Effects of Mild Hyperbaric Normoxia on Physiological Recovery After Exercise in a Hot Environment

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Sedilla, Ilham Bakri, Takafumi Maeda This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9078861/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 Background Mild hyperbaric normoxia (MHN), defined as exposure to increased ambient pressure while breathing normal air, is a non-oxygen-enriched hyperbaric recovery approach. Simulated firefighting activities impose substantial cardiovascular strain and thermal strain, providing a relevant model for recovery research. This study examined cardiac and thermoregulatory responses during recovery following simulated firefighting under MHN conditions. Methods Twelve healthy young male participants performed a 20-minute cycling exercise at 60% of age-predicted maximal heart rate in a controlled hot environment (31°C, 60% relative humidity) while wearing full firefighting gear. Post-exercise recovery was conducted under three atmospheric pressure conditions: normobaric normoxia (1.0 atm absolute [ATA] and hyperbaric normoxia (1.5 and 1.7 ATA) for 20 minutes in a randomized order. Heart rate, cardiac output, stroke volume, core and skin temperatures, and skin blood flow were continuously measured, whereas blood pressure and subjective thermal responses were obtained every 5 minutes. Results MHN at 1.5 ATA significantly reduced heart rate during recovery compared with 1.0 and 1.7 ATA ( p = 0.027), indicating enhanced autonomic recovery, whereas blood pressure showed no significant differences across conditions. Core body temperature remained stable across pressures; however, mean skin temperature was higher at 1.5 and 1.7 ATA than at 1.0 ATA ( p = 0.048). Skin blood flow declined similarly over time regardless of condition. Thermal comfort differed between pressure conditions (p < 0.001), with the 1.7 ATA condition differing from both 1.0 ATA and 1.5 ATA, whereas 1.0 ATA and 1.5 ATA showed similar ratings corresponding to comfortable conditions. Thermal sensation changed significantly over time but was generally perceived as slightly cool across all pressures. Conclusion MHN at 1.5 ATA appears to facilitate faster cardiac recovery after exercise-induced heat stress, potentially through enhanced parasympathetic activation, while altered skin temperature responses may reflect changes in peripheral heat exchange. These results provide preliminary evidence supporting the use of mild hyperbaric normoxia as a recovery strategy for firefighters and other individuals working in extreme heat, although additional studies in real operational settings are needed to confirm its practical applicability. Mild Hyperbaric Normoxia Physiological Recovery Thermoregulation Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Hyperbaric oxygen therapy (HBOT) is a well-established clinical treatment that involves breathing 100% oxygen at pressures above atmospheric levels within a sealed chamber. This therapy significantly increases the amount of oxygen dissolved in the plasma, enhances tissue oxygenation, and promotes biological processes such as angiogenesis, modulation of inflammation, and accelerated tissue repair [ 1 , 2 ]. Although traditionally used to manage decompression sickness, carbon monoxide poisoning, and chronic wounds [ 3 , 4 ], HBOT has recently shown promise in supporting recovery from physical exertion and heat-induced physiological stress [ 5 ]. Beyond clinical applications, interest has grown in the role of hyperbaric environments for supporting physiological recovery in populations exposed to high physical and environmental demands. Among occupations with extreme physical and environmental demands, firefighters are particularly vulnerable to thermal stress and cardiovascular overload. Wearing heavy protective gear during intense physical activity in high-temperature environments elevates core body temperature and circulatory load, impairing recovery and increasing health risks. Sudden cardiac death accounts for approximately 45% of line-of-duty deaths among firefighters in the United States, highlighting the urgent need for effective post-exertional recovery strategies [ 6 ]. Therefore, the National Fire Protection Association recommends that firefighters take a recovery period of approximately 10–20 minutes. Moreover, as climate change intensifies the frequency and duration of heatwaves, such interventions are becoming increasingly critical for occupational safety and resilience [ 7 ]. Various cooling strategies have been implemented to mitigate heat stress in firefighters, including forearm immersion, cold water ingestion, and the use of an ice vest. While these methods have demonstrated short-term benefits in reducing core body temperature and improving post-task performance, their practicality and effectiveness may be limited during real-world firefighting operations [ 8 ]. Recent studies have explored mild hyperbaric oxygen therapy (mHBOT), typically administered at approximately 1.25 ATA with moderately elevated oxygen concentrations (26–28% O₂). For example, Qu et al. reported that mHBOT improved muscle fatigue recovery and cardiovascular responses in male athletes, suggesting potential relevance for occupational settings exposed to heat stress [ 9 ]. However, the use of elevated oxygen concentrations makes it difficult to isolate the physiological effects of increased ambient pressure alone. To address this limitation, the present study applies a mild hyperbaric normoxia (MHN) approach using normal air (FiO₂: 20.9% O₂) to examine the physiological effects of increased ambient pressure without hyperoxia exposure. While hyperbaric oxygen therapy (HBOT) has been widely studied, evidence regarding the physiological effects of MHN in post-exercise recovery remains limited. Previous experimental studies have demonstrated that mild hyperbaric exposure while breathing normal air can elicit physiological responses, supporting the feasibility of this approach [ 10 – 12 ]. Unlike HBOT, which involves 100% oxygen exposure and fundamentally differs from the normoxic conditions examined in the present study, MHN may represent a practical approach for exploratory research in healthy populations. Building on this evidence, the present study aims to examine the potential role of MHN in enhancing physiological recovery following exercise in hot environments, with implications for occupational heat-stress management, safety, and performance. Methods Participants Twelve healthy and physically active male students aged 20–25 years participated in this study (age, 21.7 ± 0.8 years; height, 170.4 ± 5.3 cm; body mass, 59.6 ± 7.4 kg; body fat percentage, 15 ± 6%). Prior to participation, all participants completed a health-screening form, which included a medical history questionnaire and a current health declaration. All participants were fully informed about the study procedures, potential risks, and their right to withdraw at any time, without penalty. Written informed consent was obtained from each participant before participation. The study protocol was reviewed and approved by the Ethics Committee of the Faculty of Design, Kyushu University (Approval No. 600). Experimental Design All participants visited the research laboratory on four separate occasions. The initial visit involved a briefing on the experimental trial, baseline measurements, including the standard YMCA submaximal test [ 13 ] to estimate participants’ maximal oxygen consumption, and familiarization with the protocol. The subsequent visits were dedicated to the actual experimental trials. During the actual experimental trials, prior to instrumentation, participants' hydration status was assessed using a urine-specific gravity refractometer (PAL-10S; ATAGO Co., Ltd., Tokyo, Japan). Participants showing signs of dehydration were asked to rehydrate by drinking water before continuing, provided their levels were below 1,030 mg/dL. Each participant completed a standardized exercise and recovery protocol inside the climatic chamber with controlled environmental conditions (31°C and 60% relative humidity). To induce a firefighting-like heat load—based solely on thermal stress and the wearing of full personal protective equipment (PPE), rather than any actual operational firefighting tasks—participants performed a 20-minute cycling exercise on a cycle ergometer at 60% of their age-predicted maximal heart rate while wearing full firefighters' protective clothing, including a helmet and respiratory mask. This simulated occupational heat exposure reflects a typical firefighting thermal scenario. Immediately after the exercise session, participants were transferred to the adjacent climatic chamber, where further instrumentation and adjustments to the atmospheric pressure required approximately 8 minutes. Participants then sat in a mesh chair for a 20-minute recovery phase. During recovery, the protective gear was loosened, and the mask and helmet were taken off. Each participant underwent recovery under three separate atmospheric pressure conditions: 1.0 ATA, based on that day's ambient pressure, 1.5 ATA (1,130 mmHg), and 1.7 ATA (1,270 mmHg), each with 20.9% oxygen concentration. The conditions were randomized and counterbalanced across participants to control for the order effects. Measurements Physiological and subjective thermal responses were continuously monitored throughout the exercise and recovery phases. Core body temperature was measured using an ear canal thermistor (earplug type), whereas skin temperatures were recorded at seven anatomical sites: the forehead, abdomen, forearm, hand, thigh, calf, and instep. These measurements were collected at 1-second intervals using a data logger (LT-8A, Gram Corporation, Japan; precision ± 0.01°C). The mean skin temperature (MST) was calculated using the Hardy–Dubois 7-point formula [ 14 ]. Physiological measurements, including heart rate and stroke volume for estimating cardiac output (using PF07 Q-Link PhysioFlow®), as well as skin blood flow (SkBF) at the forearm and finger, measured with a blood flowmeter (Omegaflow, Omegawave, Tokyo, Japan), were continuously recorded to enable the evaluation of peripheral vasomotor responses during heat stress and subsequent recovery. Additionally, blood pressure was measured before exercise, immediately after exercise, and every 5 minutes during the recovery phase. The Borg Rating of Perceived Exertion (RPE) scale was used to evaluate physical exertion. Moreover, subjective thermal perceptions were evaluated using two standardized scales: thermal comfort (1–4; very uncomfortable to very comfortable) and thermal sensation (1–7; very cold to very hot). Statistical Analysis All data are presented as means ± standard deviation. Statistical analyses of physiological measurements were performed using JASP software (version 0.19.2.0). A two-way repeated-measures ANOVA was used to analyze heart rate, skin blood flow, systolic and diastolic blood pressure, and skin and ear canal temperatures, with Bonferroni-adjusted post-hoc pairwise comparisons applied when significant main or interaction effects were observed. Subjective thermal perceptions were analyzed separately using the Friedman test in Jamovi software (version 2.6.44), followed by post-hoc pairwise comparisons with appropriate adjustments for multiple testing when significant differences were detected. Statistical significance was set at p < 0.05. Results The results present a comparative analysis of the physiological and subjective thermal responses during recovery under three atmospheric pressure conditions (1.0, 1.5, and 1.7 ATA). The data at the end of the exercise were shown at time zero, with recovery measurements initiated eight minutes post-exercise. Cardiac Responses Heart Rate Heart rate gradually decreased during the recovery period, as evidenced by a main effect of time ( F = 21.42, p < 0.001, ηp ² = 0.401). Furthermore, significant differences in heart rate were also found across atmospheric pressure conditions ( F = 4.06, p = 0.027, ηp ² = 0.202). However, the interaction between time and condition did not reach significance ( F = 0.73, p = 0.697, ηp ² = 0.044), indicating that heart rate recovery was comparable across conditions. Bonferroni-adjusted post hoc comparisons revealed a significant difference between the 1.5 ATA and 1.7 ATA conditions ( p = 0.028). As shown in Fig. 1 a, heart rate decreased during the recovery period across all atmospheric pressure conditions; notably, the 1.5 ATA condition exhibited the most pronounced decline, the 1.0 ATA condition showed a more gradual reduction, and the 1.7 ATA condition maintained a relatively higher heart rate throughout the recovery period. Cardiac Output Cardiac output did not significantly change across atmospheric pressure conditions during recovery, as shown in Fig. 1 b. The results revealed no main effects of condition ( F = 0.171, p = 0.843, ηp ² = 0.011), time ( F = 1.076, p = 0.376, ηp ² = 0.033), or interaction effects ( F = 0.493, p = 0.893, ηp ² = 0.030). However, visual inspection of the trend revealed condition-specific temporal patterns. The 1.5 ATA condition displayed a more stable, gradually decreasing cardiac output, while the 1.0 ATA and 1.7 ATA conditions showed an initial decline, reaching a minimum around 13 minutes, followed by a slight increase during later recovery stages. Stroke Volume Stroke volume was not significantly affected by atmospheric pressure conditions during recovery, as indicated by the absence of a main effect of condition ( F = 0.07, p = 0.933, ηp ² = 0.004; Fig. 1 c). Similarly, no main effect of time was observed ( F = 0.27, p = 0.931, ηp ² = 0.008), suggesting that stroke volume remained stable across measurement intervals throughout the recovery period. Furthermore, the time-by-condition interaction was not significant ( F = 0.69, p = 0.734, ηp ² = 0.041), indicating a consistent stroke volume response across all atmospheric pressure conditions. Ear Canal Temperature The results showed a significant main effect of time on ear canal temperature, F = 104.79, p < 0.001, ηp ² = 0.766, suggesting that ear canal temperature decreased over time. The main effect of condition was not significant, F = 0.379, p = 0.687, ηp ² = 0.023; however, a significant interaction between time and condition was found, F = 1.842, p = 0.001, ηp ² = 0.103, indicating that different conditions had distinct temporal patterns. Holm-adjusted post-hoc tests found no significant differences between conditions at individual time points ( p > 0.05). As shown in Fig. 2 , the 1.0 ATA condition showed the steepest decline, the 1.5 ATA condition showed a gradual decrease with a brief drop around minute 23, and the 1.7 ATA condition showed an intermediate pattern. Mean Skin Temperature In mean skin temperature, the results revealed a significant main effect of time, F = 35.31, p < 0.001, ηp ² = 0.525, indicating a general decline throughout the recovery period. There was also a significant main effect of condition, F = 3.33, p = 0.048, ηp ² = 0.172. Furthermore, the time-by-condition interaction was significant, F = 1.62, p = 0.01, ηp ² = 0.092, suggesting that the pattern of change over time differed across conditions. Bonferroni-corrected post-hoc comparisons revealed that the 1.0 ATA condition differed significantly from the 1.5 ATA condition at minutes 18 and 28. Overall, the 1.0 ATA condition exhibited the fastest decline over time, while the 1.5 ATA condition declined more gradually (Fig. 3 ). Skin Temperatures Overall, skin temperature decreased at 5 of 7 measurement sites, with the lowest values observed at 1.0 ATA condition. In contrast, at 1.5 ATA, a pronounced decrease was observed only in three sites: the hand, forearm, and forehead. As shown in Fig. 4 a and Fig. 4 c, forehead and forearm temperatures declined more rapidly at 1.5 ATA, while abdominal and instep temperatures (Fig. 4 b and Fig. 4 g) mainly decreased at 1.0 ATA. However, a significant main effect of time ( p 0.05). Hand temperature revealed a significant main effect of time ( F = 4.75, p < 0.001, ηp ² = 0.129) and a significant time-by-condition interaction ( F = 2.42, p < 0.001, ηp ²= 0.131). No significant between-subjects effects were observed for condition ( F = 1.63, p = 0.210, ηp ²= 0.093) as shown in Fig. 4 d. Post hoc analysis indicated significant differences between 1.0 ATA and 1.5 ATA, and between 1.5 ATA and 1.7 ATA, at minute 28. The results revealed a significant main effect of time on thigh temperature ( F = 3.32, p = 0.001, ηp ² = 0.072; Fig. 4 e) and a significant main effect of condition ( F = 4.01, p = 0.022, ηp ² = 0.212), while the time-by-condition interaction did not reach statistical significance ( F = 1.79 p = 0.003, ηp ² = 0.100). Post-hoc analyses of the time effect showed significant differences between 1.0 and 1.7 ATA at minutes 13, 18, and 23 ( p < 0.05). Lastly, calf temperature (Fig. 4 f) showed a significant main effect of time ( F = 37.92, p < 0.001, ηp ² = 0.542) and a significant interaction between time and conditions ( F = 1.59, p = 0.013, ηp ² = 0.09), while no significant effect of condition was observed ( p = 0.303). Skin Blood Flow and Blood Pressure Forearm skin blood flow showed a significant main effect of time ( F = 2.86, p < 0.001, ηp ² = 0.09), but there were no significant effects of condition ( F = 0.05, p = 0.948, ηp ² = 0.004) or the interaction between time and condition ( F = 0.56, p = 0.989, ηp ² = 0.004). This suggests that skin blood flow changed over the recovery period but was not affected by different atmospheric pressure conditions. On the other hand, finger skin blood flow did not change significantly, with no notable effects of condition ( F = 0.62, p = 0.546, ηp ² = 0.041), time ( F = 0.38, p = 0.995, ηp ² = 0.013), or their interaction ( F = 0.66, p = 0.950, ηp ² = 0.044). Furthermore, blood pressure changes during recovery were primarily time dependent. Systolic blood pressure demonstrated a significant main effect of time ( F = 2.49, p = 0.046, ηp ² = 0.072), whereas neither the condition ( F = 0.20, p = 0.818, ηp ² = 0.012) nor the time-by-condition interaction ( F = 0.62, p = 0.761, ηp ² = 0.037) showed significant effects. Diastolic blood pressure followed a similar temporal pattern, exhibiting a significant effect of time ( F = 2.73, p = 0.032, ηp ²= 0.078), but was unaffected by atmospheric pressure condition ( F = 0.42, p = 0.66, ηp ² = 0.026) or the time-by-condition interaction ( F = 0.584, p = 0.789, ηp ² = 0.035). Subjective Thermal Perceptions Regarding thermal comfort, the results showed a significant effect of atmospheric pressure ( χ ² = 70.2, df = 2, p < 0.001), with post hoc analysis indicating that the 1.7 ATA condition differed from both the 1.0 ATA and 1.5 ATA conditions. No difference was found between 1.0 ATA and 1.5 ATA. Since lower scores indicate better thermal comfort, both the 1.0 ATA and 1.5 ATA conditions had median scores of 2 (“comfortable”). Descriptively, thermal comfort improved over time across all conditions, with mean scores decreasing from 2.32 to 1.83 under 1.0 ATA, from 2.13 to 1.54 under 1.5 ATA, and from 2.18 to 1.64 under 1.7 ATA, with the lower reduction observed under the 1.5 ATA condition. Moreover, thermal sensation scores decreased over time under all atmospheric pressure conditions, with lower average values observed at 1.5 ATA (from approximately 4.05 to 2.50), followed by 1.7 ATA (from about 4.20 to 2.45), and 1.0 ATA (from roughly 3.83 to 2.95). When all time points were combined, the results revealed a significant effect of pressure condition ( χ ² = 7.47, df = 2, p = 0.024). Post hoc analysis showed a significant difference only between the 1.0 ATA and 1.5 ATA groups ( p = 0.006). Nonetheless, all conditions had the same median thermal sensation score of 3.0, indicating a slightly cool thermal sensation. Discussion This study examined the effects of MHN on physiological recovery following exercise in a hot environment. The experimental trial in this study aims to simulate a firefighting-like heat load, and our findings reveal a split response. The heart rate recovered more rapidly at 1.5 ATA, whereas the skin temperature decreased more quickly at 1.0 ATA. These contrasting patterns can be further explained by distinct underlying mechanisms. Cardiac Response In this study, a significant reduction in heart rate during recovery was observed at 1.5 ATA, suggesting enhanced parasympathetic activation accompanied by a relative reduction in sympathetic nervous activity. Overall, the trend showed a consistently lower heart rate at 1.5 ATA compared with 1.0 ATA and 1.7 ATA, although the differences were driven by the trend rather than individual time points. Direct evidence examining heart rate recovery under mild hyperbaric normoxia at pressures comparable to the present study is limited; therefore, this interpretation is based on physiological plausibility. The findings suggest that 1.5 ATA may represent an optimal balance, whereby moderately increased ambient pressure enhances oxygen delivery, facilitates ATP resynthesis, and reduces cardiac strain, thereby promoting a stronger parasympathetic response as part of overall autonomic modulation, including sympathetic influences. This interpretation is consistent with reports showing reduced heart rate and autonomic modulation during hyperbaric exposure, even when breathing normal air [ 15 , 16 ]. In contrast, at higher pressure (1.7 ATA), enhanced hyperoxia-driven vasoconstriction may increase systemic vascular resistance and cardiovascular load, potentially modulating heart rate responses. Such vasoconstrictive effects are well documented during hyperoxic exposure and may be