiovascular risk factors among older adults [ 8 ]. Similarly, EI is a practical method for assessing age-related risks associated with reduced muscle quality [ 9 ]. Okura et al. found an inverse relationship between physical activity levels and rectus femoris EI in older adult patients [ 10 ]. The relationship between PF and EI has been explored previously, revealing an inverse association, even after adjusting for potential confounding factors such as sex in prepuberal children [ 11 ]. Therefore, it is essential to account for sex when examining these two variables. In research investigating the relationship between sex and PF, potential confounding factors are typically managed using statistical analyses such as ANOVA [ 4 ], linear regression [ 5 ], or logistic regression [ 12 ], selected based on the study's objectives and the nature of the outcome under investigation. Mediation, a statistical approach, is employed to elucidate the mechanisms underlying the connection between two variables and to assess to what extent another variable may mediate, modify, or distort this relationship [ 13 ]. A mediation effect occurs when a third variable, known as the mediator, elucidates how an independent variable influences a dependent variable. This study aims to address the hypothesis of how sex influences PF and whether EI serves as a potential mediator in this relationship. In other words, it seeks to determine whether EI can elucidate how an individual's sex impacts their PF. Therefore, the objectives of the present study are twofold: (i) to examine the relationship between PF and EI in schoolchildren, considering sex as a factor, and (ii) to examine whether the association between sex and PF is mediated by EI. 2. Methods 2.1 Study design and population In this cross-sectional study, we investigated PF and muscle ultrasound (US) parameters among Spanish schoolchildren, utilizing data from the "Observatorio de Actividad Física en escolares" (Physical Activity Observatory in Schoolchildren) available at https://observatorioactividadfisica.es . We analyzed data from 497 children, aged 4.0 to 10.9 years, comprising 288 boys and 209 girls, with a mean age (95% confidence interval [CI]) of 7.39 years, to assess the mediation effect of EI on sex differences in PF. To ensure a representative sample, participant recruitment locations were purposefully selected across various sites in the Metropolitan Region of Pamplona, Navarra, Spain. These locations encompassed a diverse range of settings, including a private school (Santa María la Real-Maristas), three state schools (San Juan de la Cadena, Lago de Mendillorri, and Garcia Galdeano), two sports centers (a private sports center, S.C.D.R Anaitasuna, and a football club, Gazte Berriak C.F), as well as a primary care facility (C.S Iturrama). Ethical considerations were paramount in our study. We obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad Pública de Navarra (CENEDUCA1/2019). 2.2 Measurements We conducted a thorough assessment of anthropometric measures, adhering to the CDC-NHANES Survey protocol [ 14 ]. Height measurements were taken in the Frankfurt position using a SECA 213® stadiometer, with a precision of 1 mm. Weight was measured with participants wearing light clothing and bare feet, utilizing a Tanita DC-430MAS® scale with a precision of 100 g. Waist circumference was measured in centimeters, employing a Seca 201® non-elastic flexible tape with a precision of 1 mm. Body mass index (BMI) was calculated using the standard formula (kg/m²). PF was evaluated through age-appropriate tests: the ALPHA Fitness battery [ 15 ] for school children and the PREFIT battery [ 16 ] for preschool children. Cardiovascular fitness was assessed using the Course Navette test, which measured the number of laps completed in a single trial. Muscle fitness was evaluated for both upper and lower body strength. Upper body strength was determined using a handgrip strength (HGS) test, where children underwent the test twice with a Takei 5001® analog dynamometer, providing isometric grip strength measurements in kilograms of force (kgf) with a precision of 100 g. Lower body strength was assessed through the standing long jump (SLJ) test, with children performing the test three times, and the best performance was recorded. Speed and agility were measured using the 4 × 10m shuttle run test. Children completed this test twice, and their best time was recorded, with shorter times indicating better performance. To establish an overall PF score, we combined the following parameters: CRF (Course Navette - laps), HGS, SLJ, and speed-agility (4 × 10 test). In the case of the speed-agility test, where a longer time denoted poorer performance, we inverted the time by multiplying it by -1 for consistency in the scoring process. The evaluation of EI within the rectus femoris muscle was conducted using real-time two-dimensional B-mode US, specifically utilizing the Esaote MyLabTM50 system from Genova, Italy. Participants were instructed to lie on their backs with extended knees and maintain a state of rest during the image capture process. To ensure accurate probe placement and image capture location, a point was marked at 50% of the distance between the anterior superior iliac spine and the midpoint of the patella. The image acquisition was performed by a skilled operator (YGA) employing a multi-frequency linear transducer with a range of 4–15 MHz. To minimize potential bias, enhance spatial resolution, and maintain consistency in muscle structure measurements, all measurements of muscle structure were acquired using fixed settings that remained constant for all participants. The image gain was set at 55 decibels (dB), the dynamic range was maintained at 72, and the image depth ranged from 45 to 50 mm. Any optional post-processing features within the software were deactivated, and the time gain compensation settings were kept neutral. US gel (Ultrasound GEL® Ref: 33273, Gima s.p.A Laboratories, Inc., Gessate, MI, Italy) was applied to ensure proper acoustic contact, and gentle pressure was exerted to visualize the muscle border partially. Subsequently, images were captured, followed by panoramic images. EI was determined based on the average gray level within the region of interest (ROI) in 8-bit resolution images, where gray levels ranged from 0 to 255 (with 0 representing black and 255 indicating white). To perform this analysis, ImageJ software (ImageJ, National Institutes of Health, USA, version 1.45s) was employed. Higher values on the grayscale spectrum indicated greater intramuscular fat and interstitial fibrous tissue accumulation [ 9 ]. Mean and standard deviation (SD) calculations were carried out for each histogram. Data processing was executed using internal software integrated into the Esaote MyLabTM50, employing a methodology similar to established procedures (9, 7). 2.3 Statistical analysis We employed Statistical Package for SPSS release 3.5.3 (IBM Corp., Portsmouth, Hampshire, UK) to investigate the potential mediation effect of EI in the relationship between sex (boys vs. girls) and our primary outcomes, which included various aspects of PF. Mediation analysis is a statistical approach that serves to elucidate the underlying mechanisms that connect two variables and assess the extent to which this connection may be influenced, mediated, or confounded by a third variable. A mediation effect occurs when a third factor, known as the mediator, plays a role in explaining the impact of an independent variable on a dependent one. This analytical approach employs nested models to estimate the proportion of the total association, while accounting for various covariates, that can be attributed to an indirect pathway facilitated by the mediator. Confidence intervals, calculated through a nonparametric bootstrapping method with 5000 iterations, were used to estimate the significance of these effects. Throughout our statistical analyses, a significance level of 0.05 for two-tailed P-values was considered meaningful. 