Effect of aerobic capacity on sweat rate and fluid intake during outdoor exercise in the heat

Identifying potential thermal drivers of sudomotor in camels (Camelus dromedarius)

The mechanism of sudomotor regulation in the family Camelidae, as in other mammals, is poorly understood. Five healthy dromedary bulls (400 kg and 4 years-old) were used to examine the interrelationship of sweating rate (SR) with ten thermal parameters measured (and/or estimated) every 3-hr for a 24-hr time period under natural and shaded environmental conditions, in order to subsequently identify the potential thermal drivers of sudomotor in this species. Results revealed that all parameters, including SR, had clearly (P < 0·001) exhibited monophasic circadian rhythms. Moreover, the obtained findings pointed out that strong/moderate positive correlations were existed between SR and eight parameters [i.e. ambient (T_a), rectal (T_r), body (T_b), skin (T_sk), coat (T_ct) temperatures as well as total (ambient-to-body, BTG_t), external (ambient-to-skin, BTG_ex), and internal (skin-to-body, BTG_in) body thermal gradients] suggesting that they may all be good indicators of sweating activity. Nonetheless, out of those highly-correlated parameters, merely six (i.e. T_a, T_b, T_sk, T_ct, BTG_t, and BTG_ex) showed superior coefficients of determination (R² ≥ 0·90; P < 0·000) when interrelated with SR; thereby, implying that they have the potential to drive sudomotor. Notably, however, results were more probably allude that sudomotor is regulated through BTG_t. Accordingly, the onset of sweating (i.e. threshold) and its effective level was determined using BTG_t. A method of how SR can be regulated through BTG_t was proposed according to Webb's theory of controlling body-heat content. Some shortcomings prevent confirming that BTG_t is the best thermal driver of sudomotor in this species were noted. Research dealing with this interesting physiological process requires further experimentation to fully elucidate the basic functional mechanisms of Camelidae's thermoregulatory system .

Unraveling the relationship between the topographic distribution patterns of skin temperature and perspiration response in dromedary camels

The question of how skin temperature (T_sk), measured at different regions of the skin, can affect sudomotor activity and thus show a pattern in topographic distribution for the perspiration response (PR) rate in dromedary camels was approached and examined in this experiment. Under natural summer conditions, four healthy dromedary bulls, with a mean body mass of 420 kg and age of three years, were measured for T_sk and PR in seven skin regions (i.e. head, neck, shoulder, axillary, hump, flank, and hip) twice daily [between 04:00-05:00 h with a mean ambient temperature (T_a) of 26·78 °C and relative humidity (RH) of 18·25% as well as between 13:00-14:00 h with T_a of 44·78 °C and RH of 5·90%] for two successive days. The experiment has clearly demonstrated some novel findings. In fact, results pointed out that T_sk (P < 0·05) exhibited a distinct topographic pattern that faded almost completely at a higher T_a. Meanwhile, PR unexpectedly manifested a uniform (P ≥ 0·05) distribution throughout the experiment, which appears to serve an eco-teleological purpose in dromedaries. Moreover, the obtained findings indicated that the hump and hip regions in particular can work as thermal windows, yet all seven skin regions can predict whole-skin PR fairly accurately (R² ≥ 0·90; P < 0·000). Above all, analysis indicated that T_sk in many regions can affect perspiration in camels (R² < 0·82; P < 0·000), but it failed to demonstrate a topographic pattern in perspiration response at higher or lower T_a; therefore, the data attests that no specific relationship may exist between the topography of a perspiration pattern and the level of regional T_sk. Some shortcomings were noted herein, but research dealing with this subject may very well improve our understanding of the basic functional mechanisms of the thermoregulatory system in this species.

Fluid Balance in Team Sport Athletes and the Effect of Hypohydration on Cognitive, Technical, and Physical Performance

Sweat losses in team sports can be significant due to repeated bursts of high-intensity activity, as well as the large body size of athletes, equipment and uniform requirements, and environmental heat stress often present during training and competition. In this paper we aimed to: (1) describe sweat losses and fluid balance changes reported in team sport athletes, (2) review the literature assessing the impact of hypohydration on cognitive, technical, and physical performance in sports-specific studies, (3) briefly review the potential mechanisms by which hypohydration may impact team sport performance, and (4) discuss considerations for future directions. Significant hypohydration (mean body mass loss (BML) >2%) has been reported most consistently in soccer. Although American Football, rugby, basketball, tennis, and ice hockey have reported high sweating rates, fluid balance disturbances have generally been mild (mean BML <2%), suggesting that drinking opportunities were sufficient for most athletes to offset significant fluid losses. The effect of hydration status on team sport performance has been studied mostly in soccer, basketball, cricket, and baseball, with mixed results. Hypohydration typically impaired performance at higher levels of BML (3-4%) and when the method of dehydration involved heat stress. Increased subjective ratings of fatigue and perceived exertion consistently accompanied hypohydration and could explain, in part, the performance impairments reported in some studies. More research is needed to develop valid, reliable, and sensitive sport-specific protocols and should be used in future studies to determine the effects of hypohydration and modifying factors (e.g., age, sex, athlete caliber) on team sport performance.