amplified under hyperbaric conditions due to markedly elevated arterial oxygen partial pressure [ 17 ]. Furthermore, stroke volume and cardiac output did not differ across pressure conditions, indicating that the observed cardiovascular responses were primarily driven by changes in heart rate rather than alterations in stroke volume, a pattern also observed during hyperbaric air exposure in humans [ 15 , 16 ]. Blood pressure showed no significant differences across pressure conditions but demonstrated a significant effect of time during recovery, reflecting the decline in arterial pressure. The fact that blood pressure changed similarly across all conditions indicates that the temporal pattern of blood pressure recovery was primarily driven by time rather than ambient pressure, despite previous reports of pressure-related BP elevation [ 18 ]. Skin blood flow showed no significant differences across the three pressure conditions and decreased progressively over time during recovery, indicating that recovery was primarily influenced by temporal factors rather than ambient pressure. Taken together, these findings suggest that skin blood flow recovery under mild hyperbaric normoxia follows a time-dependent pattern. Thermoregulation and Impaired Heat Dissipation The core temperature in the ear canal remained stable across hyperbaric pressures, reflecting intact central thermoregulation. Kujawski et al. reported that core body temperature exhibits only minimal, non-significant fluctuations during compression and decompression phases in a hyperbaric chamber, indicating preserved thermal balance under hyperbaric exposure [ 19 ]. Evidence from human studies further shows that heat-acclimated individuals maintain a stable core temperature, whereas peripheral and cardiovascular systems adapt to repeated thermal stress [ 20 ]. Ear canal temperature measurement is a reliable method for estimating deep-body temperature under controlled conditions [ 21 ], and its stability in the present study supports the conclusion that hyperbaric normoxia at 1.5–1.7 ATA does not impose additional heat stress or disrupt recovery of oxygen delivery, heart function, or metabolism. In contrast, the mean skin temperature was significantly higher at 1.5 and 1.7 ATA than at 1.0 ATA [ 22 ], likely attributable to pressure-induced compression of protective clothing—governed by Boyle's Law ( \({P}_{1}{V}_{1}={P}_{2}{V}_{2}\) ), which reduces air layer volumes in garments—increasing conductive heat gain while impairing convective and evaporative cooling, and slowing skin heat dissipation independent of metabolic heat changes. Elevated ambient pressure mechanically compresses clothing ensembles, diminishing the skin-fabric insulating air layer and heat loss efficiency [ 23 , 24 ]. Finger skin temperature decreased most rapidly at 1.5 ATA during recovery, indicating stronger peripheral vasoconstriction to conserve core heat. In contrast, thigh and calf skin covered by protective clothing showed the lowest temperature at 1.0 ATA, highlighting regional differences influenced by clothing coverage and vascular responses [ 22 ]. Consistent with previous evidence, uncovered skin regions, such as the hands, are more sensitive to environmental pressure because they lack insulation and can lose heat more rapidly through convection and evaporation [ 25 ]. These findings provide preliminary evidence that mild hyperbaric normoxia at 1.5 ATA may be beneficial for recovery after heat stress. However, these findings should be interpreted with consideration of methodological limitations, as skin temperature was measured while participants wore full protective clothing, and neither sweat rate nor the microclimate within the clothing was assessed, which may have influenced heat dissipation responses. Subjective Thermal Perceptions Thermal comfort showed significant changes over time across all pressure conditions, indicating that recovery duration was the main factor affecting comfort perception. This temporal shift agrees with studies examining thermal environments in hyperbaric chambers, where comfort depended more on thermal conditions and exposure time than on atmospheric pressure alone [ 26 , 27 ]. Both 1.0 ATA and 1.5 ATA conditions were perceived as comfortable, with no statistical difference between them, whereas 1.7 ATA was rated significantly less comfortable. These findings align with broader thermal comfort research, which indicates that comfort changes as individuals physiologically adjust to their environment, with time-dependent adaptation playing a key role in shaping subjective comfort evaluations [ 28 ]. Overall, the present results support the idea that temporal adaptation, rather than mild pressure variation under normoxic conditions, is the main factor influencing thermal comfort in hyperbaric conditions. Thermal sensation changed significantly over time across all pressure conditions, indicating a strong influence of exposure duration on heat perception, consistent with previous findings showing that thermal sensation is highly responsive to physiological adaptation [ 28 ]. Although the combined median thermal sensation score was the same across conditions (median = 3.0), corresponding to a slightly cool sensation, a significant difference was observed between the 1.0 ATA and 1.5 ATA conditions, while no significant differences were found involving the 1.7 ATA condition. The greater decrease in thermal sensation observed at 1.5 ATA compared with 1.0 ATA may reflect subtle pressure-related influences on peripheral heat exchange, such as changes in skin heat dissipation and clothing compression, which can become more apparent over time. These results are largely consistent with Hu et al. [ 27 ], suggesting that ambient pressure alone does not substantially alter thermal perception unless it affects heat transfer or thermal balance. Overall, these findings indicate that changes in thermal sensation under mild hyperbaric normoxic conditions are primarily driven by time-dependent physiological adaptation rather than ambient pressure itself. Conclusion These findings indicate that mild hyperbaric normoxia at 1.5 ATA facilitates a faster post-exercise reduction in heart rate than 1.7 ATA or normobaric conditions, suggesting improved autonomic vagal recovery. Although core temperature, blood pressure, and skin blood flow did not differ significantly across pressure conditions, their consistent decreases over time suggest that MHN may help maintain physiological stability during recovery. The higher skin temperatures observed at 1.5 ATA likely reflect decreased evaporative cooling due to compression of protective clothing and altered sweat evaporation under pressure, suggesting a potential effect of mild hyperbaric normoxia on peripheral heat exchange—an aspect relevant to thermal comfort and safety in occupational settings. Both thermal comfort and thermal sensation were significantly influenced by time, indicating that perceptual responses were mainly driven by temporal adaptation rather than pressure differences. Although no statistical differences were observed across pressure conditions, descriptive patterns indicated that 1.5 ATA was perceived as slightly more comfortable than 1.0 and 1.7 ATA. These findings suggest that exposure duration, rather than pressure conditions, is the primary factor influencing perceptual outcomes in hyperbaric normoxia. Overall, the results suggest that mild hyperbaric normoxia at 1.5 ATA promotes quicker vagal-mediated heart-rate recovery, supports physiological stability, affects peripheral heat exchange, and offers slightly better perceptual comfort—potentially serving as a recovery method after heat stress, although confirmation in firefighter populations and real operational settings remains necessary. Abbreviations MHN Mild hyperbaric normoxia HBOT Hyperbaric oxygen therapy mHBOT Mild hyperbaric oxygen therapy ATA Atmospheres absolute HR Heart rate CO Cardiac output SV Stroke volume SkBF Skin blood flow MST Mean skin temperature RPE Rating of perceived exertion PPE Personal protective equipment FiO₂ Fraction of inspired oxygen. Declarations Not applicable. Ethics approval and consent to participate This study was reviewed and approved by the Ethics Committee of the Graduate School of Design at Kyushu University. The approval number is 600 (Acceptance number: 833), issued on February 29, 2024. All experimental procedures were carried out in accordance with the committee's ethical standards and the Declaration of Helsinki. Consent for publication Not applicable. Competing interests The authors declare that they have no competing interests. Funding Not applicable. Author Contribution Nadzirah Ikasari Syamsul (NS), Ilham Bakri (IB), and Takafumi Maeda (TM) conceived and designed the research. NS and Keneth B. Sedilla (KS) conducted experimental trials. NS analyzed the data and wrote the manuscript. TM, KS, and IB edited the manuscript and provided critical feedback. All authors read and approved the final manuscript. Acknowledgement This research was supported by the Maeda Laboratory (Environmental Ergonomics Laboratory) and the Research Center for Human Environmental Adaptation at Kyushu University. We would like to thank the participants for their time and willingness to take part in this study. We also thank the members of the Maeda Laboratory for their contributions to this research. Furthermore, we would like to acknowledge Mr. Yasuhiko Maeda, a technical staff member at the Research Center for Human Environmental Adaptation, for his technical support throughout the experiment. 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The effects of hyperbaric exposure on immediate and delayed changes in core temperature and its circadian fluctuations. Polish Hyperbaric Research 60, 37–48 (2017). Périard, J. D., Racinais, S. & Sawka, M. N. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scand. J. Med. Sci. Sport. 25, 20–38 exposure(2015). Gasim, G. I., Musa, I. R., Abdien, M. T., & Adam, I. (2013). Accuracy of tympanic temperature measurement using an infrared thermometer. BMC Research Notes , 6 , 194. Dünnwald, T. et al. Safety and Feasibility Study of A Hyperbaric Infrared Chamber Combining Hyperbaric Oxygen Therapy and Low Temperature Infrared Radiation. International Journal of Innovative Research in Medical Science (IJIRMS) , 01, 349 to 358–349 358 (2016).26. Chen, Y. S., Fan, J., Qian, X., & Zhang, W. (2004). Effect of garment fit on thermal insulation and evaporative resistance. Textile Research Journal , 74 (8), 742–748. Havenith, G. (2002). Interaction of clothing and thermoregulation. Exogenous Dermatology , 1 (5), 221–230. Sweet, D. K. et al. Thermoregulation during a six-hour exposure to warm, humid hyperbaric conditions. Undersea Hyperb. Med. J. Undersea Hyperb. Med. Soc. Inc 49, 459–465 (2022). Mourot, L., Cluzeau, C. & Regnard, J. Hyperbaric Gaseous Cryotherapy: Effects on Skin Temperature and Systemic Vasoconstriction. Arch. Phys. Med. Rehabil. 