Acute work rate adjustments during high-intensity interval training in a hot and temperate environment

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 work rate adjustments during high-intensity interval training based on target heart rate. Seven participants (3 women) completed 5 study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HRmax) in ~22 °C and 4 trials performed in ~22 °C (TEMP) or ~35 °C (HOT), consisting of an 8-min warm-up at 70% HRmax followed by 1 (15TEMP and 15HOT) or 5 (43TEMP and 43HOT) rounds of high-intensity interval training (1 round=4 min work at 90% HRmax and 3 min recovery at 70% HRmax). Work rate was lowered 33±20 W (p=0.005) in 43TEMP and 56±30 W (p=0.003) in 43HOT between the first and fifth work intervals. Similarly, thermal strain (0.2 °C higher rectal temperature, p=0.01) and cardiovascular strain (6 b∙min–1 larger increase in heart rate, p=0.01) were greater in 43HOT versus 43TEMP. 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.