3. Results Table 1 displays the characteristics of the study sample, segmented by sex. The cohort consisted of 497 children aged 4.0 to 10.9, comprising 288 boys and 209 girls. Notably, anthropometric parameters (except for BMI) and Our analysis revealed boys exhibited significantly higher PF components and echo-intensity levels compared to girls. Future studies could explore intervention strategies that consider EI's mediating role to enhance physical fitness equally across sexes. Table 1 Descriptive characteristics of full sample, boys and girls participating in the study. Variables Full sample (n = 497) Boys (n = 288) Girls (n = 209) P value Anthropometric parameters Age (years) 7.39 (7.22; 7.56) 7.64 (7.41; 7.88) 7.03 (6.80; 7.27) < 0.001 Height (cm) 124.82 (123.65; 126.00) 126.61 (125.02; 128.21) 122.37 (120.69; 124.05) < 0.001 Weight (kg) 27.21 (26.48; 27.93) 28.16 (27.19; 29.14) 25.89 (24.83; 26.96) 0.002 Body mass index (kg) 17.08 (16.87; 17.28) 17.18 (16.92; 17.45) 16.93 (16.61; 17.25) 0.225 Waist circumference (cm) 58.27 (57.65;58.90) 58.88 (58.05; 59.70) 57.45 (56.50; 58.39) 0.026 Physical fitness components CRF (laps) 40.23 (38.36; 42.11) 46.59 (43.90; 49.29) 31.72 (29.74; 33.70) < 0.001 Absolute HGS (kgf) 11.44 (11.11; 11.76) 12.14 (11.69; 12.59) 10.47 (10.02; 10.92) < 0.001 SLJ (cm) 119.26 (116.74; 121.78) 125.79 (122.24; 129.34) 110.25 (107.15; 113.34) < 0.001 Speed agility (s) 13.52 (13.37; 13.68) 13.19 (12.98; 13.40) 13.97 (13.75; 14.20) < 0.001 Overall fitness (z-score) 0.01 (-0.07; 0.09) 0.23 (0.12; 0.34) -0.28 (-0.37; -0.19) < 0.001 Echo-intensity 44.82 (43.61; 46.03) 43.13 (41.45; 44.81) 47.15 (45.48; 48.82) 0.001 The prevalence of EI estimated through US, and PF components measured by HGS, SLJ, speed-agility, cardiorespiratory fitness (CRF), and overall fitness, were significantly higher in boys than in girls. Specifically, HGS was significantly greater in boys than in girls (mean [SD], 12.14 [3.86] vs. 10.47 [3.32] kg; difference, 1.67 kg; P < 0.001) (Fig. 1 A). Similarly, SLJ was longer in boys than in girl (mean [SD], 125.79 [30.58] vs. 110.24 [22.61] cm; difference, 15.55 cm; P < 0.001) (Fig. 1 B). Likewise, speed agility (mean [SD], 13.19 [1.75] vs. 13.97 [1.62] sec; difference, 0.78 sec; P < 0.001) and CRF (mean [SD], 46.59 [22.79] vs. 31.71 [14.47] laps; difference, 32.12 laps; P < 0.001) were significantly higher in boys than in girls (Fig. 1 , C and D). Overall PF (z-score) was also significantly greater in the boys than in girls (mean [SD], 0.23 [0.90] vs. -0.28 [0.64]; difference, 1.67; P < 0.001) (Fig. 2 ). A mediation analysis was conducted to estimate the effect of the sex on PF. The results of the mediation analysis indicated that a significant portion of the total effect of boys on HGS (1.70 kg; 95% CI, 1.05, 2.36 kg) could be attributed to an indirect effect mediated by EI (0.33 kg; 95% CI, 0.13, 0.56 kg; 19.10% mediation). Even after accounting for the EI association, a statistically significant direct effect of sex on HGS remained at 1.37 kg (95% CI, 0.74, 1.99 kg) (Fig. 1 A). Similarly, the analysis revealed that a substantial portion of the total effect of sex on SLJ (15.57 cm; 95% CI, 10.61, 20.53 cm) was mediated by EI (2.74 cm; 95% CI, 0.99, 4.80 cm; 17.43% mediation), leaving a statistically significant direct effect of sex on SLJ at 12.82 cm (95% CI, 8.14, 17.50 cm) (Fig. 1 B). Furthermore, mediation analysis indicated that the total effect of sex on speed-agility (-0.76 sec; 95% CI, -1.07 to -0.45 sec) was partially mediated by EI (-0.13 sec; 95% CI, -0.23 to -0.04 sec), with a remaining direct effect of -0.63 sec (95% CI, -0.93, -0.33 sec; 15.11% mediation) (Fig. 1 C). Similarly, the total effect of sex on CRF (15.13 laps; 95% CI, 11.55 to 18.71 laps) was mediated in part by EI (1.87 laps; 95% CI, 0.61 to 3.31 laps), leaving a statistically significant direct effect of sex on CRF at 13.26 laps (95% CI, 9.86, 16.66 laps; 12.30% mediation) (Fig. 1 D). In summary, the mediation analysis indicated that a significant portion of the total effect of sex on overall fitness (z-score) of 0.51 (95% CI, 0.36 to 0.66) was mediated by EI (0.07; 95% CI, 0.02 to 0.13), with a remaining direct effect of overall fitness (z-score) at 0.44 (95% CI, 0.30, 0.58; 15.68% mediation) (Fig. 2 ). 4. Discussion This study is the first to employ mediation analysis to explore the role of EI in elucidating the relationship between sex and PF in schoolchildren. Intriguingly, our investigation revealed that various components of PF, including HGS, SLJ, speed-agility, CRF, and the overall fitness (z-score), were mediated by EI. These findings not only reaffirm the significant disparities in PF and EI between boys and girls but also underscore the relationship between lower levels of EI and enhanced PF. Therefore, our results propose that EI serves as a comprehensive mediator in the intricate connection between sex and PF in children. These findings may be attributed to multiple factors. The associations observed between somatotype and PF assessments predominantly stem from a shared genetic foundation among children and adolescents, irrespective of sex [ 17 ]. Furthermore, the study by Zaqout et al. highlights that modifiable targets such as BMI, physical activity, dietary habits, and psychosocial factors play pivotal roles in augmenting PF [ 18 ]. Sexual dimorphism is another crucial aspect to consider. Historically, 'sexual dimorphism' has characterized systematic anatomical differences between individuals of different sexes (14, 15). These differences in fitness tests are already evident during preschool years and tend to magnify with age. Recent insights have elucidated sexual dimorphism in substrate metabolism during post-exercise recovery and the role of lipid kinetics in supporting resting metabolism during this phase [ 20 ]. linearity on running speed and aerobic capacity seem to be crucial in prepubescent boys and girls[ 21 ]. The impact of adiposity on explosive strength, musculoskeletal proportions on running speed, and relative linearity on both running speed and aerobic capacity emerge as critical factors in this context. In exploring the mediation effect of echo-intensity on sex differences in physical fitness, it is crucial to consider the role of hormonal differences. For instance, testosterone and estrogen significantly influence muscle composition and function, potentially impacting echo-intensity and, by extension, physical fitness levels [ 22 ]. Testosterone, more prevalent in boys, is known to facilitate muscle growth and strength, possibly leading to higher echo-intensity values and better physical performance compared to girls. Conversely, estrogen, which is more dominant in girls, plays a role in fat distribution, which could influence echo-intensity measurements and physical fitness outcomes differently. Acknowledging the study's observational nature, these speculations about hormonal influences invite further research into the intricate biological pathways affecting physical fitness through muscle quality, as measured by echo-intensity. While our analyses yielded robust results, it is important to acknowledge several limitations. The study is constrained by its relatively modest sample size, and the observational nature of the study design