88, 1339–1343 (2007). Hu, J., Zhang, H., & Xu, Q. (2016). Evaluation of thermal comfort in hyperbaric oxygen chamber [Conference: The 2nd Information Technology and Mechatronics Engineering Conference (ITOEC 2016) ResearchGate. Mekjavic, I. B., Eiken, O., & Tipton, M. (2021). Perception of Thermal Comfort during Skin Cooling and Heating, 11(7), 681. Additional Declarations No competing interests reported. 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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-9078861","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":621880961,"identity":"5cd83333-e751-4b65-b10f-defd4612ec9b","order_by":0,"name":"Nadzirah Ikasari Syamsul","email":"data:image/png;base64,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","orcid":"","institution":"Kyushu University","correspondingAuthor":true,"prefix":"","firstName":"Nadzirah","middleName":"Ikasari","lastName":"Syamsul","suffix":""},{"id":621880962,"identity":"13acf887-75c7-440d-a92d-4ea31415db52","order_by":1,"name":"Keneth B. Sedilla","email":"","orcid":"","institution":"Kyushu University","correspondingAuthor":false,"prefix":"","firstName":"Keneth","middleName":"B.","lastName":"Sedilla","suffix":""},{"id":621880963,"identity":"809d4a0d-8a0d-4880-8add-7700caf2434c","order_by":2,"name":"Ilham Bakri","email":"","orcid":"","institution":"Hasanuddin University","correspondingAuthor":false,"prefix":"","firstName":"Ilham","middleName":"","lastName":"Bakri","suffix":""},{"id":621880964,"identity":"ed0ad82a-e5cb-4546-97df-75709ca32026","order_by":3,"name":"Takafumi Maeda","email":"","orcid":"","institution":"Kyushu University","correspondingAuthor":false,"prefix":"","firstName":"Takafumi","middleName":"","lastName":"Maeda","suffix":""}],"badges":[],"createdAt":"2026-03-10 04:09:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9078861/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9078861/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107043066,"identity":"8e6e05f4-1216-4837-abfd-a8a0b54a8b26","added_by":"auto","created_at":"2026-04-16 06:45:50","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":43229,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in (a) heart rate, (b) cardiac output, and (c) stroke volume during the recovery period under three atmospheric pressure conditions (1.0, 1.5, and 1.7 ATA). Values are presented as mean ± SD. *Heart rate differed significantly between the 1.5 and 1.7 ATA conditions (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05). The \u003cem\u003ep\u003c/em\u003e-values for one-way repeated measures ANOVA are noted.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9078861/v1/32a1ceaa884b442dabecd795.jpg"},{"id":107043067,"identity":"bc16f45c-5c48-49db-a64c-c452989c7b51","added_by":"auto","created_at":"2026-04-16 06:45:50","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":19962,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in ear canal temperature during the recovery period under three atmospheric pressure conditions (1.0, 1.5, and 1.7 ATA). Values are presented as mean ± SD. The \u003cem\u003ep\u003c/em\u003e-values for one-way repeated measures ANOVA are noted.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9078861/v1/32e90ce92e9a1f41faf13379.jpg"},{"id":107043068,"identity":"b7ad9b9c-fe74-4a3a-a90e-132a99c64caa","added_by":"auto","created_at":"2026-04-16 06:45:50","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":21723,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in mean skin temperature during the recovery period under three atmospheric pressure conditions (1.0, 1.5, and 1.7 ATA). Values are presented as mean ± SD. *Mean skin temperature differed significantly between the 1.0 and 1.5 ATA conditions (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05). The \u003cem\u003ep\u003c/em\u003e-values for one-way repeated measures ANOVA are noted.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9078861/v1/fa8687c14088659b6fc760fa.jpg"},{"id":107043069,"identity":"9e35fc35-ff56-41af-9421-3274afec02e1","added_by":"auto","created_at":"2026-04-16 06:45:50","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":83852,"visible":true,"origin":"","legend":"\u003cp\u003eSkin temperature changes at the (a) forehead, (b) abdomen, (c) forearm, (d) hand, (e) thigh, (f) calf, and (g) instep. Values are presented as mean ± SD. *Thigh temperature differed between 1.0 ATA and 1.7 ATA (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05). *Hand temperature differed between 1.0 ATA and 1.5 ATA (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05). \u003csup\u003e\u003cstrong\u003e♱\u003c/strong\u003e\u003c/sup\u003eHand temperature differed between 1.5 ATA and 1.7 ATA (\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05). The \u003cem\u003ep\u003c/em\u003e-values for one-way repeated measures ANOVA are noted.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9078861/v1/441db95d7e2fadbe8dacb0bd.jpg"},{"id":107481273,"identity":"ad18be27-642e-4af3-b4c0-7466267f7b28","added_by":"auto","created_at":"2026-04-22 02:16:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":496467,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9078861/v1/926b3c5b-ff0a-45b8-9a13-d402497db9b0.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of Mild Hyperbaric Normoxia on Physiological Recovery After Exercise in a Hot Environment","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHyperbaric oxygen therapy (HBOT) is a well-established clinical treatment that involves breathing 100% oxygen at pressures above atmospheric levels within a sealed chamber. This therapy significantly increases the amount of oxygen dissolved in the plasma, enhances tissue oxygenation, and promotes biological processes such as angiogenesis, modulation of inflammation, and accelerated tissue repair [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Although traditionally used to manage decompression sickness, carbon monoxide poisoning, and chronic wounds [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], HBOT has recently shown promise in supporting recovery from physical exertion and heat-induced physiological stress [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Beyond clinical applications, interest has grown in the role of hyperbaric environments for supporting physiological recovery in populations exposed to high physical and environmental demands.\u003c/p\u003e \u003cp\u003eAmong occupations with extreme physical and environmental demands, firefighters are particularly vulnerable to thermal stress and cardiovascular overload. Wearing heavy protective gear during intense physical activity in high-temperature environments elevates core body temperature and circulatory load, impairing recovery and increasing health risks. Sudden cardiac death accounts for approximately 45% of line-of-duty deaths among firefighters in the United States, highlighting the urgent need for effective post-exertional recovery strategies [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Therefore, the National Fire Protection Association recommends that firefighters take a recovery period of approximately 10\u0026ndash;20 minutes. Moreover, as climate change intensifies the frequency and duration of heatwaves, such interventions are becoming increasingly critical for occupational safety and resilience [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eVarious cooling strategies have been implemented to mitigate heat stress in firefighters, including forearm immersion, cold water ingestion, and the use of an ice vest. While these methods have demonstrated short-term benefits in reducing core body temperature and improving post-task performance, their practicality and effectiveness may be limited during real-world firefighting operations [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRecent studies have explored mild hyperbaric oxygen therapy (mHBOT), typically administered at approximately 1.25 ATA with moderately elevated oxygen concentrations (26\u0026ndash;28% O₂). For example, Qu et al. reported that mHBOT improved muscle fatigue recovery and cardiovascular responses in male athletes, suggesting potential relevance for occupational settings exposed to heat stress [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. However, the use of elevated oxygen concentrations makes it difficult to isolate the physiological effects of increased ambient pressure alone.\u003c/p\u003e \u003cp\u003eTo address this limitation, the present study applies a mild hyperbaric normoxia (MHN) approach using normal air (FiO₂: 20.9% O₂) to examine the physiological effects of increased ambient pressure without hyperoxia exposure. While hyperbaric oxygen therapy (HBOT) has been widely studied, evidence regarding the physiological effects of MHN in post-exercise recovery remains limited. Previous experimental studies have demonstrated that mild hyperbaric exposure while breathing normal air can elicit physiological responses, supporting the feasibility of this approach [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Unlike HBOT, which involves 100% oxygen exposure and fundamentally differs from the normoxic conditions examined in the present study, MHN may represent a practical approach for exploratory research in healthy populations. Building on this evidence, the present study aims to examine the potential role of MHN in enhancing physiological recovery following exercise in hot environments, with implications for occupational heat-stress management, safety, and performance.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eParticipants\u003c/h2\u003e \u003cp\u003eTwelve healthy and physically active male students aged 20\u0026ndash;25 years participated in this study (age, 21.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 years; height, 170.4\u0026thinsp;\u0026plusmn;\u0026thinsp;5.3 cm; body mass, 59.6\u0026thinsp;\u0026plusmn;\u0026thinsp;7.4 kg; body fat percentage, 15\u0026thinsp;\u0026plusmn;\u0026thinsp;6%). Prior to participation, all participants completed a health-screening form, which included a medical history questionnaire and a current health declaration. All participants were fully informed about the study procedures, potential risks, and their right to withdraw at any time, without penalty. Written informed consent was obtained from each participant before participation. The study protocol was reviewed and approved by the Ethics Committee of the Faculty of Design, Kyushu University (Approval No. 600).