Work Rate Adjustments During High-intensity Interval Training In The Heat With Active And Passive Recovery

Rating of perceived exertion (RPE) drifts upward and maximal oxygen uptake (V̇O2max) declines over time during exercise in hot conditions. As such, work rate must be lowered to maintain target RPE. Recovery mode (active [ACT] or passive [PASS]) may alter the magnitude of work rate adjustments needed during high-intensity interval training (HIIT) in the heat, as well as the magnitude of decrement in V̇O2max over time. PURPOSE: To test the hypotheses that during RPE-based HIIT in the heat, ACT 1) necessitates greater decrements in work rate during the work intervals to maintain target RPE, 2) results in greater thermal and cardiovascular strain, and 3) leads to greater decrements in V̇O2max over time compared to PASS. METHODS: Eight subjects (6 men) performed a graded exercise test on a cycle ergometer to determine V̇O2max in temperate (~22 °C) conditions followed by 4 separate, counterbalanced 15- and 43-min trials (43ACT, 43PASS) in a hot (~35 °C) environment. Exercise intensity was prescribed using RPE (6-20 scale). All experimental trials began with an 8-min warm-up (RPE = 12), followed by 1 (15-min trials) or 5 (43-min trials) rounds of HIIT (4 min of work at RPE = 17, and 3 min of recovery at either RPE = 12 for ACT or sitting for PASS). V̇O2max was measured immediately following the 15- and 43-min trials. RESULTS: Between the first and final work interval, work rate had to be lowered by 14% ± 24% during 43ACT and by 1% ± 16% during 43PASS (p = 0.04) to maintain target RPE. Over the same time, rectal temperature increased by 0.9 °C (p < 0.001) with ACT and 0.7 °C with PASS (p < 0.001), with no difference between recovery mode (p = 0.11). HR during the final min of the work intervals was not different between conditions and increased from the first to final interval by 14 b·min–1 (p = 0.001). During 43PASS, HR was 35 b·min–1 lower during the final min of the first recovery interval (p < 0.001) and 27 b·min–1 lower during the final recovery interval (p < 0.002) compared to 43ACT. V̇O2max was 0.4 L·min–1 (11.5%) lower following the 43-min trials versus 15-min trials (p = 0.008). CONCLUSIONS: RPE-based HIIT in the heat with ACT resulted in increased cardiovascular strain and required larger work rate adjustments compared to PASS. However, thermal strain and declines in aerobic capacity were not different between ACT and PASS strategies.