Influence of exercise training with thigh compression on heat-loss responses

Meta-analysis of heat-induced changes in cardiac function from over 400 laboratory-based heat exposure studies

Heat waves are associated with increased fatalities from adverse cardiovascular events attributed to the negative effects of heat on cardiac function. However, scientific understanding of acute cardiac adjustments to heat has come primarily from laboratory experiments employing insulated and encapsulated heating modalities, most commonly water-perfused suits. We evaluated whether findings from those studies reflect cardiac responses during more natural exposures to hot ambient conditions simulated in climate-controlled chambers by synthesizing the findings from over 400 laboratory-based heat exposure studies (6858 participant-exposures) published between 1961-2024. Among all included studies, median (interquartile range) elevations in core temperature and heart rate from baseline to end-exposure were 0.9 (0.5-1.3)°C and 27 (15-40) beats/min. Multilevel mixed-effects meta-analyses revealed exacerbated elevations in heart rate, cardiac output, and rate pressure product (estimate of cardiac workload) and blunted falls in systolic pressure in participants heated via encapsulated modalities. Leveraging the large dataset, we also provide empirical estimates of body temperature and cardiovascular responses to a wide range of conditions experienced during heat waves. With rising global temperatures, ecologically-minded physiological research is needed to improve understanding of the effects of heat stress on cardiac responses and further the development of robust climate health models and evidence-based heat-health guidance.

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.

Effect of regular precooling on adaptation to training in the heat

PURPOSE

This study investigated whether regular precooling would help to maintain day-to-day training intensity and improve 20-km cycling time trial (TT) performed in the heat. Twenty males cycled for 10 day × 60 min at perceived exertion equivalent to 15 in the heat (35 °C, 50% relative humidity), preceded by no cooling (CON, n = 10) or 30-min water immersion at 22 °C (PRECOOL, n = 10).

METHODS

19 participants (n = 9 and 10 for CON and PRECOOL, respectively) completed heat stress tests (25-min at 60% [Formula: see text] and 20-km TT) before and after heat acclimation.

RESULTS

Changes in mean power output (∆MPO, P = 0.024) and heart rate (∆HR, P = 0.029) during heat acclimation were lower for CON (∆MPO - 2.6 ± 8.1%, ∆HR - 7 ± 7 bpm), compared with PRECOOL (∆MPO + 2.9 ± 6.6%, ∆HR - 1 ± 8 bpm). HR during constant-paced cycling was decreased from the pre-acclimation test in both groups (P < 0.001). Only PRECOOL demonstrated lower rectal temperature (T_re) during constant-paced cycling (P = 0.002) and lower T_re threshold for sweating (P = 0.042). However, skin perfusion and total sweat output did not change in either CON or PRECOOL (all P > 0.05). MPO (P = 0.016) and finish time (P = 0.013) for the 20-km TT were improved in PRECOOL but did not change in CON (P = 0.052 for MPO, P = 0.140 for finish time).

CONCLUSION

Precooling maintains day-to-day training intensity and does not appear to attenuate adaptation to training in the heat.