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental Design\u003c/h3\u003e\n\u003cp\u003eAll participants visited the research laboratory on four separate occasions. The initial visit involved a briefing on the experimental trial, baseline measurements, including the standard YMCA submaximal test [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] to estimate participants\u0026rsquo; maximal oxygen consumption, and familiarization with the protocol. The subsequent visits were dedicated to the actual experimental trials. During the actual experimental trials, prior to instrumentation, participants' hydration status was assessed using a urine-specific gravity refractometer (PAL-10S; ATAGO Co., Ltd., Tokyo, Japan). Participants showing signs of dehydration were asked to rehydrate by drinking water before continuing, provided their levels were below 1,030 mg/dL.\u003c/p\u003e \u003cp\u003eEach participant completed a standardized exercise and recovery protocol inside the climatic chamber with controlled environmental conditions (31\u0026deg;C and 60% relative humidity). To induce a firefighting-like heat load\u0026mdash;based solely on thermal stress and the wearing of full personal protective equipment (PPE), rather than any actual operational firefighting tasks\u0026mdash;participants performed a 20-minute cycling exercise on a cycle ergometer at 60% of their age-predicted maximal heart rate while wearing full firefighters' protective clothing, including a helmet and respiratory mask. This simulated occupational heat exposure reflects a typical firefighting thermal scenario.\u003c/p\u003e \u003cp\u003eImmediately after the exercise session, participants were transferred to the adjacent climatic chamber, where further instrumentation and adjustments to the atmospheric pressure required approximately 8 minutes. Participants then sat in a mesh chair for a 20-minute recovery phase. During recovery, the protective gear was loosened, and the mask and helmet were taken off. Each participant underwent recovery under three separate atmospheric pressure conditions: 1.0 ATA, based on that day's ambient pressure, 1.5 ATA (1,130 mmHg), and 1.7 ATA (1,270 mmHg), each with 20.9% oxygen concentration. The conditions were randomized and counterbalanced across participants to control for the order effects.\u003c/p\u003e\n\u003ch3\u003eMeasurements\u003c/h3\u003e\n\u003cp\u003ePhysiological and subjective thermal responses were continuously monitored throughout the exercise and recovery phases. Core body temperature was measured using an ear canal thermistor (earplug type), whereas skin temperatures were recorded at seven anatomical sites: the forehead, abdomen, forearm, hand, thigh, calf, and instep. These measurements were collected at 1-second intervals using a data logger (LT-8A, Gram Corporation, Japan; precision\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u0026deg;C). The mean skin temperature (MST) was calculated using the Hardy\u0026ndash;Dubois 7-point formula [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePhysiological measurements, including heart rate and stroke volume for estimating cardiac output (using PF07 Q-Link PhysioFlow\u0026reg;), as well as skin blood flow (SkBF) at the forearm and finger, measured with a blood flowmeter (Omegaflow, Omegawave, Tokyo, Japan), were continuously recorded to enable the evaluation of peripheral vasomotor responses during heat stress and subsequent recovery. Additionally, blood pressure was measured before exercise, immediately after exercise, and every 5 minutes during the recovery phase. The Borg Rating of Perceived Exertion (RPE) scale was used to evaluate physical exertion. Moreover, subjective thermal perceptions were evaluated using two standardized scales: thermal comfort (1\u0026ndash;4; very uncomfortable to very comfortable) and thermal sensation (1\u0026ndash;7; very cold to very hot).\u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eAll data are presented as means\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation. Statistical analyses of physiological measurements were performed using JASP software (version 0.19.2.0). A two-way repeated-measures ANOVA was used to analyze heart rate, skin blood flow, systolic and diastolic blood pressure, and skin and ear canal temperatures, with Bonferroni-adjusted post-hoc pairwise comparisons applied when significant main or interaction effects were observed. Subjective thermal perceptions were analyzed separately using the Friedman test in Jamovi software (version 2.6.44), followed by post-hoc pairwise comparisons with appropriate adjustments for multiple testing when significant differences were detected. Statistical significance was set at \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eThe results present a comparative analysis of the physiological and subjective thermal responses during recovery under three atmospheric pressure conditions (1.0, 1.5, and 1.7 ATA). The data at the end of the exercise were shown at time zero, with recovery measurements initiated eight minutes post-exercise.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eCardiac Responses\u003c/h2\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003eHeart Rate\u003c/h2\u003e \u003cp\u003eHeart rate gradually decreased during the recovery period, as evidenced by a main effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;21.42, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.401). Furthermore, significant differences in heart rate were also found across atmospheric pressure conditions (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.06, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.027, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.202). However, the interaction between time and condition did not reach significance (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.73, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.697, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.044), indicating that heart rate recovery was comparable across conditions. Bonferroni-adjusted post hoc comparisons revealed a significant difference between the 1.5 ATA and 1.7 ATA conditions (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.028). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, heart rate decreased during the recovery period across all atmospheric pressure conditions; notably, the 1.5 ATA condition exhibited the most pronounced decline, the 1.0 ATA condition showed a more gradual reduction, and the 1.7 ATA condition maintained a relatively higher heart rate throughout the recovery period.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e\n\u003ch3\u003eCardiac Output\u003c/h3\u003e\n\u003cp\u003eCardiac output did not significantly change across atmospheric pressure conditions during recovery, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. The results revealed no main effects of condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.171, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.843, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.011), time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.076, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.376, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.033), or interaction effects (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.493, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.893, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.030). However, visual inspection of the trend revealed condition-specific temporal patterns. The 1.5 ATA condition displayed a more stable, gradually decreasing cardiac output, while the 1.0 ATA and 1.7 ATA conditions showed an initial decline, reaching a minimum around 13 minutes, followed by a slight increase during later recovery stages.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eStroke Volume\u003c/h2\u003e \u003cp\u003eStroke volume was not significantly affected by atmospheric pressure conditions during recovery, as indicated by the absence of a main effect of condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.07, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.933, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.004; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). Similarly, no main effect of time was observed (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.27, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.931, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.008), suggesting that stroke volume remained stable across measurement intervals throughout the recovery period. Furthermore, the time-by-condition interaction was not significant (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.69, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.734, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.041), indicating a consistent stroke volume response across all atmospheric pressure conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eEar Canal Temperature\u003c/h2\u003e \u003cp\u003eThe results showed a significant main effect of time on ear canal temperature, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;104.79, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.766, suggesting that ear canal temperature decreased over time. The main effect of condition was not significant, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.379, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.687, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.023; however, a significant interaction between time and condition was found, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.842, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.103, indicating that different conditions had distinct temporal patterns. Holm-adjusted post-hoc tests found no significant differences between conditions at individual time points (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the 1.0 ATA condition showed the steepest decline, the 1.5 ATA condition showed a gradual decrease with a brief drop around minute 23, and the 1.7 ATA condition showed an intermediate pattern.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eMean Skin Temperature\u003c/h2\u003e \u003cp\u003eIn mean skin temperature, the results revealed a significant main effect of time, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;35.31, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.525, indicating a general decline throughout the recovery period. There was also a significant main effect of condition, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3.33, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.048, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.172. Furthermore, the time-by-condition interaction was significant, \u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.62, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.01, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.092, suggesting that the pattern of change over time differed across conditions. Bonferroni-corrected post-hoc comparisons revealed that the 1.0 ATA condition differed significantly from the 1.5 ATA condition at minutes 18 and 28. Overall, the 1.0 ATA condition exhibited the fastest decline over time, while the 1.5 ATA condition declined more gradually (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eSkin Temperatures\u003c/h2\u003e \u003cp\u003eOverall, skin temperature decreased at 5 of 7 measurement sites, with the lowest values observed at 1.0 ATA condition. In contrast, at 1.5 ATA, a pronounced decrease was observed only in three sites: the hand, forearm, and forehead.