Work rate adjustments needed to maintain heart rate and RPE during high-intensity interval training in the heat

Higher work rates may be sustainable when maintaining target rating of perceived exertion (RPE) compared to maintaining target heart rate (THR) during high-intensity interval training (HIIT) exercise in hot conditions, but may also result in greater thermal strain and cardiovascular drift, as well as greater decrements in maximal oxygen uptake ( $$ O_2max).

To test the hypotheses that maintaining target RPE compared to THR during HIIT in the heat results in 1) smaller work rate adjustments, 2) greater thermal and cardiovascular strain, and 3) larger decreases in $$ O_2max.

Eight adults (4 women) completed a graded exercise test on a cycle ergometer in 22°C and then 4 cycling trials in 35°C, consisting of an 8-min warm-up at 70% maximal heart rate (HR_max) or 12 RPE followed by 1 (15_HR and 15_RPE) or 5 (43_HR and 43_RPE) rounds of HIIT (1 round = 4 min work at 90% HR_max or 17 RPE and 3 min recovery at 70% HR_max or 12 RPE) totaling 15 min or 43 min of exercise, respectively. Each trial ended with a GXT to measure $$ O_2max.

In the 43-min trials work rate decreased from the first to the fifth work interval in both conditions, but by a non-significant, yet moderately larger (ES = 0.53) amount during 43_HR (46 ± 29 W) compared to 43_RPE (30 ± 28 W). From the first to fifth work interval HR increased over time by 12 b⋅min^-1 in 43_RPE (p < 0.001), but did not increase during 43_HR (p = 0.36). Rectal temperature increases were not different between conditions (43_HR = 0.7°C, p < 0.001; 43_RPE = 0.8°C, p < 0.001). $$ O_2max decreased 15.6% (ES = 0.41) between 15_RPE and 43_RPE (p = 0.005), but it was not different over time during the HR-based trials [6.5%, ES = 0.16 (α adjusted for multiple comparisons = 0.0125) p = 0.03].

Maintaining target RPE and THR require considerable declines in work rate during HIIT in the heat, with ∼53% larger declines needed to maintain THR. The mitigation of cardiovascular drift in the THR trial may have contributed to the preservation of $$ O_2max.