Muscle mechanoreceptor modulation of sweat rate during recovery from moderate exercise

The incremental significance of heart rate recovery as a predictor during exercise-stress myocardial perfusion SPECT imaging in individuals with suspected coronary artery disease

Background

Major adverse cardiac events (MACE) are more likely to occur when abnormal heart rate recovery (HRR). This study aimed to assess the incremental predictive significance of HRR over exercise stress myocardial perfusion single-photon emission computed tomography (MPS) results for MACE in individuals with suspected coronary artery disease (CAD).

Methods

Between January 2014 and December 2017, we continually gathered data on 595 patients with suspected CAD who received cycling exercise stress MPS. HRR at 1, 2, 3, and 4 min were used as study variables to obtain the optimal cut-off values of HRR for MACE. The difference between the peak heart rate achieved during exercise and the heart rate at 1, 2, 3, and 4 min was used to calculate the HRR, as shown in HRR3. Heart rate variations between two locations in time, such as HRR_{2 min-1 min}, were used to establish the slope of HRR. All patients were followed for a minimum of 4 years, with MACE as the follow-up goal. The associations between HRR and MACE were assessed using Cox proportional hazards analyses.

Results

Patients with MACE were older (P = 0.001), and they also had higher rates of hypertension, dyslipidemia, diabetes, abnormal MPS findings (SSS ≥ 5%), medication history (all P < 0.001), and lower HRR values (all P < 0.01). Patients with and without MACE did not significantly vary in their HRR_{4 min-3 min}. The optimal cut-off of HRR1, 2, and 3 combined with SSS can stratify the risk of MACE in people with suspected CAD (all P < 0.001). HRR 1, 2, and 3 and its slope were linked to MACE in multivariate analysis, where HRR3 was the most significant risk predictor. With a global X² increase from 101 to 126 (P < 0.0001), HRR3 demonstrated the greatest improvement in the model's predictive capacity, incorporating clinical data and MPS outcomes.

Conclusions

HRR at 3 min has a more excellent incremental prognostic value for predicting MACE in patients with suspected CAD following cycling exercise stress MPS. Therefore, incorporating HRR at 3 min into known predictive models may further improve the risk stratification of the patients.

Does Exercise Training Improve Cardiac-Parasympathetic Nervous System Activity in Sedentary People? A Systematic Review with Meta-Analysis

The aim of this study was to investigate the training-induced effect on cardiac parasympathetic nervous system (PNS) activity, assessed by resting heart rate variability (HRV) and post-exercise heart rate recovery (HRR), in sedentary healthy people. Electronic searches were carried out in PubMed, Embase, and Web of Science. Random-effects models of between-group standardised mean difference (SMD) were estimated. Heterogeneity analyses were performed by means of the chi-square test and I² index. Subgroup analyses and meta-regressions were performed to investigate the influence of potential moderator variables on the training-induced effect. The results showed a small increase in RMSSD (SMD₊ = 0.57 [95% confidence interval (CI) = 0.23, 0.91]) and high frequency (HF) (SMD₊ = 0.21 [95% CI = 0.01, 0.42]) in favour of the intervention group. Heterogeneity tests reached statistical significance for RMSSD and HF (p ≤ 0.001), and the inconsistency was moderate (I² = 68% and 60%, respectively). We found higher training-induced effects on HF in studies that performed a shorter intervention or lower number of exercise sessions (p ≤ 0.001). Data were insufficient to investigate the effect of exercise training on HRR. Exercise training increases cardiac PNS modulation in sedentary people, while its effect on PNS tone requires future study.

Heart Rate Variability-Guided Training for Improving Mortality Predictors in Patients with Coronary Artery Disease

The objective of this research was to investigate whether heart rate variability (HRV)-guided training improves mortality predictors to a greater extent than predefined training in coronary artery disease patients. Twenty-one patients were randomly allocated to the HRV-guided training group (HRV-G) or the predefined training group (PRED-G). They measured their HRV at home daily and trained three times a week for six weeks. Resting heart rate, isolated vagal-related HRV indices (i.e., RMSSD, HF, and SD₁), weekly averaged RMSSD, heart rate recovery, and maximum oxygen uptake were assessed before and after the training period. There was a statistically significant difference (p = 0.034) in the change in weekly averaged RMSSD in favor of the HRV-G, while no differences were found in the remaining analyzed variables (p > 0.050). Regardless of the training prescription method, exercise training decreased resting heart rate (p = 0.001; -4.10 [95% CI = -6.37--1.82] beats per minute (bpm)), and increased heart rate recovery at 2 min (p = 0.010; 4.33 [95% CI = 1.15-7.52] bpm) and maximum oxygen uptake (p < 0.001; 3.04 [95% CI = 1.70-4.37] mL·kg^-1·min^-1). HRV-guided training is superior to predefined training in improving vagal-related HRV when methodological factors are accounted for.