limits causal inferences. Conclusion EI in the rectus femoris emerges as a crucial factor that elucidates how a child's sex influences their PF. Importantly, our findings reveal a significant difference in physical fitness levels between boys and girls. This disparity can be partially attributed to echo-intensity, underscoring its potential role as a mediator. These insights hold potential implications for public health strategies targeting children and adolescents. Abbreviations PF Physical fitness US Ultrasound EI echo-intensity HGS Handgrip strength SLJ Standing Long Jump CRF Cardiorespiratory fitness AU Arbitrary Units Declarations Ethics approval and consent to participate We obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad Pública de Navarra (CENEDUCA1/2019). Consent for publication Not applicable Availability of data and material The datasets used and/or analyzed during the current study can be made available from the corresponding author on reasonable request. Competing interests All authors have nothing to declare. Funding Funding for the study was provided by the Gobierno de Navarra–Departamento de Salud, co-financed by the European Regional Development Fund through the ERDF Operational Program 2014–2020 (Navarra). Authors' contributions GLG, YGA, RRV, AMA, MI designed research; GLG, YGA, RRV, AMA conducted research; GLG, YGA, RRV, AMA analyzed data; GLG, YGA, RRV, MI AMA wrote the first draft of manuscript; GLG, AMA, RRV and MI had primary responsibility for final content. All authors read and approved the final manuscript. Acknowledgments None References Regitz-Zagrosek V (2012) Sex and gender differences in health. EMBO Rep 13:596–603 Wells JCK (2007) Sexual dimorphism of body composition. Best Pract Res Clin Endocrinol Metab 21:415–430 Tyler R, Mackintosh KA, Spacey HL, Stratton G (2020) A cross-sectional study on the deprivation and sex differences in health-related fitness measures in school children. J Sports Sci 38:70–78 Latorre Román PÁ, Del Moreno R, Lucena Zurita M, Salas Sánchez J, García-Pinillos F (2017) Mora López D. Physical fitness in preschool children: association with sex, age and weight status. Child Care Health Dev 43:267–273 Tomkinson GR, Carver KD, Atkinson F, Daniell ND, Lewis LK, Fitzgerald JS et al (2018) European normative values for physical fitness in children and adolescents aged 9–17 years: results from 2 779 165 Eurofit performances representing 30 countries. Br J Sports Med 52:1445–14563 García-Alonso Y, Alonso-Martínez AM, García-Hermoso A, Legarra-Gorgoñon G, Izquierdo M, Ramírez-Vélez R (2023) Centile reference curves of the ultrasound-based characteristics of the rectus femoris muscle composition in children at 4–11 years old. Front Pediatr 11:1168253 del Fonseca FJ, Alonso JV, Álvarez MV, Orr S, Cantarero FJL (2017) Physical fitness as an indicator of health status and its relationship to academic performance during the prepubertal period. Health Promot Perspect 7:197–204 Peterson MD, McGrath R, Zhang P, Markides KS, Al Snih S, Wong R (2016) Muscle Weakness Is Associated With Diabetes in Older Mexicans: The Mexican Health and Aging Study. J Am Med Dir Assoc 17:933–938 Stock MS, Thompson BJ (2021) Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions. Eur J Appl Physiol 121:369–380 Okura K, Iwakura M, Kawagoshi A, Sugawara K, Takahashi H, Shioya T (2022) Objective physical activity level is associated with rectus femoris muscle echo-intensity in patients with chronic obstructive pulmonary disease. Clin Respir J 16:572–580 García-Alonso Y, García-Hermoso A, Alonso-Martínez AM, Legarra-Gorgoñon G, Izquierdo M, Ramírez-Vélez R (2022) Associations between physical fitness components with muscle ultrasound parameters in prepuberal children. Int J Obes 46:960–968 Lu KD, Billimek J, Bar-Yoseph R, Radom-Aizik S, Cooper DM, Anton-Culver H (2016) Sex Differences in the Relationship between Fitness and Obesity on Risk for Asthma in Adolescents. J Pediatr 176:36–42 MacKinnon D (2012) Introduction to Statistical Mediation Analysis. Routledge Questionnaires NHANES, Datasets, and, Documentation R (2023) accessed September 5, n.d. https://wwwn.cdc.gov/nchs/nhanes/Default.aspx Ruiz JR, Castro-Piñero J, España-Romero V, Artero EG, Ortega FB, Cuenca MM et al (2011) Field-based fitness assessment in young people: the ALPHA health-related fitness test battery for children and adolescents. Br J Sports Med 45:518–524 Ortega FB, Cadenas-Sánchez C, Sánchez-Delgado G, Mora-González J, Martínez-Téllez B, Artero EG et al (2015) Systematic review and proposal of a field-based physical fitness-test battery in preschool children: the PREFIT battery. Sports Med Auckl NZ 45:533–555 Silventoinen K, Maia J, Jelenkovic A, Pereira S, Gouveia É, Antunes A et al (2021) Genetics of somatotype and physical fitness in children and adolescents. Am J Hum Biol Off J Hum Biol Counc 33:e23470 Zaqout M, Vyncke K, Moreno LA, De Miguel-Etayo P, Lauria F, Molnar D et al (2016) Determinant factors of physical fitness in European children. Int J Public Health 61:573–582 Ortega-Avila JG, García-Muñoz H, Segura Ordoñez A, Salazar Contreras BC (2022) Sexual dimorphism of leptin and adiposity in children between 0 and 10 years: a systematic review and meta-analysis. Biol Sex Differ 13:47 Henderson GC (2014) Sexual Dimorphism in the Effects of Exercise on Metabolism of Lipids to Support Resting Metabolism. Front Endocrinol ;5 Marta CC, Marinho DA, Barbosa TM, Carneiro AL, Izquierdo M, Marques MC (2013) Effects of body fat and dominant somatotype on explosive strength and aerobic capacity trainability in prepubescent children. J Strength Cond Res 27:3233–3244 Handelsman DJ, Hirschberg AL, Bermon S (2018) Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocr Rev 39:803–829 Baron RM, Kenny DA (1986) The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations. J Pers Soc Psychol 51:1173–1182 Additional Declarations No competing interests reported. Supplementary Files GRAPHICALABSTRACT.docx Cite Share Download PDF Status: Posted Version 1 posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4257280","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":291881305,"identity":"6f90c717-5717-4c87-81c1-6c2bf8997b39","order_by":0,"name":"Yesenia García-Alonso","email":"","orcid":"","institution":"Universidad Pública de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Yesenia","middleName":"","lastName":"García-Alonso","suffix":""},{"id":291881306,"identity":"20c673b1-0ac3-4884-bbc3-4c30c2919f96","order_by":1,"name":"Robinson Ramírez-Vélez","email":"","orcid":"","institution":"Universidad Pública de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Robinson","middleName":"","lastName":"Ramírez-Vélez","suffix":""},{"id":291881307,"identity":"fc7415c0-1b29-40e2-ac55-9b9499331cdc","order_by":2,"name":"Gaizka Legarra-Gorgoñon","email":"","orcid":"","institution":"Universidad Pública de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Gaizka","middleName":"","lastName":"Legarra-Gorgoñon","suffix":""},{"id":291881308,"identity":"1e092eb6-4536-46f1-ba27-d553cce0a188","order_by":3,"name":"Mikel Izquierdo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBUlEQVRIiWNgGAWjYDACZjYGhgcMEkAWYwMDAxuDHEjwwANCWhKQtBiDtSTgtQasBcFJbADR+LTotrMlPkhgsLDnu5Hc+LiizCZ9ftjhh0Bb7OR0G7BrMTvMdtgA6DBmyRuJzYZnzqXlbrydZgDUkmxsdgCXFvY2CaAWNoMbiW2SjW2HczfOTgBpOZC4DbeW9h9ALTxALe0/gVrSDWenfyCghe0YKMQkQLYwArUkyEvnELKFLVkiwUDCQPLMw2bJhnNphhukcwoOJBjg8cv5Y4YfPlTU2fMdT3/4saHMRl5+dvpmoIidHC4tEGAAxDAFBgdgIgQBTIt8AzGqR8EoGAWjYCQBAOwtYSkWVxtuAAAAAElFTkSuQmCC","orcid":"","institution":"Universidad Pública de