Neural and non-neural control of skin blood flow during isometric handgrip exercise in the heat stressed human

Dose-dependent nonthermal modulation of whole body heat exchange during dynamic exercise in humans

To maintain heat balance during exercise, humans rely on skin blood flow and sweating to facilitate whole body dry and evaporative heat exchange. These responses are modulated by the rise in body temperature (thermal factors), as well as several nonthermal factors implicated in the cardiovascular response to exercise (i.e., central command, mechanoreceptors, and metaboreceptors). However, the way these nonthermal factors interact with thermal factors to maintain heat balance remains poorly understood. We therefore used direct calorimetry to quantify the effects of dose-dependent increases in the activation of these nonthermal stimuli on whole body dry and evaporative heat exchange during dynamic exercise. In a randomized crossover design, eight participants performed 45-min cycling at a fixed metabolic heat production (200 W/m2) in warm, dry conditions (30°C, 20% relative humidity) on four separate occasions, differing only in the level of lower-limb compression applied via bilateral thigh cuffs pressurized to 0, 30, 60, or 90 mmHg. This model provoked increments in nonthermal activation while ensuring the heat loss required to balance heat production was matched across trials. At end-exercise, dry heat loss was 2 W/m2 [1, 3] lower per 30-mmHg pressure increment (P = 0.006), whereas evaporative heat loss was elevated 5 W/m2 [3, 7] with each pressure increment (P < 0.001). Body heat storage and esophageal temperature did not differ across conditions (both P ≥ 0.600). Our findings indicate that the nonthermal factors engaged during exercise exert dose-dependent, opposing effects on whole body dry and evaporative heat exchange, which do not significantly alter heat balance.NEW & NOTEWORTHY To maintain heat balance during exercise, humans rely on skin blood flow and sweating to facilitate dry and evaporative heat exchange. These responses are modulated by body temperatures (thermal factors) and several nonthermal factors (e.g., central command, metaboreceptors), although the way thermal and nonthermal factors interact to regulate body temperature is poorly understood. We demonstrate that nonthermal factors exert dose-dependent, opposing effects on dry and evaporative heat loss, without altering heat storage during dynamic exercise.

SWEAT RATE MEASUREMENTS AFTER HIGH INTENSITY INTERVAL TRAINING USING BODY WEIGHT

ABSTRACT Introduction: Physical activity raises body temperature, increases the sweat rate and accelerates fluid loss during exercise, thereby impairing exercise performance. However, studies using the high intensity interval training (HIIT) approach and its effects on rates of perspiration and hydration are still inconclusive. Objectives: The objective of this study was to assess sweating and water loss during an HIIT workout session, using body weight, with healthy college students. Methods: Twenty male individuals (31 ± 07 years) were split into two groups: Active group (AG) and Inactive group (IG). The HIIT workout protocol, using body weight, consisted of a single bout with 1:1 stimuli, being: 30” “all out” intensity, involving jumping jack, mountain climber, burpee and squat jump exercises; and 30” of passive recovery, totaling 20 minutes of exercises. For comparison purposes, after 48 hours all the individuals underwent the continuous running protocol with intensity corresponding to 75% of maximum heart rate for 40 minutes. The intensity of the session was monitored continuously, at each 30”, using the perceived exertion scale for both protocols. To ensure euhydration status, all individuals ingested 500 ml of water 120 minutes before the training session. Results: Significant differences (p= 0.01) were found in body mass after HIIT compared to the Moderate session in both Active (HIIT: -0.60 ± 0.29 kg, Moderate: -0.26 ± 0.12 kg) and Inactive (HIIT: -0.92 ± 0.30 kg, Moderate: -0.26 ± 0.26 kg) groups, however, no differences were found between groups. Absolute sweating rate values comparing moderate and HIIT single bout in Inactive (Moderate: 10.55 ± 10.59 ml/min; HIIT: 28.90 ± 13.88 ml/min) and Active (Moderate: 9.60 ± 4.52 ml/min; HIIT: 26.00 ± 15.06 ml/min) groups were different between types of exercise, but not between groups. Conclusions: The sweating rate is influenced by the intensity of the exercise, being higher after HIIT than after a moderate exercise session. However, the sweating rate variation is not affected by the subjects’ physical activity level. Level of Evidence II; Diagnostic studies-Investigating a diagnostic test.