Activation of Mechanoreflex, but not Central Command, Delays Heart Rate Recovery after Exercise in Healthy Men

This study tested the hypotheses that activation of central command and muscle mechanoreflex during post-exercise recovery delays fast-phase heart rate recovery with little influence on the slow phase. Twenty-five healthy men underwent three submaximal cycling bouts, each followed by a different 5-min recovery protocol: active (cycling generated by the own subject), passive (cycling generated by external force) and inactive (no-cycling). Heart rate recovery was assessed by the heart rate decay from peak exercise to 30 s and 60 s of recovery (HRR30s, HRR60s fast phase) and from 60 s-to-300 s of recovery (HRR60−300s slow phase). The effect of central command was examined by comparing active and passive recoveries (with and without central command activation) and the effect of mechanoreflex was assessed by comparing passive and inactive recoveries (with and without mechanoreflex activation). Heart rate recovery was similar between active and passive recoveries, regardless of the phase. Heart rate recovery was slower in the passive than inactive recovery in the fast phase (HRR60s=20±8vs.27 ±10 bpm, p<0.01), but not in the slow phase (HRR60−300s=13±8vs.10±8 bpm, p=0.11). In conclusion, activation of mechanoreflex, but not central command, during recovery delays fast-phase heart rate recovery. These results elucidate important neural mechanisms behind heart rate recovery regulation.

Sweating as a heat loss thermoeffector

In humans, sweating is the most powerful autonomic thermoeffector. The evaporation of sweat provides by far the greatest potential for heat loss and it represents the only means of heat loss when air temperature exceeds skin temperature. Sweat production results from the integration of afferent neural information from peripheral and central thermoreceptors which leads to an increase in skin sympathetic nerve activity. At the neuroglandular junction, acetylcholine is released and binds to muscarinic receptors which stimulate the secretion of a primary fluid by the secretory coil of eccrine glands. The primary fluid subsequently travels through a duct where ions are reabsorbed. The end result is the expulsion of hypotonic sweat on to the skin surface. Sweating increases in proportion with the intensity of the thermal challenge in an attempt of the body to attain heat balance and maintain a stable internal body temperature. The control of sweating can be modified by biophysical factors, heat acclimation, dehydration, and nonthermal factors. The purpose of this article is to review the role of sweating as a heat loss thermoeffector in humans.

Passive Heating Attenuates Post-exercise Cardiac Autonomic Recovery in Healthy Young Males

Post-exercise heart rate (HR) recovery (HRR) presents a biphasic pattern, which is mediated by parasympathetic reactivation and sympathetic withdrawal. Several mechanisms regulate these post-exercise autonomic responses and thermoregulation has been proposed to play an important role. The aim of this study was to test the effects of heat stress on HRR and HR variability (HRV) after aerobic exercise in healthy subjects. Twelve healthy males (25 ± 1 years, 23.8 ± 0.5 kg/m²) performed 14 min of moderate-intensity cycling exercise (40–60% HR_reserve) followed by 5 min of loadless active recovery in two conditions: heat stress (HS) and normothermia (NT). In HS, subjects dressed in a whole-body water-perfused tube-lined suit to increase internal temperature (T_c) by ~1°C. In NT, subjects did not wear the suit. HR, core and skin temperatures (T_c and T_sk), mean arterial pressure (MAP) skin blood flow (SKBF), and cutaneous vascular conductance (CVC) were measured throughout and analyzed during post-exercise recovery. HRR was assessed through calculations of HR decay after 60 and 300 s of recovery (HRR60s and HRR300s), and the short- and long-term time constants of HRR (T30 and HRRt). Post-exercise HRV was examined via calculations of RMSSD (root mean square of successive RR intervals) and RMS (root mean square residual of RR intervals). The HS protocol promoted significant thermal stress and hemodynamic adjustments during the recovery (HS-NT differences: T_c = +0.7 ± 0.3°C; T_sk = +3.2 ± 1.5°C; MAP = −12 ± 14 mmHg; SKBF = +90 ± 80 a.u; CVC = +1.5 ± 1.3 a.u./mmHg). HRR and post-exercise HRV were significantly delayed in HS (e.g., HRR60s = 27 ± 9 vs. 44 ± 12 bpm, P < 0.01; HRR300s = 39 ± 12 vs. 59 ± 16 bpm, P < 0.01). The effects of heat stress (e.g., the HS-NT differences) on HRR were associated with its effects on thermal and hemodynamic responses. In conclusion, heat stress delays HRR, and this effect seems to be mediated by an attenuated parasympathetic reactivation and sympathetic withdrawal after exercise. In addition, the impact of heat stress on HRR is related to the magnitude of the heat stress-induced thermal stress and hemodynamic changes.

