Thermoregulatory and aerobic changes after endurance training in a hypobaric hypoxic and warm environment

Abstract

Plasma volume (PV) expansion by endurance training and/or heat acclimatization is known to increase aerobic and thermoregulatory capacities in humans. Also, higher erythrocyte volume (EV) fractions in blood are known to improve these capacities. We tested the hypothesis that training in a hypobaric hypoxic and warm environment would increase peak aerobic power (VO(2)(peak)) and forearm skin vascular conductance (FVC) response to increased esophageal temperature (T(es)) more than training in either environment alone, by increasing both PV and EV. Twenty men were divided into four training regimens (n = 5 each): low-altitude cool (610-m altitude, 20 degrees C ambient temperature, 50% relative humidity), high-altitude cool (2,000 m, 20 degrees C), low-altitude warm (610 m, 30 degrees C), and high-altitude warm (HW; 2,000 m, 30 degrees C). They exercised on a cycle ergometer at 60% VO(2)(peak) for 1 h/day for 10 days in a climate chamber. After training, PV increased in all trials, but EV increased in only high-altitude trials (both P < 0.05). VO(2)(peak) increased in all trials (P < 0.05) but without any significant differences among trials. FVC response to increased T(es) was measured during exercise at 60% of the pretraining VO(2)(peak) at 610 m and 30 degrees C. After the training, T(es) threshold for increasing FVC decreased in warm trials (P < 0.05) but not in cool trials and was significantly lower in HW than in cool trials (P < 0.05). The slope of FVC increase/T(es) increase increased in all trials (P < 0.05) except for high-altitude cool (P > 0.4) and was significantly higher in HW than in cool trials (P < 0.05). Thus, against our hypothesis, the VO(2)(peak) for HW did not increase more than in other trials. Moreover, slope of FVC increase/T(es) increase in HW increased most, despite the similar increase in blood volume, suggesting that factors other than blood volume were involved in the highest FVC response in HW.

Heat Illness during Working and Preventive Considerations from Body Fluid Homeostasis

“…Blood volume fluctuates with seasonal variation of T a ; 5% expansion in the hottest months and ~3% contraction in the coldest months 56) . We recently reported that 3.5% PV expansion occurred after 10-d endurance training (at 60% VO 2max for 1 h/d) in warm environments (T a =30°C) with concomitant improvement of thermoregulatory responses 57) . More recently, Ichinose et al 59) suggested that the increases in blood volume and cardiac stroke volume after endurance training for 10 d are closely associated with the enhanced sensitivity of cutaneous vasodilation to increased T es during exercise in a hot environment.…”

“…5). Since the reduction in each subject was significantly correlated with the increase in PV after training, they suggested that the stretch of baroreceptors due to PV expansion 57,58) was involved in the reduction. However, they also suggested that there was no causal relationship between the reduction in osmotic suppression of cutaneous vasodilation and plasma expansion but they were caused by different mechanisms associated with heat acclimation.…”

“…The increased SkBF and sweating responses are characterized by the low T es threshold for cutaneous vasodilation and sweating 57,58) and their enhanced sensitivity to increased T es 57,59) . The magnitude of adaptation depends on the intensity, duration, and frequency of heat exposures, but the most effective maneuver to acquire the adaptation should include 'exercise in the heat' 56) .…”

Body Temperature Regulation During Exercise and Hyperthermia in Diabetics

“…After a subsequent 10-day training period in a hot condition (T a , 35°C), these responses improved further [27]. The increased SkBF and sweating responses are characterized by the early start of cutaneous vasodilation and sweating responses to increased T es [28,29] and the increased sensitivity to increase SkBF and sweat rate in response to an increased T es compared with before exercise training [28,29]. It is suggested that the mechanisms of the increase in thermoregulatory responses with exercise training are similar to those following acclimations to repeated heat exposures, including adaptations of the thermoregulatory center and thermoregulatory effectors [1,12] as well as an increase in VO 2max [30] and PV [28,31,32].…”

“…Several longitudinal studies have reported that heat dissipative responses during exercise in a hot environment are enhanced when aerobic training is performed under hot or even cool to thermoneutral conditions by initially sedentary elderly individuals [30,60,84] and young individuals [27][28][29]. Thomas et al [30] reported that a 16-week aerobic training program increased VO 2max by ≥5% and advanced onset of cutaneous vasodilation in response to an increased mean body temperature during exercise in a hot environment (T a , 36°C) both in young and in elderly individuals.…”