The mechanisms underlying the muscle metaboreflex modulation of sweating and cutaneous blood flow in passively heated humans

Metaboreceptors can modulate cutaneous blood flow and sweating during heat stress but the mechanisms remain unknown. Fourteen participants (31 ± 13 years) performed 1‐min bout of isometric handgrip (IHG) exercise at 60% of their maximal voluntary contraction followed by a 3‐min occlusion (OCC), each separated by 10 min, initially under low (LHS, to activate sweating without changes in core temperature) and high (HHS, whole‐body heating to a core temperature increase of 1.0°C) heat stress conditions. Cutaneous vascular conductance (CVC) and sweat rate were measured continuously at four forearm skin sites perfused with 1) lactated Ringer's solution (Control), 2) 10 mmol L‐NAME [inhibits nitric oxide synthase (NOS)], 3) 10 mmol Ketorolac [inhibits cyclooxygenase (COX)], or 4) 4 mmol theophylline (THEO; inhibits adenosine receptors). Relative to pre‐IHG levels with Control, NOS inhibition attenuated the metaboreceptor‐mediated increase in sweating under LHS and HHS (P ≤ 0.05), albeit the attenuation was greater under LHS (P ≤ 0.05). In addition, a reduction from baseline was observed with THEO under LHS during OCC (P ≤ 0.05), but not HHS (both P > 0.05). In contrast, CVC was lower than Control with L‐NAME during OCC in HHS (P ≤ 0.05), but not LHS (P > 0.05). We show that metaboreceptor activation modulates CVC via the stimulation of NOS and adenosine receptors, whereas NOS, but not COX or adenosine receptors, contributes to sweating at all levels of heating.

Modulation of muscle metaboreceptor activation upon sweating and cutaneous vascular responses to rising core temperature in humans

The present study investigated the role of muscle metaboreceptor activation on human thermoregulation by measuring core temperature thresholds and slopes for sweating and cutaneous vascular responses during passive heating associated with central and peripheral mechanisms. Six male and eight female subjects inserted their lower legs into hot water (43°C) while wearing a water perfusion suit on the upper body (34°C). One minute after immersion, an isometric handgrip exercise--40% of maximum voluntary contraction-was conducted for 1.5 min in both control and experimental conditions, while postexercise occlusion was performed in the experimental condition only for 9 min. The postexercise forearm occlusion during passive heating consistently stimulated muscle metaboreceptors, as implicated by significantly elevated mean arterial blood pressure throughout the experimental period (P <0.05). Stimulation of the forearm muscle metaboreceptors increased sweating and cutaneous vascular responses during passive heating, and was associated with significant reductions in esophageal temperature threshold of sweating and cutaneous vasodilation (Δ threshold, sweating: 0.33 ± 0.05 and 0.16 ± 0.04°C, cutaneous vascular conductance: 0.38 ± 0.08 and 0.16 ± 0.05°C for control and experimental groups, respectively, P < 0.05). The slopes of these responses were not different between the conditions. These results suggest that muscle metaboreceptor activation in the forearm accelerates sweating and cutaneous vasodilation during passive heating associated with a reduction in core temperature thresholds and may be related to central mechanisms controlling heat loss responses.

Do metaboreceptors alter heat loss responses following dynamic exercise?

Metaboreceptor activation during passive heating is known to influence cutaneous vascular conductance (CVC) and sweat rate (SR). However, whether metaboreceptors modulate the suppression of heat loss following dynamic exercise remains unclear. On separate days, before and after 15 min of high-intensity treadmill running in the heat (35°C), eight males underwent either 1) no isometric handgrip exercise (IHG) or ischemia (CON), 2) 1 min IHG (60% of maximum, IHG), 3) 1 min IHG followed by 2 min of ischemia (IHG+OCC), 4) 2 min of ischemia (OCC), or 5) 1 min IHG followed by 2 min of ischemia with application of lower body negative pressure (IHG+LBNP). SR (ventilated capsule), cutaneous blood flow (Laser-Doppler), and mean arterial pressure (Finometer) were measured continuously before and after dynamic exercise. Following dynamic exercise, CVC was reduced with IHG exercise ( P < 0.05) and remained attenuated with post-IHG ischemia during IHG+OCC relative to CON (39 ± 2 vs. 47 ± 6%, P < 0.05). Furthermore, the reduction in CVC was exacerbated by application of LBNP during post-IHG ischemia (35 ± 3%, P < 0.05) relative to IHG+OCC. SR increased during IHG exercise ( P < 0.05) and remained elevated during post-IHG ischemia relative to CON following dynamic exercise (0.94 ± 0.15 vs. 0.53 ± 0.09 mg·min−1·cm−2, P < 0.05). In contrast, application of LBNP during post-IHG ischemia had no effect on SR (0.93 ± 0.09 mg·min−1·cm−2, P > 0.05) relative to post-IHG ischemia during IHG+OCC. We show that CVC is reduced and that SR is increased by metaboreceptor activation following dynamic exercise. In addition, we show that the metaboreflex-induced loading of the baroreceptors can influence the CVC response, but not the sweating response.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Changes in eccrine sweating on the glabrous skin of the palm and finger during isometric exercise

AIM

The goals of this study were to investigate changes in the sweating and cutaneous vascular responses on the palm and the volar aspect of the index finger during sustained static exercise of increasing intensity and to determine whether the former can be attributed to altered sweat gland activity.