Restoration of thermoregulation after exercise

Performing exercise, especially in hot conditions, can heat the body, causing significant increases in internal body temperature. To offset this increase, powerful and highly developed autonomic thermoregulatory responses (i.e., skin blood flow and sweating) are activated to enhance whole body heat loss; a response mediated by temperature-sensitive receptors in both the skin and the internal core regions of the body. Independent of thermal control of heat loss, nonthermal factors can have profound consequences on the body's ability to dissipate heat during exercise. These include the activation of the body's sensory receptors (i.e., baroreceptors, metaboreceptors, mechanoreceptors, etc.) as well as phenotypic factors such as age, sex, acclimation, fitness, and chronic diseases (e.g., diabetes). The influence of these factors extends into recovery such that marked impairments in thermoregulatory function occur, leading to prolonged and sustained elevations in body core temperature. Irrespective of the level of hyperthermia, there is a time-dependent suppression of the body's physiological ability to dissipate heat. This delay in the restoration of postexercise thermoregulation has been associated with disturbances in cardiovascular function which manifest most commonly as postexercise hypotension. This review examines the current knowledge regarding the restoration of thermoregulation postexercise. In addition, the factors that are thought to accelerate or delay the return of body core temperature to resting levels are highlighted with a particular emphasis on strategies to manage heat stress in athletic and/or occupational settings.

Effect of stride frequency on thermoregulatory responses during endurance running in distance runners

Changing stride frequency may influence oxygen uptake and heart rate during running as a function of running economy and central command. This study investigated the influence of stride frequency manipulation on thermoregulatory responses during endurance running. Seven healthy endurance runners ran on a treadmill at a velocity of 15km/h for 60min in a controlled environmental chamber (ambient temperature 27°C and relative humidity 50%), and stride frequency was manipulated. Stride frequency was intermittently manipulated by increasing and decreasing frequency by 10% from the pre-determined preferred frequency. These periods of increase or decrease were separated by free frequency running in the order of free stride frequency, stride frequency manipulation (increase or decrease), free stride frequency, and stride frequency manipulation (increase or decrease) for 15min each. The increased and decreased stride frequencies were 110% and 91% of the free running frequency, respectively (196±6, 162±5, and 178±5steps/min, respectively, P<0.01). Compared to the control, stride frequency manipulation did not affect rectal temperature, heart rate, or the rate of perceived exhaustion during running. Whole-body sweat loss increased significantly when stride frequency was manipulated (1.48±0.11 and 1.57±0.11kg for control and manipulated stride frequencies, respectively, P<0.05), but stride frequency had a small effect on sweat loss overall (Cohen's d=0.31). A higher mean skin temperature was also observed under mixed frequency conditions compared to that in the control (P<0.05). While the precise mechanisms underlying these changes remain unknown (e.g. running economy or central command), our results suggest that manipulation of stride frequency does not have a large effect on sweat loss or other physiological variables, but does increase mean skin temperature during endurance running.

Influence of forearm muscle metaboreceptor activation on sweating and cutaneous vascular responses during dynamic exercise

We examined whether the sustained activation of metaboreceptor in forearm during cycling exercise can modulate sweating and cutaneous vasodilation. On separate days, 12 young participants performed a 1.5-min isometric handgrip exercise at 40% maximal voluntary contraction followed by 1) 9-min forearm ischemia (Occlusion, to activate metaboreceptor) or 2) no ischemia (Control) in thermoneutral conditions (27°C, 50%) with mean skin temperature clamped at 34°C. Thirty seconds after the handgrip exercise, participants cycled for 13.5 min at 40% V̇o2 max For Occlusion, forearm ischemia was maintained for 9 min followed by no ischemia thereafter. Local sweat rate (SR, ventilated capsule) and cutaneous vascular conductance (CVC, laser-Doppler perfusion units/mean arterial pressure) on the contralateral nonischemic arm as well as esophageal and skin temperatures were measured continuously. The period of ischemia in the early stages of exercise increased SR (+0.03 mg·cm(-2)·min(-1), P < 0.05) but not CVC (P > 0.05) above Control levels. No differences were measured in the esophageal temperature at which onset of sweating (Control 37.19 ± 0.09 vs. Occlusion 37.07 ± 0.09°C) or CVC (Control 37.21 ± 0.08 vs. Occlusion 37.08 ± 0.10°C) as well as slopes for these responses (all P > 0.05). However, a greater elevation in SR occurred thereafter such that SR was significantly elevated at the end of the ischemic period relative to Control (0.37 ± 0.05 vs. 0.23 ± 0.05 mg·cm(-2)·min(-1), respectively, P < 0.05) despite no differences in esophageal temperature. We conclude that the activation of forearm muscle metaboreceptor can modulate sweating, but not CVC, during cycling exercise without affecting the core temperature-SR relationship.

