Exercise-rest cycles do not alter local and whole body heat loss responses

Abstract

Previous studies have suggested that greater core temperatures during intermittent exercise (Ex) are due to attenuated sweating [upper back sweat rate (SR)] and skin blood flow (SkBF) responses. We evaluated the hypothesis that heat loss is not altered during exercise-rest cycles (ER). Ten male participants randomly performed four 120-min trials: 1) 60-min Ex and 60-min recovery (60ER); 2) 3 ϫ 20-min Ex separated by 20-min recoveries (20ER); 3) 6 ϫ 10-min Ex separated by 10-min recoveries (10ER), or 4) 12 ϫ 5-min Ex separated by 5-min recoveries (5ER In contrast, the slope of the SkBF response against esophageal temperature did not significantly change from the first to the last Ex (5ER: 51 Ϯ 23 vs. 54 Ϯ 19%/°C, P ϭ 0.848; 10ER: 53 Ϯ 8 vs. 56 Ϯ 21%/°C, P ϭ 0.786; 20ER: 44 Ϯ 20 vs. 50 Ϯ 27%/°C, P ϭ 0.432). Overall, no differences in body heat content and core temperature were observed. These results suggest that altered local and whole body heat loss responses do not explain the previously observed greater core temperatures during intermittent exercise. calorimetry; core temperature; heat stress; skin blood flow; sweat rate INTERMITTENT EXERCISE IS TYPICAL of many occupational and military activities. In most cases, the intermittent aspect of these activities is inherent to the task(s) being performed. However, intermittent exercise is also used by many health and safety industries (1, 18), as well as the US military (24), as a strategy to minimize the risk of exertional heat strain when these activities are performed in hot and/or humid environments.The benefit of exercise-rest cycles is to decrease the timeweighted average rate of metabolic heat production for a given task, which, when combined with sufficient recovery periods, should maintain core temperature below 38°C (1, 18). However, studies examining differences in core temperature between continuous and intermittent exercise have yielded conflicting results. In general, when the exercise periods are of long duration and/or performed under compensable conditions, continuous and intermittent exercise result in similar increases in core temperature (2,3,5,(13)(14)(15)22). In contrast, greater increases in core temperature have been reported during intermittent exercise consisting of short exercise-rest cycles performed under both compensable (4, 17) and uncompensable (13) conditions. Although the specific reason(s) for these discrepancies is unclear, studies that report greater core temperatures during intermittent exercise have consistently attributed this response to altered heat loss responses (4,13,17).It is surprising to note, however, that there exists relatively limited research that directly compares the control of heat loss responses between intermittent and continuous exercise, as most studies only examine core temperature responses. Ekblom et al. (4) attributed a greater increase in core temperature during intermittent exercise to a lower overall loss of body weight (i.e., evaporative heat loss), while Kraning and Gonzalez (13) implied reduced sk...

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

“…Mean skin temperature (Tsk) was calculated using a modified version of the formula reported by Nadel et al The SR on the forehead (center of the forehead), chest (below the clavicle), forearm (center of the left ventral forearm), and palm (between the left thumb and wrist) were measured continuously using the ventilated-capsule method. Because it has been reported that the SR caused by isolated muscle mechanoreflex activation is small (below 0.1 mg·cm 2 ·min Ϫ1 ) (23), we used smaller sweat capsules (1.54 cm 2 ) and greater dry nitrogen gas flow (500 ml/min) to the capsules (nitrogen gas rate/capsule area ϭ 324 ml/cm 2 ) than previous studies [147-248 ml/cm 2 (13,25,28,39,45)] to detect small changes in the response. The capsules were carefully attached to the skin with glue.…”

Whole body heat loss is reduced in older males during short bouts of intermittent exercise

“…Previous studies have demonstrated that the time taken to balance the differential rates of whole body heat production and heat loss (i.e., thermal inertia) is reduced when core temperature is elevated and the body is already warm (7,21,22). The greater rate of whole body heat loss with repeated exercise, known as the priming effect (7), has been shown to reduce the amount of heat gained during the subsequent exercise periods (21).…”

Evidence for cyclooxygenase‐dependent sweating in young males during intermittent exercise in the heat

“…There is also a lack of information on the underlying pathways governing the control of sweating following exercise and during a subsequent exercise bout. Studies show that there is a more rapid increase in sweating during subsequent exercise compared to the first bout, and this has been termed the priming effect (Gagnon & Kenny, ). However, this is not paralleled by a more pronounced rate of heat dissipation in recovery.…”