METHODS

Five male and five female subjects performed maximal voluntary handgrip contractions (MVC: right hand) for 60 s at 20, 35 and 50% MVC (ambient temperature 25 °C, relative humidity 50%).

RESULTS

The sweat rate and the number of activated sweat glands on the non-exercised hand showed intensity-dependent increases (P < 0.05). At 35 and 50% MVC, finger sweat secretion was significantly higher than on the palm, which was primarily associated with the number of activated sweat glands (P < 0.05). In addition, there was a marked simultaneous decrease in the cutaneous vascular conductance for the finger at 35 and 50% MVC (P < 0.05), but not for the palm.

CONCLUSION

Our results suggest that a difference exists between intensity-dependent increases of sudomotor responses within more than one glabrous skin site. Specifically, markedly greater sweating occurs on the volar finger than on the palmar surface during sustained static exercise. These differences in sweat rate mainly resulted from changes in the number of activated sweat glands. In addition, intra-segment variations in cutaneous blood flow on the glabrous hand are shown.

Sweating responses and the muscle metaboreflex under mildly hyperthermic conditions in sprinters and distance runners

To investigate the effects of different training methods on nonthermal sweating during activation of the muscle metaboreflex, we compared sweating responses during postexercise muscle occlusion in endurance runners, sprinters, and untrained men under mild hyperthermia (ambient temperature, 35°C; relative humidity, 50%). Ten endurance runners, nine sprinters, and ten untrained men (maximal oxygen uptakes: 57.5 ± 1.5, 49.3 ± 1.5, and 36.6 ± 1.6 ml·kg(-1)·min(-1), respectively; P < 0.05) performed an isometric handgrip exercise at 40% maximal voluntary contraction for 2 min, and then a pressure of 280 mmHg was applied to the forearm to occlude blood circulation for 2 min. The Δ change in mean arterial blood pressure between the resting level and the occlusion was significantly higher in sprinters than in untrained men (32.2 ± 4.4 vs. 17.3 ± 2.6 mmHg, respectively; P < 0.05); however, no difference was observed between distance runners and untrained men. The Δ mean sweating rate (averaged value of the forehead, chest, forearm, and thigh) during the occlusion was significantly higher in distance runners than in sprinters and untrained men (0.38 ± 0.07, 0.19 ± 0.03, and 0.11 ± 0.04 mg·cm(-2)·min(-1), respectively; P < 0.05) and did not differ between sprinters and untrained men. Our results suggest that the specificity of training modalities influences the sweating response during activation of the muscle metaboreflex. In addition, these results imply that a greater activation of the muscle metaboreflex does not cause a greater sweating response in sprinters.

Combined heat and mental stress alters neurovascular control in humans

This study examined the effect of combined heat and mental stress on neurovascular control. We hypothesized that muscle sympathetic nerve activity (MSNA) and forearm vascular responses to mental stress would be augmented during heat stress. Thirteen subjects performed 5 min of mental stress during normothermia (Tcore; 37 ± 0°C) and heat stress (38 ± 0°C). Heart rate, mean arterial pressure (MAP), MSNA, forearm vascular conductance (FVC; venous occlusion plethysmography), and forearm skin vascular conductance (SkVCf; via laser-Doppler) were analyzed. Heat stress increased heart rate, MSNA, SkVCf, and FVC at rest but did not change MAP. Mental stress increased MSNA and MAP during both thermal conditions; however, the increase in MAP during heat stress was blunted, whereas the increase in MSNA was accentuated, compared with normothermia (time × condition; P < 0.05 for both). Mental stress decreased SkVCf during heat stress but not during normothermia (time × condition, P < 0.01). Mental stress elicited similar increases in heart rate and FVC during both conditions. In one subject combined heat and mental stress induced presyncope coupled with atypical blood pressure and cutaneous vascular responses. In conclusion, these findings indicate that mental stress elicits a blunted increase of MAP during heat stress, despite greater increases in total MSNA and cutaneous vasoconstriction. The neurovascular responses to combined heat and mental stress may be clinically relevant to individuals frequently exposed to mentally demanding tasks in hyperthermic environmental conditions (i.e., soldiers, firefighters, and athletes).

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.