Human skin wetness perception: psychophysical and neurophysiological bases

The ability to perceive thermal changes in the surrounding environment is critical for survival. However, sensing temperature is not the only factor among the cutaneous sensations to contribute to thermoregulatory responses in humans. Sensing skin wetness (i.e. hygrosensation) is also critical both for behavioral and autonomic adaptations. Although much has been done to define the biophysical role of skin wetness in contributing to thermal homeostasis, little is known on the neurophysiological mechanisms underpinning the ability to sense skin wetness. Humans are not provided with skin humidity receptors (i.e., hygroreceptors) and psychophysical studies have identified potential sensory cues (i.e. thermal and mechanosensory) which could contribute to sensing wetness. Recently, a neurophysiological model of human wetness sensitivity has been developed. In helping clarifying the peripheral and central neural mechanisms involved in sensing skin wetness, this model has provided evidence for the existence of a specific human hygrosensation strategy, which is underpinned by perceptual learning via sensory experience. Remarkably, this strategy seems to be shared by other hygroreceptor-lacking animals. However, questions remain on whether these sensory mechanisms are underpinned by specific neuromolecular pathways in humans. Although the first study on human wetness perception dates back to more than 100 years, it is surprising that the neurophysiological bases of such an important sensory feature have only recently started to be unveiled. Hence, to provide an overview of the current knowledge on human hygrosensation, along with potential directions for future research, this review will examine the psychophysical and neurophysiological bases of human skin wetness perception.

Sweating response to passive stretch of the calf muscle during activation of forearm muscle metaboreceptors in heated humans

Activation of muscle metaboreceptors and mechanoreceptors has been shown to independently influence the sweating response, while their integrative control effects remain unclear. We examined the sweating response when the two muscle receptors are concurrently activated in different limbs, as well as the blood pressure response. In total, 27 young males performed passive calf muscle stretches (muscle mechanoreceptor activation) for 30 s in a semisupine position with and without postisometric handgrip exercise muscle ischemia (PEMI, muscle metaboreceptor activation) at exercise intensities of 35 and 50% of maximum voluntary contraction (MVC) under hot conditions (ambient temperature, 35°C, relative humidity, 50%). Passive calf muscle stretching alone increased the mean sweating rate significantly on the forehead, chest, and thigh (SRmean) and mean arterial blood pressure (MAP), but not the heart rate (HR), from prestretching levels by 0.04 ± 0.01 mg·cm(2)·min(-1), 4.0 ± 1.3 mmHg (P < 0.05), and -1.0 ± 0.5 beats/min (P > 0.05), respectively. The SRmean and MAP during PEMI were significantly higher than those at rest. The passive calf muscle stretch during PEMI increased MAP significantly by 3.4 ± 1.0 and 2.0 ± 0.7 mmHg for 35 and 50% of MVC, respectively (P < 0.05), but not that of SRmean or HR at either exercise intensity. These results suggest that sweating and blood pressure responses to concurrent activation of the two muscle receptors in different limbs differ and that the influence of calf muscle mechanoreceptor activation alone on the sweating response disappears during forearm muscle metaboreceptor activation.

Non-thermal modification of heat-loss responses during exercise in humans

This review focuses on the characteristics of heat-loss responses during exercise with respect to non-thermal factors. In addition, the effects of physical training on non-thermal heat-loss responses are discussed. When a subject is already sweating the sweating rate increases at the onset of dynamic exercise without changes in core temperature, while cutaneous vascular conductance (skin blood flow) is temporarily decreased. Although exercise per se does not affect the threshold for the onset of sweating, it is possible that an increase in exercise intensity induces a higher sensitivity of the sweating response. Exercise increases the threshold for cutaneous vasodilation, and at higher exercise intensities, the sensitivity of the skin-blood-flow response decreases. Facilitation of the sweating response with increased exercise intensity may be due to central command, peripheral reflexes in the exercising muscle, and mental stimuli, whereas the attenuation of skin-blood-flow responses with decreased cutaneous vasodilation is related to many non-thermal factors. Most non-thermal factors have negative effects on magnitude of cutaneous vasodilation; however, several of these factors have positive effects on the sweating response. Moreover, thermal and non-thermal factors interact in controlling heat-loss responses, with non-thermal factors having a greater impact until core temperature elevations become significant, after which core temperature primarily would control heat loss. Finally, as with thermally induced sweating responses, physical training seems to also affect sweating responses governed by non-thermal factors.