Navarra","correspondingAuthor":true,"prefix":"","firstName":"Mikel","middleName":"","lastName":"Izquierdo","suffix":""},{"id":291881309,"identity":"495b2360-6424-4425-9c5a-3e7674187db1","order_by":4,"name":"Alicia M Alonso-Martínez","email":"","orcid":"","institution":"Universidad Pública de Navarra","correspondingAuthor":false,"prefix":"","firstName":"Alicia","middleName":"M","lastName":"Alonso-Martínez","suffix":""}],"badges":[],"createdAt":"2024-04-12 10:59:39","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4257280/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4257280/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":55250900,"identity":"a06fa21f-1eb5-4651-9625-2159cc82f357","added_by":"auto","created_at":"2024-04-24 17:37:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":170947,"visible":true,"origin":"","legend":"\u003cp\u003eEffects for the relationship between boys vs. girls for HGS (A), SLJ (B), speed-agility (C) and CRF (D), mediated through EI. Linear regressions models were fitted according to the procedures outlined by Baron and Kenny [23] to assess whether the association between boys vs. girls and HGS, SLJ, speed-agility and CRF was mediated by EI. The first equation (β\u003csub\u003e1\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003ea\u003c/strong\u003e\u003c/em\u003e) regressed the mediator (EI) on the independent variable (boys vs. girls). The second equation (β\u003csub\u003e3\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/em\u003e) regressed the dependent variable (HGS, SLJ, speed-agility and CRF) on the independent variable. The third equation regressed (btotal, path c) the dependent variable on both the independent variable and the mediator variable. The direct effect, denoted as (β\u003csub\u003edir\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003ec´\u003c/strong\u003e\u003c/em\u003e), is the effect coefficient of independent variable in a linear regression and the mediator on the dependent variable. The indirect effect is the component of the total effect that is mediated through EI (\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e–\u003cem\u003e\u003cstrong\u003ec’\u003c/strong\u003e\u003c/em\u003e) in the relationship between dependent variable and the independent variable; it is denoted by β\u003csub\u003e1\u003c/sub\u003e_ β\u003csub\u003e3 \u003c/sub\u003e(β\u003csub\u003eind\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"Figure11.png","url":"https://assets-eu.researchsquare.com/files/rs-4257280/v1/f9a80f0e1ee9ffbb7761f294.png"},{"id":55250901,"identity":"10207579-b327-4844-997e-915111e42050","added_by":"auto","created_at":"2024-04-24 17:37:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":78994,"visible":true,"origin":"","legend":"\u003cp\u003eEffects for the relationship between boys vs. girls for overall fitness \u003cem\u003e(z-score)\u003c/em\u003e, mediated through EI. Linear regressions models were fitted according to the procedures outlined by Baron and Kenny\u0026nbsp; [23] to assess whether the association between boys vs. girls and overall fitness \u003cem\u003e(z-score)\u003c/em\u003e, was mediated by EI. The first equation (β\u003csub\u003e1\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003ea\u003c/strong\u003e\u003c/em\u003e) regressed the mediator (EI) on the independent variable (boys vs. girls). The second equation (β\u003csub\u003e3\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/em\u003e) regressed the dependent variable overall fitness \u003cem\u003e(z-score)\u003c/em\u003e, on the independent variable. The third equation regressed (btotal, path c) the dependent variable on both the independent variable and the mediator variable. The direct effect, denoted as (β\u003csub\u003edir\u003c/sub\u003e, path \u003cem\u003e\u003cstrong\u003ec´\u003c/strong\u003e\u003c/em\u003e), is the effect coefficient of independent variable in a linear regression and the mediator on the dependent variable. The indirect effect is the component of the total effect that is mediated through EI (\u003cem\u003e\u003cstrong\u003ec\u003c/strong\u003e\u003c/em\u003e–\u003cem\u003e\u003cstrong\u003ec’\u003c/strong\u003e\u003c/em\u003e) in the relationship between dependent variable and the independent variable; it is denoted by β\u003csub\u003e1\u003c/sub\u003e_ β\u003csub\u003e3 \u003c/sub\u003e(β\u003csub\u003eind\u003c/sub\u003e).\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4257280/v1/c069a08b6f74d2898f899381.png"},{"id":59439957,"identity":"124078ea-59bc-41fe-9875-0b4bff6c8266","added_by":"auto","created_at":"2024-07-01 20:46:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":618477,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4257280/v1/697ece3b-d989-466c-be31-5aabc1b0ebe7.pdf"},{"id":55250902,"identity":"266a6436-aab2-4f06-ad2e-c1ba618edc06","added_by":"auto","created_at":"2024-04-24 17:37:45","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":164253,"visible":true,"origin":"","legend":"","description":"","filename":"GRAPHICALABSTRACT.docx","url":"https://assets-eu.researchsquare.com/files/rs-4257280/v1/a7cdbe2a1a86d0a823ca4139.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSex differences can be attributed to specific aspects of human biology [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Significant sexual dimorphism in body composition concerning lean mass, fat mass, and bone mass becomes evident very early in life and undergoes the most pronounced changes during puberty [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Numerous studies have investigated disparities in Physical Fitness (PF) between boys and girls [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. For instance, Latorre et al. reported that boys exhibited superior performance in CRF endurance, reaction time, strength, and running speed across various preschooler groups [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Consequently, reference values for PF have been established for each age group, differentiated by sex [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn contrast, limited research has explored gender-based disparities in muscle structure during early childhood. Garc\u0026iacute;a-Alonso et al. are among the few who have established age-specific normal ranges for echo-intensity (EI) and developed reference values for different age groups and sexes [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Both PF and EI are potent indicators of children's health [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. This association has also been observed in conditions such as type 2 diabetes and other cardiovascular risk factors among older adults [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Similarly, EI is a practical method for assessing age-related risks associated with reduced muscle quality [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Okura et al. found an inverse relationship between physical activity levels and rectus femoris EI in older adult patients [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe relationship between PF and EI has been explored previously, revealing an inverse association, even after adjusting for potential confounding factors such as sex in prepuberal children [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Therefore, it is essential to account for sex when examining these two variables.