Plateau in Core Temperature during Shorter but Not Longer Work/Rest Cycles in Heat

This study compared physiological responses to two work/rest cycles of a 2:1 work-to-rest ratio in a hot environment. In a randomized crossover design, fourteen participants completed 120 min of walking and rest in the heat (36.3 ± 0.6 °C, 30.2 ± 4.0% relative humidity). Work/rest cycles were (1) 40 min work/20 min rest [40/20], or (2) 20 min work/10 min rest [20/10], both completing identical work. Core temperature (T_c), skin temperature (T_sk), heart rate (HR), nude body mass, and perception of work were collected. Comparisons were made between trials at equal durations of work using three-way mixed model ANOVA. T_c plateaued in [20/10] during the second hour of work (p = 0.93), while T_c increased in [40/20] (p < 0.01). There was no difference in maximum T_c ([40/20]: 38.08 ± 0.35 °C, [20/10]: 37.99 ± 0.27 °C, p = 0.22) or end-of-work T_sk ([40/20]: 36.1 ± 0.8 °C, [20/10]: 36.0 ± 0.7 °C, p = 0.45). End-of-work HR was greater in [40/20] (145 ± 25 b·min^-1) compared to [20/10] (141 ± 27 b·min^-1, p = 0.04). Shorter work/rest cycles caused a plateau in T_c while longer work/rest cycles resulted in a continued increase in T_c throughout the work, indicating that either work structure could be used during shorter work tasks, while work greater than 2 h in duration may benefit from shorter work/rest cycles to mitigate hyperthermia.