Is there evidence for nonthermal modulation of whole body heat loss during intermittent exercise?

This study compared the effect of active, passive, and inactive recoveries on whole body evaporative and dry heat loss responses during intermittent exercise at an air temperature of 30 degrees C and a relative humidity of 20%. Nine males performed three 15-min bouts of upright seated cycling at a fixed external workload of 150 W. The exercise bouts were separated by three 15-min recoveries during which participants 1) performed loadless pedaling (active recovery), 2) had their lower limbs passively compressed with inflatable sleeves (passive recovery), or 3) remained upright seated on the cycle ergometer (inactive recovery). Combined direct and indirect calorimetry was employed to measure rates of whole body evaporative heat loss (EHL) and metabolic heat production (M-W). Mean body temperature (T(b)) was calculated from esophageal and mean skin temperatures, and mean arterial pressure (MAP) was measured continuously. Active and passive recoveries both reversed the reduction in MAP associated with inactive recovery (P <or= 0.05). This response was paralleled by greater levels of EHL during active (207 +/- 53 W) and passive recoveries (203 +/- 55 W) compared with the inactive condition (168 +/- 53 W, P <or= 0.05). However, the greater rate of EHL during active recovery was paralleled by a greater M-W (194 +/- 16 W) compared with inactive recovery (149 +/- 27 W, P <or= 0.001). In contrast, M-W during passive recovery (139 +/- 20 W) was not significantly different from the inactive condition (P = 0.468). Furthermore, there were no differences in T(b) between inactive and passive conditions during the recovery periods (P = 0.820). As such, passive recovery resulted in greater levels of EHL for a given change in T(b) compared with inactive recovery (P <or= 0.05). These results strongly suggest that the progressive increase in core temperature during successive exercise/rest cycles is primarily the result of a baroreflex-mediated attenuation of postexercise whole body evaporative heat loss.

Mechanisms and controllers of eccrine sweating in humans

Human body temperature is regulated within a very narrow range. When exposed to hyperthermic conditions, via environmental factors and/or increased metabolism, heat dissipation becomes vital for survival. In humans, the primary mechanism of heat dissipation, particularly when ambient temperature is higher than skin temperature, is evaporative heat loss secondary to sweat secretion from eccrine glands. While the primary controller of sweating is the integration between internal and skin temperatures, a number of non-thermal factors modulate the sweating response. In addition to summarizing the current understanding of the neural pathways from the brain to the sweat gland, as well as responses at the sweat gland, this review will highlight findings pertaining to studies of proposed non-thermal modifiers of sweating, namely, exercise, baroreceptor loading state, and body fluid status. Information from these studies not only provides important insight pertaining to the basic mechanisms of sweating, but also perhaps could be useful towards a greater understanding of potential mechanisms and consequences of disease states as well as aging in altering sweating responses and thus temperature regulation.

Can supine recovery mitigate the exercise intensity dependent attenuation of post-exercise heat loss responses?

Cutaneous vascular conductance (CVC) and sweat rate are subject to non-thermal baroreflex-mediated attenuation post-exercise. Various recovery modalities have been effective in attenuating these decreases in CVC and sweat rate post-exercise. However, the interaction of recovery posture and preceding exercise intensity on post-exercise thermoregulation remains unresolved. We evaluated the combined effect of supine recovery and exercise intensity on post-exercise cardiovascular and thermal responses relative to an upright seated posture. Seven females performed 15 min of cycling ergometry at low- (LIE, 55% maximal oxygen consumption) or high-(HIE, 85% maximal oxygen consumption) intensity followed by 60 min of recovery in either an upright seated or supine posture. Esophageal temperature, CVC, sweat rate, cardiac output, stroke volume, heart rate, total peripheral resistance, and mean arterial pressure (MAP) were measured at baseline, at end-exercise, and at 2, 5, 12, 20, and every 10 min thereafter until the end of recovery. MAP and stroke volume were maintained during supine recovery to a greater extent relative to an upright seated recovery following HIE (p <or= 0.05) and were paralleled by an elevated CVC and sweat rate response (p <or= 0.05). A significantly lower esophageal temperature was subsequently observed when supine throughout recovery (p <or= 0.05). Although we observed a reflex bradycardia and increased stoke volume with supine recovery following LIE, no differences were observed for MAP, CVC, sweat rate or esophageal temperature. Supine recovery attenuates the post-exercise reductions in MAP, CVC, and sweat rate in a manner dependent directly on exercise intensity. This effect is likely attributable to a non-thermal baroreceptor mechanism.