\u003c/p\u003e \u003cp\u003eIn research investigating the relationship between sex and PF, potential confounding factors are typically managed using statistical analyses such as ANOVA [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], linear regression [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], or logistic regression [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], selected based on the study's objectives and the nature of the outcome under investigation. Mediation, a statistical approach, is employed to elucidate the mechanisms underlying the connection between two variables and to assess to what extent another variable may mediate, modify, or distort this relationship [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. A mediation effect occurs when a third variable, known as the mediator, elucidates how an independent variable influences a dependent variable.\u003c/p\u003e \u003cp\u003eThis study aims to address the hypothesis of how sex influences PF and whether EI serves as a potential mediator in this relationship. In other words, it seeks to determine whether EI can elucidate how an individual's sex impacts their PF. Therefore, the objectives of the present study are twofold: (i) to examine the relationship between PF and EI in schoolchildren, considering sex as a factor, and (ii) to examine whether the association between sex and PF is mediated by EI.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study design and population\u003c/h2\u003e \u003cp\u003eIn this cross-sectional study, we investigated PF and muscle ultrasound (US) parameters among Spanish schoolchildren, utilizing data from the \"Observatorio de Actividad F\u0026iacute;sica en escolares\" (Physical Activity Observatory in Schoolchildren) available at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://observatorioactividadfisica.es\u003c/span\u003e\u003cspan address=\"https://observatorioactividadfisica.es\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. We analyzed data from 497 children, aged 4.0 to 10.9 years, comprising 288 boys and 209 girls, with a mean age (95% confidence interval [CI]) of 7.39 years, to assess the mediation effect of EI on sex differences in PF.\u003c/p\u003e \u003cp\u003eTo ensure a representative sample, participant recruitment locations were purposefully selected across various sites in the Metropolitan Region of Pamplona, Navarra, Spain. These locations encompassed a diverse range of settings, including a private school (Santa Mar\u0026iacute;a la Real-Maristas), three state schools (San Juan de la Cadena, Lago de Mendillorri, and Garcia Galdeano), two sports centers (a private sports center, S.C.D.R Anaitasuna, and a football club, Gazte Berriak C.F), as well as a primary care facility (C.S Iturrama).\u003c/p\u003e \u003cp\u003eEthical considerations were paramount in our study. We obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad P\u0026uacute;blica de Navarra (CENEDUCA1/2019).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Measurements\u003c/h2\u003e \u003cp\u003eWe conducted a thorough assessment of anthropometric measures, adhering to the CDC-NHANES Survey protocol [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Height measurements were taken in the Frankfurt position using a SECA 213\u0026reg; stadiometer, with a precision of 1 mm. Weight was measured with participants wearing light clothing and bare feet, utilizing a Tanita DC-430MAS\u0026reg; scale with a precision of 100 g. Waist circumference was measured in centimeters, employing a Seca 201\u0026reg; non-elastic flexible tape with a precision of 1 mm. Body mass index (BMI) was calculated using the standard formula (kg/m\u0026sup2;).\u003c/p\u003e \u003cp\u003ePF was evaluated through age-appropriate tests: the ALPHA Fitness battery [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e] for school children and the PREFIT battery [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] for preschool children. Cardiovascular fitness was assessed using the Course Navette test, which measured the number of laps completed in a single trial. Muscle fitness was evaluated for both upper and lower body strength. Upper body strength was determined using a handgrip strength (HGS) test, where children underwent the test twice with a Takei 5001\u0026reg; analog dynamometer, providing isometric grip strength measurements in kilograms of force (kgf) with a precision of 100 g. Lower body strength was assessed through the standing long jump (SLJ) test, with children performing the test three times, and the best performance was recorded. Speed and agility were measured using the 4 \u0026times; 10m shuttle run test. Children completed this test twice, and their best time was recorded, with shorter times indicating better performance. To establish an overall PF score, we combined the following parameters: CRF (Course Navette - laps), HGS, SLJ, and speed-agility (4 \u0026times; 10 test). In the case of the speed-agility test, where a longer time denoted poorer performance, we inverted the time by multiplying it by -1 for consistency in the scoring process.\u003c/p\u003e \u003cp\u003eThe evaluation of EI within the rectus femoris muscle was conducted using real-time two-dimensional B-mode US, specifically utilizing the Esaote MyLabTM50 system from Genova, Italy. Participants were instructed to lie on their backs with extended knees and maintain a state of rest during the image capture process. To ensure accurate probe placement and image capture location, a point was marked at 50% of the distance between the anterior superior iliac spine and the midpoint of the patella. The image acquisition was performed by a skilled operator (YGA) employing a multi-frequency linear transducer with a range of 4\u0026ndash;15 MHz.\u003c/p\u003e \u003cp\u003eTo minimize potential bias, enhance spatial resolution, and maintain consistency in muscle structure measurements, all measurements of muscle structure were acquired using fixed settings that remained constant for all participants. The image gain was set at 55 decibels (dB), the dynamic range was maintained at 72, and the image depth ranged from 45 to 50 mm. Any optional post-processing features within the software were deactivated, and the time gain compensation settings were kept neutral. US gel (Ultrasound GEL\u0026reg; Ref: 33273, Gima s.p.A Laboratories, Inc., Gessate, MI, Italy) was applied to ensure proper acoustic contact, and gentle pressure was exerted to visualize the muscle border partially. Subsequently, images were captured, followed by panoramic images.\u003c/p\u003e \u003cp\u003eEI was determined based on the average gray level within the region of interest (ROI) in 8-bit resolution images, where gray levels ranged from 0 to 255 (with 0 representing black and 255 indicating white). To perform this analysis, ImageJ software (ImageJ, National Institutes of Health, USA, version 1.45s) was employed. Higher values on the grayscale spectrum indicated greater intramuscular fat and interstitial fibrous tissue accumulation [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Mean and standard deviation (SD) calculations were carried out for each histogram. Data processing was executed using internal software integrated into the Esaote MyLabTM50, employing a methodology similar to established procedures (9, 7).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Statistical analysis\u003c/h2\u003e \u003cp\u003eWe employed Statistical Package for SPSS release 3.5.3 (IBM Corp., Portsmouth, Hampshire, UK) to investigate the potential mediation effect of EI in the relationship between sex (boys vs. girls) and our primary outcomes, which included various aspects of PF.