Improved sweat gland function during active heating in tennis athletes

BACKGROUND

Relatively few studies on the peripheral sweating mechanisms of trained tennis athletes have been conducted. The purpose of this study was to compare the sweating capacities of tennis athletes against untrained subjects (controls).

METHODS

Thirty-five healthy male volunteers participated including 15 untrained subjects and 20 trained tennis athletes (nationally ranked). Active heat generation was performed for 30 min (running at 60% VO 2 max ) in a climate chamber (temperature, 25.0°C ± 0.5°C; relative humidity, 60% ± 3%, termed active heating). Sweating data (local sweat onset time, local sweat volume, activated sweat glands, sweat output per gland, whole body sweat loss volume) were measured by the capacitance hygrometer-ventilated capsule method and starch-iodide paper. Mean body temperature was calculated from tympanic and skin temperatures.

RESULTS

Local sweat onset time was shorter for tennis athletes (p < 0.001). Local sweat volume, activated sweat glands of the torso and limbs, sweat output per gland, and whole body sweat loss volume were significantly higher for tennis athletes than control subjects after active heating (p < 0.001). Tympanic and mean body temperatures were lower among tennis athletes than controls (p < 0.05).

CONCLUSION

These results indicate that tennis athletes had increased regulatory capacity of their sweat gland function.

Responses of Motor-Sport Athletes to V8 Supercar Racing in Hot Conditions

Background:

Despite the thermal challenge of demanding workloads performed in high cabin temperatures while wearing heavy heat-retardant clothing, information on physiological responses to racing V8 Supercars in hot conditions is not readily available.

Purpose:

To describe the thermal, cardiovascular, and perceptual strain on V8 Supercar drivers competing in hot conditions.

Methods:

Thermal strain was indicated by body-core temperature using an ingested thermosensitive pill. Cardiovascular strain was assessed from heart rate, hydration status, and sweat rate. Perceptual strain was estimated from self-rated thermal sensation, thermal discomfort (modified Gagge scales), perceived exertion (Borg scale), and perceptual strain index.

Results:

Prerace body-core temperatures were (mean ± SD) 37.7°C ± 0.4°C (range 37.0°C to 38.2°C), rising to 39.0°C ± 0.4°C (range 38.4°C to 39.7°C) postrace. Driver heart rates were >160 and >170 beats/min for 85.3% and 46.7% of racing, respectively. Sweat rates were 1.06 ± 0.12 L/h or 13.4 ± 1.2 mL · kg−1 · h−1, and postrace dehydration was 0.6% ± 0.6% of prerace body mass. Drivers rated thermal sensation as hot (10.3 ± 0.9), thermal discomfort as uncomfortable (3.1 ± 1.0), and perceived exertion as very hard to very, very hard (8.7 ± 1.7) after the races. Overall physiological and perceptual strain were 7.4 ± 1.0 and 7.1 ± 1.2, respectively.

Conclusions:

Despite the use of cooling, V8 Supercar drivers endure thermal, cardiovascular, and perceptual strain during brief driving bouts in hot conditions.

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.

Relationship between aerobic power, blood volume, and thermoregulatory responses to exercise-heat stress

To clarify the relationship between aerobic power (VO2max), blood volume (BV), and thermoregulatory responses to exercise-heat stress, we analyzed the cross-sectional relationship between the resting BV, plasma volume (PV), erythrocyte volume (EV), VO2max, forearm blood flow (FBF), and sweating responses during exercise in a hot environment (31 degrees C, 50% relative humidity). Twelve college-aged male subjects with a mean maximal oxygen uptake of 48 (range 42-59) mL.kg-1.min-1, a mean PV of 54 (range 42-72) mL.kg-1, a mean EV of 31 (range 23-43) mL.kg-1, and a mean BV of 85 (range 67-115) mL.kg-1 (measured by the Evans Blue dye dilution method) performed three sessions of 20-min cycle exercise at two levels of intensity (40% and 60% VO2max). The BV, PV and EV correlated positively with peak FBF (r = 0.596-0.711, P < 0.05), the increase of FBF in response to a unit rise in esophageal temperature (Tes; peak delta FBF/peak delta Tes) (r = 0.592-0.656, P < 0.05) and with total sweat loss (TSL) (r = 0.599-0.634, P < 0.05) during the exercise. The VO2max correlated with TSL during exercise at 40% VO2max (r = 0.578, P < 0.05), but not with peak FBF and peak delta FBF/peak delta Tes. The VO2max per lean body mass also showed a significant positive correlation with BV (r = 0.769, P < 0.01), PV (r = 0.706, P < 0.05), and with EV (r = 0.841, P < 0.001). The peak delta FBF/peak delta Tes was correlated positively with peak FBF (r = 0.597-0.830, P < 0.05-0.01) and negatively with peak Tes (r = 0.641-0.769, P < 0.05-0.01) during the exercise at the two levels. However, the chest sweat rate (CSR), TSL, and the increase of CSR in response to a unit rise in Tes (peak delta CSR/peak delta Tes) showed no correlation with peak Tes during the exercise at the two levels. These findings suggest that 1) heat dissipation responses during exercise were related more to blood volume than aerobic power and 2) skin blood flow was related more to body temperature than sweating responses during exercise under mild heat stress.