Hyperthermia modifies the nonthermal contribution to postexercise heat loss responses

PURPOSE

This study investigated the nonthermoregulatory control of cutaneous vascular conductance (CVC) and sweating during recovery from exercise-induced hyperthermia as well as possible sex-related differences in these responses. Two hypotheses were tested in this study: 1) active and passive recovery would be more effective in attenuating the fall in mean arterial pressure (MAP) than inactive recovery, but CVC and sweat rate responses would be similar between all recovery modes; and 2) the magnitude of the change in postexercise heat loss and hemodynamic responses between recovery modes would be similar between sexes.

METHODS

Nine males and nine females were rendered hyperthermic (esophageal temperature = 39.5 degrees C) by exercise, followed by 60 min of 1) active, 2) inactive, and 3) passive recovery. CVC, sweat rate, and MAP were recorded at baseline, after 2, 5, 12, and 20 min, and at every 10 min until the end of recovery.

RESULTS

MAP was elevated above inactive recovery by 6 +/- 2 and 4 +/- 1 mm Hg for active and passive recovery, respectively (P < 0.001). No differences were observed between modes during the initial 10 min of recovery for CVC and 50 min of recovery for sweat rate. However, relative to inactive recovery CVC and sweat rate were subsequently greater by 16.2 +/- 5.8% of CVCpeak and 0.28 +/- 0.04 mg.min.cm, respectively, during active recovery, and by 11.6 +/- 2.9% of CVCpeak and 0.23 +/- 0.03 mg.min.cm, respectively, during passive recovery.

CONCLUSION

We conclude that in the presence of a greater thermal drive associated with hyperthermia, the influence of nonthermal input on postexercise heat loss responses is still observed. However, thermal control predominates over nonthermal factors in the first 10 min of recovery for CVC and for up to 50 min postexercise for sweating. Sex did not influence the effect of recovery mode on any variable.

Human heat balance during postexercise recovery: separating metabolic and nonthermal effects

Previous studies report greater postexercise heat loss responses during active recovery relative to inactive recovery despite similar core temperatures between conditions. Differences have been ascribed to nonthermal factors influencing heat loss response control since elevations in metabolism during active recovery are assumed to be insufficient to change core temperature and modify heat loss responses. However, from a heat balance perspective, different rates of total heat loss with corresponding rates of metabolism are possible at any core temperature. Seven male volunteers cycled at 75% of V̇o2peak in the Snellen whole body air calorimeter regulated at 25.0°C, 30% relative humidity (RH), for 15 min followed by 30 min of active (AR) or inactive (IR) recovery. Relative to IR, a greater rate of metabolic heat production (Ṁ − Ẇ) during AR was paralleled by a greater rate of total heat loss (ḢL) and a greater local sweat rate, despite similar esophageal temperatures between conditions. At end-recovery, rate of body heat storage, that is, [(Ṁ − Ẇ) − ḢL] approached zero similarly in both conditions, with Ṁ − Ẇ and ḢL elevated during AR by 91 ± 26 W and 93 ± 25 W, respectively. Despite a higher Ṁ − Ẇ during AR, change in body heat content from calorimetry was similar between conditions due to a slower relative decrease in ḢL during AR, suggesting an influence of nonthermal factors. In conclusion, different levels of heat loss are possible at similar core temperatures during recovery modes of different metabolic rates. Evidence for nonthermal influences upon heat loss responses must therefore be sought after accounting for differences in heat production.

Postexercise heat loss and hemodynamic responses during head-down tilt are similar between genders

PURPOSE

We evaluated the hypothesis that during recovery from dynamic exercise in the 15 degrees head-down tilt (HDT) position, the attenuation of the fall in mean arterial pressure (MAP), cutaneous vascular conductance (CVC), and sweat rate, and the augmentation of the rate of esophageal temperature (T(es)) decay relative to the upright seated (URS) posture, would be different between males and females.

METHODS

Fourteen subjects (seven males, seven females) performed two experimental protocols: 1) 15 min of cycle ergometry at 75% VO2peak and then 60 min of recovery in the URS posture; or 2) 15 min of cycle ergometry at 75% VO2peak and then 60 min of recovery in the 15 degrees HDT position. Mean skin temperature, Tes, CVC, sweat rate, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and MAP were recorded at baseline; end of exercise; 2 min, 5 min, 8 min, 12 min, 15 min, and 20 min after exercise; and every 5 min until the end of recovery (60 min).