\u003c/p\u003e \u003cp\u003eMediation analysis is a statistical approach that serves to elucidate the underlying mechanisms that connect two variables and assess the extent to which this connection may be influenced, mediated, or confounded by a third variable. A mediation effect occurs when a third factor, known as the mediator, plays a role in explaining the impact of an independent variable on a dependent one. This analytical approach employs nested models to estimate the proportion of the total association, while accounting for various covariates, that can be attributed to an indirect pathway facilitated by the mediator. Confidence intervals, calculated through a nonparametric bootstrapping method with 5000 iterations, were used to estimate the significance of these effects.\u003c/p\u003e \u003cp\u003eThroughout our statistical analyses, a significance level of 0.05 for two-tailed P-values was considered meaningful.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e displays the characteristics of the study sample, segmented by sex. The cohort consisted of 497 children aged 4.0 to 10.9, comprising 288 boys and 209 girls. Notably, anthropometric parameters (except for BMI) and Our analysis revealed boys exhibited significantly higher PF components and echo-intensity levels compared to girls. Future studies could explore intervention strategies that consider EI's mediating role to enhance physical fitness equally across sexes.\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\u003eDescriptive characteristics of full sample, boys and girls participating in the study.\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=\"char\" char=\".\" 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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariables\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFull sample\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;497)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBoys\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;288)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGirls\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;209)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eP value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eAnthropometric parameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAge (years)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.39 (7.22; 7.56)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e7.64 (7.41; 7.88)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.03 (6.80; 7.27)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eHeight (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e124.82 (123.65; 126.00)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e126.61 (125.02; 128.21)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e122.37 (120.69; 124.05)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eWeight (kg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e27.21 (26.48; 27.93)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e28.16 (27.19; 29.14)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25.89 (24.83; 26.96)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.002\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBody mass index (kg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e17.08 (16.87; 17.28)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.18 (16.92; 17.45)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16.93 (16.61; 17.25)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.225\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWaist circumference (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e58.27 (57.65;58.90)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e58.88 (58.05; 59.70)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e57.45 (56.50; 58.39)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.026\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003e\u003cb\u003ePhysical fitness components\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCRF (laps)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40.23 (38.36; 42.11)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e46.59 (43.90; 49.29)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e31.72 (29.74; 33.70)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eAbsolute HGS (kgf)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e11.44 (11.11; 11.76)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.14 (11.69; 12.59)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10.47 (10.02; 10.92)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eSLJ (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e119.26 (116.74; 121.78)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e125.79 (122.24; 129.34)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e110.25 (107.15; 113.34)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eSpeed agility (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13.52 (13.37; 13.68)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13.19 (12.98; 13.40)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13.97 (13.75; 14.20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003eOverall fitness (z-score)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.01 (-0.07; 0.09)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.23 (0.12; 0.34)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-0.28 (-0.37; -0.19)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\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\u003e\u003cb\u003eEcho-intensity\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e44.82 (43.61; 46.03)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e43.13 (41.45; 44.81)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e47.15 (45.48; 48.82)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe prevalence of EI estimated through US, and PF components measured by HGS, SLJ, speed-agility, cardiorespiratory fitness (CRF), and overall fitness, were significantly higher in boys than in girls. Specifically, HGS was significantly greater in boys than in girls (mean [SD], 12.14 [3.86] vs. 10.47 [3.32] kg; difference, 1.67 kg; P\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). Similarly, SLJ was longer in boys than in girl (mean [SD], 125.79 [30.58] vs. 110.24 [22.61] cm; difference, 15.55 cm; P\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). Likewise, speed agility (mean [SD], 13.19 [1.75] vs. 13.97 [1.62] sec; difference, 0.78 sec; P\u0026thinsp;\u0026lt;\u0026thinsp;0.001) and CRF (mean [SD], 46.59 [22.79] vs. 31.71 [14.47] laps; difference, 32.12 laps; P\u0026thinsp;\u0026lt;\u0026thinsp;0.001) were significantly higher in boys than in girls (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, C and D). Overall PF (z-score) was also significantly greater in the boys than in girls (mean [SD], 0.23 [0.90] vs. -0.28 [0.64]; difference, 1.67; P\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA mediation analysis was conducted to estimate the effect of the sex on PF. The results of the mediation analysis indicated that a significant portion of the total effect of boys on HGS (1.70 kg; 95% CI, 1.05, 2.36 kg) could be attributed to an indirect effect mediated by EI (0.33 kg; 95% CI, 0.13, 0.56 kg; 19.10% mediation). Even after accounting for the EI association, a statistically significant direct effect of sex on HGS remained at 1.37 kg (95% CI, 0.74, 1.99 kg) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). Similarly, the analysis revealed that a substantial portion of the total effect of sex on SLJ (15.57 cm; 95% CI, 10.61, 20.53 cm) was mediated by EI (2.74 cm; 95% CI, 0.99, 4.80 cm; 17.43% mediation), leaving a statistically significant direct effect of sex on SLJ at 12.82 cm (95% CI, 8.14, 17.50 cm) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB).\u003c/p\u003e \u003cp\u003eFurthermore, mediation analysis indicated that the total effect of sex on speed-agility (-0.76 sec; 95% CI, -1.07 to -0.45 sec) was partially mediated by EI (-0.13 sec; 95% CI, -0.23 to -0.04 sec), with a remaining direct effect of -0.63 sec (95% CI, -0.93, -0.33 sec; 15.11% mediation) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC). Similarly, the total effect of sex on CRF (15.13 laps; 95% CI, 11.55 to 18.71 laps) was mediated in part by EI (1.87 laps; 95% CI, 0.61 to 3.31 laps), leaving a statistically significant direct effect of sex on CRF at 13.26 laps (95% CI, 9.86, 16.66 laps; 12.30% mediation) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD).\u003c/p\u003e \u003cp\u003eIn summary, the mediation analysis indicated that a significant portion of the total effect of sex on overall fitness (z-score) of 0.51 (95% CI, 0.36 to 0.66) was mediated by EI (0.07; 95% CI, 0.02 to 0.13), with a remaining direct effect of overall fitness (z-score) at 0.44 (95% CI, 0.30, 0.58; 15.68% mediation) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThis study is the first to employ mediation analysis to explore the role of EI in elucidating the relationship between sex and PF in schoolchildren. Intriguingly, our investigation revealed that various components of PF, including HGS, SLJ, speed-agility, CRF, and the overall fitness (z-score), were mediated by EI. These findings not only reaffirm the significant disparities in PF and EI between boys and girls but also underscore the relationship between lower levels of EI and enhanced PF. Therefore, our results propose that EI serves as a comprehensive mediator in the intricate connection between sex and PF in children.\u003c/p\u003e \u003cp\u003eThese findings may be attributed to multiple factors. The associations observed between somatotype and PF assessments predominantly stem from a shared genetic foundation among children and adolescents, irrespective of sex [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Furthermore, the study by Zaqout et al. highlights that modifiable targets such as BMI, physical activity, dietary habits, and psychosocial factors play pivotal roles in augmenting PF [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Sexual dimorphism is another crucial aspect to consider. Historically, 'sexual dimorphism' has characterized systematic anatomical differences between individuals of different sexes (14, 15). These differences in fitness tests are already evident during preschool years and tend to magnify with age. Recent insights have elucidated sexual dimorphism in substrate metabolism during post-exercise recovery and the role of lipid kinetics in supporting resting metabolism during this phase [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. linearity on running speed and aerobic capacity seem to be crucial in prepubescent boys and girls[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The impact of adiposity on explosive strength, musculoskeletal proportions on running speed, and relative linearity on both running speed and aerobic capacity emerge as critical factors in this context.\u003c/p\u003e \u003cp\u003eIn exploring the mediation effect of echo-intensity on sex differences in physical fitness, it is crucial to consider the role of hormonal differences. For instance, testosterone and estrogen significantly influence muscle composition and function, potentially impacting echo-intensity and, by extension, physical fitness levels [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Testosterone, more prevalent in boys, is known to facilitate muscle growth and strength, possibly leading to higher echo-intensity values and better physical performance compared to girls. Conversely, estrogen, which is more dominant in girls, plays a role in fat distribution, which could influence echo-intensity measurements and physical fitness outcomes differently. Acknowledging the study's observational nature, these speculations about hormonal influences invite further research into the intricate biological pathways affecting physical fitness through muscle quality, as measured by echo-intensity.\u003c/p\u003e \u003cp\u003eWhile our analyses yielded robust results, it is important to acknowledge several limitations. The study is constrained by its relatively modest sample size, and the observational nature of the study design limits causal inferences.\u003c/p\u003e \u003cp\u003e \u003cb\u003eConclusion\u003c/b\u003e \u003c/p\u003e \u003cp\u003eEI in the rectus femoris emerges as a crucial factor that elucidates how a child's sex influences their PF. Importantly, our findings reveal a significant difference in physical fitness levels between boys and girls. This disparity can be partially attributed to echo-intensity, underscoring its potential role as a mediator. These insights hold potential implications for public health strategies targeting children and adolescents.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ePF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ePhysical fitness\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUltrasound\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eEI\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eecho-intensity\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHGS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eHandgrip strength\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSLJ\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eStanding Long Jump\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCRF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCardiorespiratory fitness\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eAU\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eArbitrary Units\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe obtained informed consent from parents or guardians, and children provided informed assent before participating. The study protocol adhered to the principles outlined in the Helsinki Declaration and received approval from the Ethics Committee of the Universidad P\u0026uacute;blica de Navarra (CENEDUCA1/2019).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analyzed during the current study can be made available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors have nothing to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFunding for the study was provided by the Gobierno de Navarra\u0026ndash;Departamento de Salud, co-financed by the European Regional Development Fund through the ERDF Operational Program 2014\u0026ndash;2020 (Navarra).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eGLG, YGA, RRV, AMA, MI designed research; GLG, YGA, RRV, AMA conducted research; GLG, YGA, RRV, AMA analyzed data; GLG, YGA, RRV, MI AMA wrote the first draft of manuscript; GLG, AMA, RRV and MI had primary responsibility for final content. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNone\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRegitz-Zagrosek V (2012) Sex and gender differences in health. EMBO Rep 13:596\u0026ndash;603\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWells JCK (2007) Sexual dimorphism of body composition. Best Pract Res Clin Endocrinol Metab 21:415\u0026ndash;430\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTyler R, Mackintosh KA, Spacey HL, Stratton G (2020) A cross-sectional study on the deprivation and sex differences in health-related fitness measures in school children. J Sports Sci 38:70\u0026ndash;78\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLatorre Rom\u0026aacute;n P\u0026Aacute;, Del Moreno R, Lucena Zurita M, Salas S\u0026aacute;nchez J, Garc\u0026iacute;a-Pinillos F (2017) Mora L\u0026oacute;pez D. Physical fitness in preschool children: association with sex, age and weight status. Child Care Health Dev 43:267\u0026ndash;273\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTomkinson GR, Carver KD, Atkinson F, Daniell ND, Lewis LK, Fitzgerald JS et al (2018) European normative values for physical fitness in children and adolescents aged 9\u0026ndash;17 years: results from 2 779 165 Eurofit performances representing 30 countries. Br J Sports Med 52:1445\u0026ndash;14563\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGarc\u0026iacute;a-Alonso Y, Alonso-Mart\u0026iacute;nez AM, Garc\u0026iacute;a-Hermoso A, Legarra-Gorgo\u0026ntilde;on G, Izquierdo M, Ram\u0026iacute;rez-V\u0026eacute;lez R (2023) Centile reference curves of the ultrasound-based characteristics of the rectus femoris muscle composition in children at 4\u0026ndash;11 years old. Front Pediatr 11:1168253\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003edel Fonseca FJ, Alonso JV, \u0026Aacute;lvarez MV, Orr S, Cantarero FJL (2017) Physical fitness as an indicator of health status and its relationship to academic performance during the prepubertal period. Health Promot Perspect 7:197\u0026ndash;204\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePeterson MD, McGrath R, Zhang P, Markides KS, Al Snih S, Wong R (2016) Muscle Weakness Is Associated With Diabetes in Older Mexicans: The Mexican Health and Aging Study. J Am Med Dir Assoc 17:933\u0026ndash;938\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStock MS, Thompson BJ (2021) Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions. Eur J Appl Physiol 121:369\u0026ndash;380\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOkura K, Iwakura M, Kawagoshi A, Sugawara K, Takahashi H, Shioya T (2022) Objective physical activity level is associated with rectus femoris muscle echo-intensity in patients with chronic obstructive pulmonary disease. Clin Respir J 16:572\u0026ndash;580\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGarc\u0026iacute;a-Alonso Y, Garc\u0026iacute;a-Hermoso A, Alonso-Mart\u0026iacute;nez AM, Legarra-Gorgo\u0026ntilde;on G, Izquierdo M, Ram\u0026iacute;rez-V\u0026eacute;lez R (2022) Associations between physical fitness components with muscle ultrasound parameters in prepuberal children. Int J Obes 46:960\u0026ndash;968\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLu KD, Billimek J, Bar-Yoseph R, Radom-Aizik S, Cooper DM, Anton-Culver H (2016) Sex Differences in the Relationship between Fitness and Obesity on Risk for Asthma in Adolescents. J Pediatr 176:36\u0026ndash;42\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMacKinnon D (2012) Introduction to Statistical Mediation Analysis. Routledge\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQuestionnaires NHANES, Datasets, and, Documentation R (2023) accessed September 5, n.d. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://wwwn.cdc.gov/nchs/nhanes/Default.aspx\u003c/span\u003e\u003cspan address=\"https://wwwn.cdc.gov/nchs/nhanes/Default.aspx\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRuiz JR, Castro-Pi\u0026ntilde;ero J, Espa\u0026ntilde;a-Romero V, Artero EG, Ortega FB, Cuenca MM et al (2011) Field-based fitness assessment in young people: the ALPHA health-related fitness test battery for children and adolescents. Br J Sports Med 45:518\u0026ndash;524\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOrtega FB, Cadenas-S\u0026aacute;nchez C, S\u0026aacute;nchez-Delgado G, Mora-Gonz\u0026aacute;lez J, Mart\u0026iacute;nez-T\u0026eacute;llez B, Artero EG et al (2015) Systematic review and proposal of a field-based physical fitness-test battery in preschool children: the PREFIT battery. Sports Med Auckl NZ 45:533\u0026ndash;555\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSilventoinen K, Maia J, Jelenkovic A, Pereira S, Gouveia \u0026Eacute;, Antunes A et al (2021) Genetics of somatotype and physical fitness in children and adolescents. Am J Hum Biol Off J Hum Biol Counc 33:e23470\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZaqout M, Vyncke K, Moreno LA, De Miguel-Etayo P, Lauria F, Molnar D et al (2016) Determinant factors of physical fitness in European children. Int J Public Health 61:573\u0026ndash;582\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eOrtega-Avila JG, Garc\u0026iacute;a-Mu\u0026ntilde;oz H, Segura Ordo\u0026ntilde;ez A, Salazar Contreras BC (2022) Sexual dimorphism of leptin and adiposity in children between 0 and 10 years: a systematic review and meta-analysis. Biol Sex Differ 13:47\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHenderson GC (2014) Sexual Dimorphism in the Effects of Exercise on Metabolism of Lipids to Support Resting Metabolism. Front Endocrinol ;5\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMarta CC, Marinho DA, Barbosa TM, Carneiro AL, Izquierdo M, Marques MC (2013) Effects of body fat and dominant somatotype on explosive strength and aerobic capacity trainability in prepubescent children. J Strength Cond Res 27:3233\u0026ndash;3244\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHandelsman DJ, Hirschberg AL, Bermon S (2018) Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocr Rev 39:803\u0026ndash;829\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaron RM, Kenny DA (1986) The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations. J Pers Soc Psychol 51:1173\u0026ndash;1182\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Physical fitness, echo-intensity, sex-differences and children","lastPublishedDoi":"10.21203/rs.3.rs-4257280/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4257280/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNumerous studies have documented disparities in Physical Fitness (PF) between boys and girls. This cross-sectional study investigates if echo-intensity (EI) explains the relationship between sex and PF among children. The study cohort consisted of 497 children aged 4.0\u0026ndash;10.9, including 288 boys and 209 girls. Anthropometric measures, including height, weight, and waist circumference, were assessed. PF was evaluated using age-appropriate tests: the ALPHA Fitness battery for school children and the PREFIT battery for preschool children. Ultrasound (US) was employed to evaluate EI in the rectus femoris muscle. Our analysis revealed boys exhibited significantly higher PFcan components and echo-intensity levels compared to girls (all PF p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, EI p\u0026thinsp;=\u0026thinsp;0.001). Mediation analysis revealed that the total effect of sex on overall fitness (z-score) was 0.51 (95% CI, 0.36 to 0.66), with an indirect effect mediated by EI of 0.07 (95% CI, 0.02 to 0.13), and a direct effect on overall fitness (z-score) of 0.44 (95% CI, 0.30, 0.58; 15.68% mediation). EI in the rectus femoris muscle serves as a factor elucidating how a child's sex influences their PF. These findings emphasize the importance of considering sex in exercise planning and interpreting the results of fitness assessments. These insights hold potential implications for public health strategies targeting children and adolescents.\u003c/p\u003e","manuscriptTitle":"Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-24 17:37:40","doi":"10.21203/rs.3.rs-4257280/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9140dfdb-116a-4568-80cd-0163a9577d5e","owner":[],"postedDate":"April 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-01T20:38:23+00:00","versionOfRecord":[],"versionCreatedAt":"2024-04-24 17:37:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4257280","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4257280","identity":"rs-4257280","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}