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, forehead and forearm temperatures declined more rapidly at 1.5 ATA, while abdominal and instep temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg) mainly decreased at 1.0 ATA. However, a significant main effect of time (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001) was observed, but there was no significant time-by-condition interaction or main effect of condition (both \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003eHand temperature revealed a significant main effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.75, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.129) and a significant time-by-condition interaction (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.42, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2;= 0.131). No significant between-subjects effects were observed for condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.63, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.210, \u003cem\u003eηp\u003c/em\u003e\u0026sup2;= 0.093) as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. Post hoc analysis indicated significant differences between 1.0 ATA and 1.5 ATA, and between 1.5 ATA and 1.7 ATA, at minute 28.\u003c/p\u003e \u003cp\u003eThe results revealed a significant main effect of time on thigh temperature (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3.32, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.072; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee) and a significant main effect of condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.01, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.022, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.212), while the time-by-condition interaction did not reach statistical significance (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.79 \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.003, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.100). Post-hoc analyses of the time effect showed significant differences between 1.0 and 1.7 ATA at minutes 13, 18, and 23 (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003eLastly, calf temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef) showed a significant main effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;37.92, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.542) and a significant interaction between time and conditions (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.59, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.013, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.09), while no significant effect of condition was observed (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.303).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eSkin Blood Flow and Blood Pressure\u003c/h2\u003e \u003cp\u003eForearm skin blood flow showed a significant main effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.86, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.09), but there were no significant effects of condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.05, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.948, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.004) or the interaction between time and condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.56, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.989, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.004). This suggests that skin blood flow changed over the recovery period but was not affected by different atmospheric pressure conditions. On the other hand, finger skin blood flow did not change significantly, with no notable effects of condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.62, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.546, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.041), time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.38, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.995, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.013), or their interaction (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.66, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.950, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.044).\u003c/p\u003e \u003cp\u003eFurthermore, blood pressure changes during recovery were primarily time dependent. Systolic blood pressure demonstrated a significant main effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.49, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.046, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.072), whereas neither the condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.20, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.818, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.012) nor the time-by-condition interaction (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.62, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.761, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.037) showed significant effects. Diastolic blood pressure followed a similar temporal pattern, exhibiting a significant effect of time (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.73, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.032, \u003cem\u003eηp\u003c/em\u003e\u0026sup2;= 0.078), but was unaffected by atmospheric pressure condition (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.42, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.66, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.026) or the time-by-condition interaction (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.584, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.789, \u003cem\u003eηp\u003c/em\u003e\u0026sup2; = 0.035).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eSubjective Thermal Perceptions\u003c/h2\u003e \u003cp\u003eRegarding thermal comfort, the results showed a significant effect of atmospheric pressure (\u003cem\u003eχ\u003c/em\u003e\u0026sup2; = 70.2, df\u0026thinsp;=\u0026thinsp;2, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), with post hoc analysis indicating that the 1.7 ATA condition differed from both the 1.0 ATA and 1.5 ATA conditions. No difference was found between 1.0 ATA and 1.5 ATA. Since lower scores indicate better thermal comfort, both the 1.0 ATA and 1.5 ATA conditions had median scores of 2 (\u0026ldquo;comfortable\u0026rdquo;). Descriptively, thermal comfort improved over time across all conditions, with mean scores decreasing from 2.32 to 1.83 under 1.0 ATA, from 2.13 to 1.54 under 1.5 ATA, and from 2.18 to 1.64 under 1.7 ATA, with the lower reduction observed under the 1.5 ATA condition.\u003c/p\u003e \u003cp\u003eMoreover, thermal sensation scores decreased over time under all atmospheric pressure conditions, with lower average values observed at 1.5 ATA (from approximately 4.05 to 2.50), followed by 1.7 ATA (from about 4.20 to 2.45), and 1.0 ATA (from roughly 3.83 to 2.95). When all time points were combined, the results revealed a significant effect of pressure condition (\u003cem\u003eχ\u003c/em\u003e\u0026sup2; = 7.47, df\u0026thinsp;=\u0026thinsp;2, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.024). Post hoc analysis showed a significant difference only between the 1.0 ATA and 1.5 ATA groups (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.006). Nonetheless, all conditions had the same median thermal sensation score of 3.0, indicating a slightly cool thermal sensation.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study examined the effects of MHN on physiological recovery following exercise in a hot environment. The experimental trial in this study aims to simulate a firefighting-like heat load, and our findings reveal a split response. The heart rate recovered more rapidly at 1.5 ATA, whereas the skin temperature decreased more quickly at 1.0 ATA. These contrasting patterns can be further explained by distinct underlying mechanisms.\u003c/p\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eCardiac Response\u003c/h2\u003e \u003cp\u003eIn this study, a significant reduction in heart rate during recovery was observed at 1.5 ATA, suggesting enhanced parasympathetic activation accompanied by a relative reduction in sympathetic nervous activity. Overall, the trend showed a consistently lower heart rate at 1.5 ATA compared with 1.0 ATA and 1.7 ATA, although the differences were driven by the trend rather than individual time points. Direct evidence examining heart rate recovery under mild hyperbaric normoxia at pressures comparable to the present study is limited; therefore, this interpretation is based on physiological plausibility.\u003c/p\u003e \u003cp\u003eThe findings suggest that 1.5 ATA may represent an optimal balance, whereby moderately increased ambient pressure enhances oxygen delivery, facilitates ATP resynthesis, and reduces cardiac strain, thereby promoting a stronger parasympathetic response as part of overall autonomic modulation, including sympathetic influences. This interpretation is consistent with reports showing reduced heart rate and autonomic modulation during hyperbaric exposure, even when breathing normal air [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. In contrast, at higher pressure (1.7 ATA), enhanced hyperoxia-driven vasoconstriction may increase systemic vascular resistance and cardiovascular load, potentially modulating heart rate responses. Such vasoconstrictive effects are well documented during hyperoxic exposure and may be amplified under hyperbaric conditions due to markedly elevated arterial oxygen partial pressure [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Furthermore, stroke volume and cardiac output did not differ across pressure conditions, indicating that the observed cardiovascular responses were primarily driven by changes in heart rate rather than alterations in stroke volume, a pattern also observed during hyperbaric air exposure in humans [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBlood pressure showed no significant differences across pressure conditions but demonstrated a significant effect of time during recovery, reflecting the decline in arterial pressure. The fact that blood pressure changed similarly across all conditions indicates that the temporal pattern of blood pressure recovery was primarily driven by time rather than ambient pressure, despite previous reports of pressure-related BP elevation [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSkin blood flow showed no significant differences across the three pressure conditions and decreased progressively over time during recovery, indicating that recovery was primarily influenced by temporal factors rather than ambient pressure. Taken together, these findings suggest that skin blood flow recovery under mild hyperbaric normoxia follows a time-dependent pattern.