RESULTS

During recovery from exercise, we observed significantly greater values for MAP, CVC, and sweat rate with HDT in comparison with the URS recovery posture (P <or= 0.05). The magnitude of these responses to HDT did not differ between genders, and a significantly lower T(es) was subsequently observed with HDT for the duration of recovery (P <or= 0.05) for both males and females. In the URS posture, females showed a greater decrease of postexercise MAP than did males (P <or= 0.05). At the end of 60 min of recovery, T(es) remained significantly elevated above baseline with the URS recovery posture (P <or= 0.05). With HDT, T(es) returned to baseline after 20 min.

CONCLUSION

HDT attenuates the reductions in MAP, CVC, and sweat rate observed after exercise in a gender-independent manner, and this likely is attributable to a nonthermal baroreceptor influence.

Evidence of a greater onset threshold for sweating in females following intense exercise

We evaluated the hypothesis that females would show a greater postexercise hypotension and concurrently a greater increase in the onset threshold for sweating. Fourteen subjects (7 males and 7 females) of similar age, body composition, and fitness status participated in the study. Esophageal temperature was monitored as an index of core temperature while sweat rate was measured by using a ventilated capsule placed on the upper back. Subjects cycled at either 60% (moderate) or 80% (intense) of peak oxygen consumption (VO2speak) followed by 20-min recovery. Subjects then donned a liquid-conditioned suit used to regulate mean skin temperature. The skin was then heated (approximately 4.3 degrees C.h(-1)) until sweating occurred. Esophageal temperatures were similar to baseline before the start of whole body warming for all conditions. The postexercise threshold values for sweating following moderate and intense exercise were an esophageal temperature increase of 0.10+/-0.02 and 0.22+/-0.04 degrees C, respectively for males, and 0.15+/-0.03 and 0.34+/-0.01 degrees C, respectively for females. All were elevated above baseline resting (P<0.05) and a significant sex-related difference was observed for sweating threshold values following intense exercise (P<0.05). This was paralleled by a greater decrease in mean arterial pressure in females at the end of the 20-min recovery (P<0.05). In conclusion, females demonstrate a greater postexercise onset threshold for sweating, which is paralleled by a greater postexercise hypotensive response following intense exercise.

Disturbance of thermal homeostasis following dynamic exercise

Recovery from dynamic exercise results in significant perturbations of thermoregulatory control. These perturbations evoke a prolonged elevation in core body temperature and a concomitant decrease in sweating, skin blood flow, and skin temperature to pre-exercise baseline values within the early stages of recovery. Cutaneous vasodilation and sweating are critical responses necessary for effective thermoregulation during heat stress in humans. The ability to modulate the rate of heat loss through adjustments in vasomotor and sudomotor activity is a fundamental mechanism of thermoregulatory homeostasis. There is a growing body of evidence in support of a possible relationship between hemodynamic changes postexercise and heat loss responses. Specifically, nonthermoregulatory factors, such as baroreceptors, associated with hemodynamic changes, influence the regulation of core body temperature during exercise recovery. The following review will examine the etiology of the post-exercise disturbance in thermal homeostasis and evaluate possible thermal and nonthermal factors associated with a prolonged hyperthermic state following exercise.

Sex differences in postexercise esophageal and muscle tissue temperature response

Factors associated with blood pressure regulation during recovery from exercise dramatically influence core temperature regulation. However, it is unknown whether sex-related differences in postexercise hemodynamics affect core and muscle temperature response. Sixteen participants (8 males, 8 females) completed an incremental isotonic test on a Kin-Com isokinetic apparatus to determine their activity-specific peak oxygen consumption during bilateral knee extensions (V̇o2sp). On a separate day, participants performed 15 min of isolated bilateral knee extensions at a moderate (60% V̇o2sp) exercise intensity followed by a 90-min recovery. Esophageal temperature (Tes), mean arterial pressure (MAP), muscle temperature at four depths in the active vastus medialis (TVM) and three depths in the inactive triceps brachii (TTB) were measured concurrently with sweat rate and cutaneous vascular conductance (CVC). Relative to the preexercise resting Tes of 36.7°C (SD 0.1), between 10 and 50-min of recovery Tes was 0.19°C (SD 0.02) higher for females than males ( P = 0.037). All measurements of TVM (0.036 > P > 0.014) and TTB (0.048 > P > 0.008) were higher for females during the initial 30 min of recovery by between 0.46°C and 0.64°C for TVM and by between 0.53°C and 0.70°C for TTB. In parallel, females showed a 5 to 7 mmHg greater reduction in MAP during recovery relative to males ( P = 0.002) and a significantly lower CVC ( P = 0.020) and sweat rate ( P = 0.034). Therefore, it is concluded that females demonstrate a greater and more prolonged elevation in postexercise esophageal temperature and active and inactive muscle temperatures, which is paralleled by a greater postexercise hypotensive response.

