Cardiovascular drift—a progressive increase in heart rate (HR) and decrease in stroke volume (SV) during prolonged exercise—is exacerbated by heat stress and thermal strain, and often accompanied by a decrease in work capacity (indexed as maximal oxygen uptake [$\stackrel{.}{\mathrm{V}}$O_2max]). To attenuate physiological strain during work in the heat, use of work:rest ratios is recommended by the National Institute for Occupational Safety and Health. The purpose of this study was to test the hypothesis that during moderate work in hot conditions, utilizing the recommended 45:15 min work:rest ratio would result in cardiovascular drift ‘accumulating’ over consecutive work:rest cycles and accompanying decrements in $\stackrel{.}{\mathrm{V}}$O_2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; $\stackrel{.}{\mathrm{V}}$O_2max = 42.9 ± 5.6 mL·kg⁻¹·min⁻¹) performed 120 min of simulated moderate work (201–300 kcal·h⁻¹) in hot conditions (indoor wet-bulb globe temperature = 29.0 ± 0.6 °C). Participants completed two 45:15 min work:rest cycles. Cardiovascular drift was evaluated at 15 and 45 min of each work bout; $\stackrel{.}{\mathrm{V}}$O_2max was measured after 120 min. On a separate day, $\stackrel{.}{\mathrm{V}}$O_2max was measured after 15 min under identical conditions for comparison before and after cardiovascular drift occurred. HR increased 16.7% (18 ± 9 beats·min⁻¹, p = 0.004) and SV decreased 16.9% (−12.3 ± 5.9 mL, p = 0.003) between 15 and 105 min, but $\stackrel{.}{\mathrm{V}}$O_2max was unaffected after 120 min (p = 0.14). Core body temperature increased 0.5 ± 0.2 °C (p = 0.006) over 2 h. Recommended work:rest ratios preserved work capacity but did not prevent the accumulation of cardiovascular and thermal strain.

Heart rate drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target heart rate. The purpose was to characterize acute work rate adjustments during high-intensity interval training based on target heart rate. Seven humans (three females) completed five study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HR_max) in ∼22 °C and four trials performed in ∼22 °C (TEMP) or ∼35 °C (HOT), consisting of an 8 min warm-up at 70% HR_max followed by one (15_TEMP and 15_HOT) or five (43_TEMP and 43_HOT) rounds of high-intensity interval training (one round = 4 min work at 90% HR_max and 3 min recovery at 70% HR_max) totaling 15 min or 43 min of exercise, respectively. Work rate was lowered 33 ± 20 W ( p = 0.005) in 43_TEMP and 56 ± 30 W ( p = 0.003) in 43_HOT between the first and fifth work intervals. Thermal strain (0.2 °C higher rectal temperature, p = 0.01) and cardiovascular strain (6 beats·min^–1 larger increase in heart rate from first to fifth recovery interval, p = 0.01) were greater in 43_HOT versus 43_TEMP. Using target heart rate during high-intensity interval training may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout despite the heat.