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eThermoregulation and Impaired Heat Dissipation\u003c/h2\u003e \u003cp\u003eThe core temperature in the ear canal remained stable across hyperbaric pressures, reflecting intact central thermoregulation. Kujawski et al. reported that core body temperature exhibits only minimal, non-significant fluctuations during compression and decompression phases in a hyperbaric chamber, indicating preserved thermal balance under hyperbaric exposure [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Evidence from human studies further shows that heat-acclimated individuals maintain a stable core temperature, whereas peripheral and cardiovascular systems adapt to repeated thermal stress [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Ear canal temperature measurement is a reliable method for estimating deep-body temperature under controlled conditions [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], and its stability in the present study supports the conclusion that hyperbaric normoxia at 1.5\u0026ndash;1.7 ATA does not impose additional heat stress or disrupt recovery of oxygen delivery, heart function, or metabolism.\u003c/p\u003e \u003cp\u003eIn contrast, the mean skin temperature was significantly higher at 1.5 and 1.7 ATA than at 1.0 ATA [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], likely attributable to pressure-induced compression of protective clothing\u0026mdash;governed by Boyle's Law (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({P}_{1}{V}_{1}={P}_{2}{V}_{2}\\)\u003c/span\u003e\u003c/span\u003e), which reduces air layer volumes in garments\u0026mdash;increasing conductive heat gain while impairing convective and evaporative cooling, and slowing skin heat dissipation independent of metabolic heat changes. Elevated ambient pressure mechanically compresses clothing ensembles, diminishing the skin-fabric insulating air layer and heat loss efficiency [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFinger skin temperature decreased most rapidly at 1.5 ATA during recovery, indicating stronger peripheral vasoconstriction to conserve core heat. In contrast, thigh and calf skin covered by protective clothing showed the lowest temperature at 1.0 ATA, highlighting regional differences influenced by clothing coverage and vascular responses [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Consistent with previous evidence, uncovered skin regions, such as the hands, are more sensitive to environmental pressure because they lack insulation and can lose heat more rapidly through convection and evaporation [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThese findings provide preliminary evidence that mild hyperbaric normoxia at 1.5 ATA may be beneficial for recovery after heat stress. However, these findings should be interpreted with consideration of methodological limitations, as skin temperature was measured while participants wore full protective clothing, and neither sweat rate nor the microclimate within the clothing was assessed, which may have influenced heat dissipation responses.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003eSubjective Thermal Perceptions\u003c/h2\u003e \u003cp\u003eThermal comfort showed significant changes over time across all pressure conditions, indicating that recovery duration was the main factor affecting comfort perception. This temporal shift agrees with studies examining thermal environments in hyperbaric chambers, where comfort depended more on thermal conditions and exposure time than on atmospheric pressure alone [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Both 1.0 ATA and 1.5 ATA conditions were perceived as comfortable, with no statistical difference between them, whereas 1.7 ATA was rated significantly less comfortable. These findings align with broader thermal comfort research, which indicates that comfort changes as individuals physiologically adjust to their environment, with time-dependent adaptation playing a key role in shaping subjective comfort evaluations [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Overall, the present results support the idea that temporal adaptation, rather than mild pressure variation under normoxic conditions, is the main factor influencing thermal comfort in hyperbaric conditions.\u003c/p\u003e \u003cp\u003eThermal sensation changed significantly over time across all pressure conditions, indicating a strong influence of exposure duration on heat perception, consistent with previous findings showing that thermal sensation is highly responsive to physiological adaptation [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Although the combined median thermal sensation score was the same across conditions (median\u0026thinsp;=\u0026thinsp;3.0), corresponding to a slightly cool sensation, a significant difference was observed between the 1.0 ATA and 1.5 ATA conditions, while no significant differences were found involving the 1.7 ATA condition. The greater decrease in thermal sensation observed at 1.5 ATA compared with 1.0 ATA may reflect subtle pressure-related influences on peripheral heat exchange, such as changes in skin heat dissipation and clothing compression, which can become more apparent over time. These results are largely consistent with Hu et al. [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], suggesting that ambient pressure alone does not substantially alter thermal perception unless it affects heat transfer or thermal balance. Overall, these findings indicate that changes in thermal sensation under mild hyperbaric normoxic conditions are primarily driven by time-dependent physiological adaptation rather than ambient pressure itself.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThese findings indicate that mild hyperbaric normoxia at 1.5 ATA facilitates a faster post-exercise reduction in heart rate than 1.7 ATA or normobaric conditions, suggesting improved autonomic vagal recovery. Although core temperature, blood pressure, and skin blood flow did not differ significantly across pressure conditions, their consistent decreases over time suggest that MHN may help maintain physiological stability during recovery. The higher skin temperatures observed at 1.5 ATA likely reflect decreased evaporative cooling due to compression of protective clothing and altered sweat evaporation under pressure, suggesting a potential effect of mild hyperbaric normoxia on peripheral heat exchange\u0026mdash;an aspect relevant to thermal comfort and safety in occupational settings. Both thermal comfort and thermal sensation were significantly influenced by time, indicating that perceptual responses were mainly driven by temporal adaptation rather than pressure differences. Although no statistical differences were observed across pressure conditions, descriptive patterns indicated that 1.5 ATA was perceived as slightly more comfortable than 1.0 and 1.7 ATA. These findings suggest that exposure duration, rather than pressure conditions, is the primary factor influencing perceptual outcomes in hyperbaric normoxia. Overall, the results suggest that mild hyperbaric normoxia at 1.5 ATA promotes quicker vagal-mediated heart-rate recovery, supports physiological stability, affects peripheral heat exchange, and offers slightly better perceptual comfort\u0026mdash;potentially serving as a recovery method after heat stress, although confirmation in firefighter populations and real operational settings remains necessary.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eMHN\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMild hyperbaric normoxia\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHBOT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eHyperbaric oxygen therapy\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003emHBOT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMild hyperbaric oxygen therapy\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eATA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eAtmospheres absolute\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHR\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eHeart rate\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCO\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCardiac output\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSV\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eStroke volume\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSkBF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eSkin blood flow\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eMST\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMean skin temperature\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eRPE\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eRating of perceived exertion\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ePPE\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ePersonal protective equipment\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eFiO₂\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eFraction of inspired oxygen.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":" \u003cp\u003eNot applicable.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e \u003cp\u003e This study was reviewed and approved by the Ethics Committee of the Graduate School of Design at Kyushu University. The approval number is 600 (Acceptance number: 833), issued on February 29, 2024. All experimental procedures were carried out in accordance with the committee's ethical standards and the Declaration of Helsinki.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent for publication\u003c/strong\u003e \u003cp\u003eNot applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCompeting interests\u003c/strong\u003e \u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eNot applicable.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eNadzirah Ikasari Syamsul (NS), Ilham Bakri (IB), and Takafumi Maeda (TM) conceived and designed the research. NS and Keneth B. Sedilla (KS) conducted experimental trials. NS analyzed the data and wrote the manuscript. TM, KS, and IB edited the manuscript and provided critical feedback. