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 (Tc), skin temperature (Tsk), 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. Tc plateaued in 20/10 during the second hour of work (p = 0.93), while Tc increased in 40/20 (p < 0.01). There was no difference in maximum Tc (40/20: 38.08 ± 0.35 °C, 20/10: 37.99 ± 0.27 °C, p = 0.22) or end-of-work Tsk (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 Tc while longer work/rest cycles resulted in a continued increase in Tc 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.