15° Head-down tilt attenuates the postexercise reduction in cutaneous vascular conductance and sweating and decreases esophageal temperature recovery time

The following study examined the effect of 15° head-down tilt (HDT) on postexercise heat loss and hemodynamic responses. We tested the hypothesis that recovery from dynamic exercise in the HDT position would attenuate the reduction in the heat loss responses of cutaneous vascular conductance (CVC) and sweating relative to upright seated (URS) recovery in association with an augmented hemodynamic response and an increased rate of core temperature decay. Seven male subjects performed the following three experimental protocols: 1) 60 min in the URS posture followed by 60 min in the 15° HDT position; 2) 15 min of cycle ergometry at 75% of their predetermined V̇o2 peak followed by 60 min of recovery in the URS posture; or 3) 15 min of cycle ergometry at 75% of their predetermined V̇o2 peak followed by 60 min of recovery in the 15° HDT position. Mean skin temperature, esophageal temperature (Tes), skin blood flow, sweat rate, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance, and mean arterial pressure (MAP) were recorded at baseline, end exercise, 2, 5, 8, 12, 15, and 20 min, and every 5 min until end of recovery (60 min). Without preceding exercise, HDT decreased HR and increased SV ( P ≤ 0.05). During recovery after exercise, a significantly greater MAP, SV, CVC, and sweat rate and a significantly lower HR were found with HDT compared with URS posture ( P ≤ 0.05). Subsequently, a significantly lower Tes was observed with HDT after 15 min of recovery onward ( P ≤ 0.05). At the end of 60 min of recovery, Tes remained significantly elevated above baseline with URS ( P ≤ 0.05); however, Tes returned to baseline with HDT. In conclusion, extended recovery from dynamic exercise in the 15° HDT position attenuates the reduction in CVC and sweating, thereby significantly increasing the rate of Tes decay compared with recovery in the URS posture.

Neural control and mechanisms of eccrine sweating during heat stress and exercise

In humans, evaporative heat loss from eccrine sweat glands is critical for thermoregulation during exercise and/or exposure to hot environmental conditions, particularly when environmental temperature is greater than skin temperature. Since the time of the ancient Greeks, the significance of sweating has been recognized, whereas our understanding of the mechanisms and controllers of sweating has largely developed during the past century. This review initially focuses on the basic mechanisms of eccrine sweat secretion during heat stress and/or exercise along with a review of the primary controllers of thermoregulatory sweating (i.e., internal and skin temperatures). This is followed by a review of key nonthermal factors associated with prolonged heat stress and exercise that have been proposed to modulate the sweating response. Finally, mechanisms pertaining to the effects of heat acclimation and microgravity exposure are presented.

Nonthermoregulatory control of cutaneous vascular conductance and sweating during recovery from dynamic exercise in women

The purpose of the study was to examine the effect of 1) active (loadless pedaling), 2) passive (assisted pedaling), and 3) inactive (motionless) recovery modes on mean arterial pressure (MAP), cutaneous vascular conductance (CVC), and sweat rate during recovery after 15 min of dynamic exercise in women. It was hypothesized that an active recovery mode would be most effective in attenuating the fall in MAP, CVC, and sweating during exercise recovery. Ten female subjects performed 15 min of cycle ergometer exercise at 70% of their predetermined peak oxygen consumption followed by 20 min of 1) active, 2) passive, or 3) inactive recovery. Mean skin temperature (Tsk), esophageal temperature (Tes), skin blood flow, sweating, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and MAP were recorded at baseline, end exercise, and 2, 5, 8, 12, 15, and 20 min postexercise. Cutaneous vascular conductance (CVC) was calculated as the ratio of laser-Doppler blood flow to MAP. In the active recovery mode, CVC, sweat rate, MAP, CO, and SV remained elevated over inactive values (P < 0.05). The passive mode was equally as effective as the active mode in maintaining MAP. Sweat rate was different among all modes after 12 min of recovery (P < 0.05). TPR during active recovery remained significantly lower than during recovery in the inactive mode (P < 0.05). No differences in either Tes or Tsk were observed among conditions. The results indicate that CVC can be modulated by central command and possibly cardiopulmonary baroreceptors in women. However, differences in sweat rate may be influenced by factors such as central command, mechanoreceptor stimulation, or cardiopulmonary baroreceptors.