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis research was supported by the Maeda Laboratory (Environmental Ergonomics Laboratory) and the Research Center for Human Environmental Adaptation at Kyushu University. We would like to thank the participants for their time and willingness to take part in this study. We also thank the members of the Maeda Laboratory for their contributions to this research. Furthermore, we would like to acknowledge Mr. Yasuhiko Maeda, a technical staff member at the Research Center for Human Environmental Adaptation, for his technical support throughout the experiment.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eOrtega, M. A. \u003cem\u003eet al.\u003c/em\u003e A General Overview on the Hyperbaric Oxygen Therapy: Applications, Mechanisms and Translational Opportunities. \u003cem\u003eMedicina (B. Aires).\u003c/em\u003e 57, 864 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLindenmann, J., Kamolz, L., Graier, W., Smolle, J. \u0026amp; Smolle-Juettner, F. M. Hyperbaric oxygen therapy and tissue regeneration: A literature survey. \u003cem\u003eBiomedicines\u003c/em\u003e 10, 3145 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTsai, H. M., Gao, C. J., Li, W. X., Lin, M. T. \u0026amp; Niu, K. C. Resuscitation from experimental heatstroke by hyperbaric oxygen therapy. \u003cem\u003eCrit. Care Med.\u003c/em\u003e 33, 813\u0026ndash;818 (2005).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNiu, K. C., Lin, M. T. \u0026amp; Chang, C. P. Hyperbaric oxygen improves survival in heatstroke rats by reducing multiorgan dysfunction and brain oxidative stress. \u003cem\u003eEur. J. Pharmacol.\u003c/em\u003e 569, 94\u0026ndash;102 (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNiu, K. C., Chang, C. K., Lin, M. T. \u0026amp; Huang, K. F. A hyperbaric oxygen therapy approach to heat stroke with multiple organ dysfunction: A case report. \u003cem\u003eChin. J. Physiol.\u003c/em\u003e 52, 169\u0026ndash;172 (2009).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSmith, D. L., Barr, D. A. \u0026amp; Kales, S. N. Extreme sacrifice: Sudden cardiac death in the US fire service. \u003cem\u003eExtrem. Physiol. \\\u0026amp; Med.\u003c/em\u003e 2, 6 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLeon, L. R. \u0026amp; Bouchama, A. Heat stroke. \u003cem\u003eCompr. Physiol.\u003c/em\u003e 5, 611\u0026ndash;647 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFullagar, H., Govus, S., Taylor, M. G., Siegler, M. B. \u0026amp; Kellmann, M. R. Cooling strategies for firefighters: Effects on physiological, physical, and visuo-motor outcomes following fire-fighting tasks in the heat. \u003cem\u003eJ. Therm. Biol.\u003c/em\u003e 106, 103236 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQu, C., Zhao, Y., Guo, H., Zhang, X. \u0026amp; Liu, D. Effects of mild hyperbaric oxygen therapy on timing sequence recovery of muscle fatigue in Chinese university male athletes. \u003cem\u003eJ. Exerc. Sci. \\\u0026amp; Fit.\u003c/em\u003e 22, 305\u0026ndash;315 (2024).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFujita, S., Tomioka, Y., Ono, Y., \u0026amp; Deie, M. (2016). 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Human Sci.\u003c/em\u003e 24, 439\u0026ndash;443 (2005).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePougnet, R., Pougnet, L., Lucas, D., Henckes, A., Lodd\u0026eacute;, B., \u0026amp; Dewitte, J.-D. (2018). Health effects of hyperbaric exposure on chamber attendants: A literature review. \u003cem\u003eInternational Maritime Health\u003c/em\u003e, \u003cem\u003e69\u003c/em\u003e(1), 58\u0026ndash;62.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLund, V., Kentala, E., Scheinin, H., Klossner, J., Sariola-Heinonen, K., \u0026amp; Jalonen, J. (2000). Hyperbaric oxygen increases parasympathetic activity in professional divers. \u003cem\u003eActa Physiologica Scandinavica\u003c/em\u003e, \u003cem\u003e170\u003c/em\u003e (1), 39\u0026ndash;44.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHelmerhorst, H. J. F., Schultz, M. J., van der Voort, P. H. J., de Jonge, E., \u0026amp; van Westerloo, D. J. (2015). Hyperoxia-induced vasoconstriction: Mechanisms and clinical implications. \u003cem\u003eCritical Care\u003c/em\u003e, \u003cem\u003e19\u003c/em\u003e, 439..\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTakemura, A. Exposure to a mild hyperbaric oxygen environment elevates blood pressure. \u003cem\u003eJ. Phys. Ther. Sci.\u003c/em\u003e 34, 360\u0026ndash;364 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKujawski, S., Słomko, J., Zawadka-Kunikowska, M., Kozakiewicz, M., Klawe, J. J., Tafil-Klawe, M. \u0026amp; Zalewski, P. The effects of hyperbaric exposure on immediate and delayed changes in core temperature and its circadian fluctuations. \u003cem\u003ePolish Hyperbaric Research\u003c/em\u003e 60, 37\u0026ndash;48 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eP\u0026eacute;riard, J. D., Racinais, S. \u0026amp; Sawka, M. N. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. \u003cem\u003eScand. J. Med. Sci. Sport.\u003c/em\u003e 25, 20\u0026ndash;38 exposure(2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGasim, G. I., Musa, I. R., Abdien, M. T., \u0026amp; Adam, I. (2013). Accuracy of tympanic temperature measurement using an infrared thermometer. \u003cem\u003eBMC Research Notes\u003c/em\u003e, \u003cem\u003e6\u003c/em\u003e, 194.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD\u0026uuml;nnwald, T. \u003cem\u003eet al.\u003c/em\u003e Safety and Feasibility Study of A Hyperbaric Infrared Chamber Combining Hyperbaric Oxygen Therapy and Low Temperature Infrared Radiation. \u003cem\u003eInternational Journal of Innovative Research in Medical Science (IJIRMS)\u003c/em\u003e, 01, 349 to 358\u0026ndash;349 358 (2016).26.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen, Y. S., Fan, J., Qian, X., \u0026amp; Zhang, W. (2004). Effect of garment fit on thermal insulation and evaporative resistance. \u003cem\u003eTextile Research Journal\u003c/em\u003e, \u003cem\u003e74\u003c/em\u003e(8), 742\u0026ndash;748.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHavenith, G. (2002). Interaction of clothing and thermoregulation. \u003cem\u003eExogenous Dermatology\u003c/em\u003e, \u003cem\u003e1\u003c/em\u003e(5), 221\u0026ndash;230.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSweet, D. K. \u003cem\u003eet al.\u003c/em\u003e Thermoregulation during a six-hour exposure to warm, humid hyperbaric conditions. \u003cem\u003eUndersea Hyperb. Med. J. Undersea Hyperb. Med. Soc. Inc\u003c/em\u003e 49, 459\u0026ndash;465 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMourot, L., Cluzeau, C. \u0026amp; Regnard, J. Hyperbaric Gaseous Cryotherapy: Effects on Skin Temperature and Systemic Vasoconstriction. \u003cem\u003eArch. Phys. Med. Rehabil.\u003c/em\u003e 88, 1339\u0026ndash;1343 (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHu, J., Zhang, H., \u0026amp; Xu, Q. (2016). Evaluation of thermal comfort in hyperbaric oxygen chamber [Conference: The 2nd Information Technology and Mechatronics Engineering Conference (ITOEC 2016) ResearchGate.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMekjavic, I. B., Eiken, O., \u0026amp; Tipton, M. (2021). Perception of Thermal Comfort during Skin Cooling and Heating, 11(7), 681.\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":"Mild Hyperbaric Normoxia, Physiological Recovery, Thermoregulation","lastPublishedDoi":"10.21203/rs.3.rs-9078861/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9078861/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003eMild hyperbaric normoxia (MHN), defined as exposure to increased ambient pressure while breathing normal air, is a non-oxygen-enriched hyperbaric recovery approach. Simulated firefighting activities impose substantial cardiovascular strain and thermal strain, providing a relevant model for recovery research. This study examined cardiac and thermoregulatory responses during recovery following simulated firefighting under MHN conditions.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eTwelve healthy young male participants performed a 20-minute cycling exercise at 60% of age-predicted maximal heart rate in a controlled hot environment (31\u0026deg;C, 60% relative humidity) while wearing full firefighting gear. Post-exercise recovery was conducted under three atmospheric pressure conditions: normobaric normoxia (1.0 atm absolute [ATA] and hyperbaric normoxia (1.5 and 1.7 ATA) for 20 minutes in a randomized order. Heart rate, cardiac output, stroke volume, core and skin temperatures, and skin blood flow were continuously measured, whereas blood pressure and subjective thermal responses were obtained every 5 minutes.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eMHN at 1.5 ATA significantly reduced heart rate during recovery compared with 1.0 and 1.7 ATA (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.027), indicating enhanced autonomic recovery, whereas blood pressure showed no significant differences across conditions. Core body temperature remained stable across pressures; however, mean skin temperature was higher at 1.5 and 1.7 ATA than at 1.0 ATA (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.048). Skin blood flow declined similarly over time regardless of condition. Thermal comfort differed between pressure conditions (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), with the 1.7 ATA condition differing from both 1.0 ATA and 1.5 ATA, whereas 1.0 ATA and 1.5 ATA showed similar ratings corresponding to comfortable conditions. Thermal sensation changed significantly over time but was generally perceived as slightly cool across all pressures.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eMHN at 1.5 ATA appears to facilitate faster cardiac recovery after exercise-induced heat stress, potentially through enhanced parasympathetic activation, while altered skin temperature responses may reflect changes in peripheral heat exchange. These results provide preliminary evidence supporting the use of mild hyperbaric normoxia as a recovery strategy for firefighters and other individuals working in extreme heat, although additional studies in real operational settings are needed to confirm its practical applicability.\u003c/p\u003e","manuscriptTitle":"Effects of Mild Hyperbaric Normoxia on Physiological Recovery After Exercise in a Hot Environment","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-16 06:45:39","doi":"10.21203/rs.3.rs-9078861/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":"2f79c965-357c-4a92-bf9b-a1332fdf6035","owner":[],"postedDate":"April 16th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"202681086168082043404494352154982134111","date":"2026-05-04T01:35:00+00:00","index":14,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-16T06:45:39+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-16 06:45:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9078861","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9078861","identity":"rs-9078861","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}