Abstract:
Cardiovascular (CV) strain, indexed as CV drift—a progressive increase in heart rate (HR) and decrease in stroke volume (SV) during prolonged exercise—is exacerbated by environmental heat stress and is accompanied by a decrease in maximal work capacity (V?O2max). To attenuate CV strain, work:rest ratios have been recommended by the National Institute for Occupational Safety and Health (NIOSH). Whether these guidelines sufficiently mitigate CV drift and preserve V?O2max is unknown. PURPOSE: To test the hypothesis that during moderate work (201-300 kcal/h) in hot conditions [indoor wet-bulb globe temperature (WBGTin)=29 °C] utilizing the recommended 45:15 work:rest ratio, CV drift will ‘accumulate’ over time, and the magnitude of accumulated CV drift will be proportional to decrements in V?O2max after 120 min. METHODS: Eight subjects [5 women; (mean±SD) age=25±5 y; body mass=74.8±11.6 kg; V?O2max=42.9±5.6 mL/kg/min] completed 3 sessions on different days. The first visit involved measurement of V?O2max (WBGTin=18.1±1.2 °C). The following 2 experimental trials were counterbalanced (WBGTin=29.0±0.6 °C). Moderate work was achieved by 2.5 min of arm curls (4.5 kg at 20/min) and 20 min of walking (V?O2=1.0–1.1 L/min) on a treadmill, repeated once for a total of 45 min of work, and then followed by 15 min of seated rest. HR and SV were measured at 15 and 45 min of each work bout to evaluate CV drift. The 120-min trial consisted of 2 full work-rest cycles followed by measurement of V?O2max; the 15-min trial replicated the first 15 min of the 120-min visit followed by measurement of V?O2max and was necessary to measure V?O2max before CV drift occurred. RESULTS: CV drift accumulated between 15 and 105 min: HR increased 16.7% (18±9 bpm, P=0.004) and SV decreased 16.9% (-12.3±5.9 mL, P=0.003), but V?O2max was not affected after 2 full work-rest cycles (P=0.14). Core body temperature increased by 0.5±0.2 °C (P=0.006) over 2 h. CONCLUSION: Although CV drift occurred after two 45:15 work:rest cycles, V?O2max was unaffected. Work capacity is preserved during 2 hours of work in the heat with rest cycles patterned like those in this study, but CV and thermal strain persist unabated.
