Hillary A. Yoder, Anne M. Mulholland, Nick Barefoot, Katie Sullivan, Hayley V. MacDonald, Jonathan E. Wingo, FACSM. University of Alabama, Tuscaloosa, AL.

Heart rate (HR) and rating of perceived exertion (RPE) drift upward over time during exercise in hot conditions. As such, work rate must be lowered to maintain target intensity. PURPOSE: To characterize work rate adjustments during high-intensity interval training (HIIT) exercise in the heat based on target HR and target RPE and to test the hypotheses that 1) work rate must be lowered to a greater extent to maintain target HR than to maintain target RPE and 2) greater thermal and cardiovascular strain result from maintaining target RPE compared to target HR during HIIT in hot conditions. METHODS: 4 adults [3 men, (mean±SD) age=26±8 y] completed two 43-min trials on a cycle ergometer in 35 °C. Exercise intensity was prescribed based on a target HR (HR-based) or target RPE (RPE-based). After an 8-min warm-up at 70% HRmaxor RPE of 12, subjects completed 5 rounds of HIIT (4 min of work at 90% HRmax or an RPE of 17 and 3 min of recovery at 70% HRmax or an RPE of 12). RESULTS: Change in power output from the first to the last high-intensity bout was not different between trials (HR-based=-61±33 W, RPE-based =-37±18 W; p=0.15). Since HR was essentially clamped during HR-based, on average HR increased over twice as much from the first to final high-intensity bout during RPE-based (15±5 beats/min) compared to HR-based (7±6 beats/min), although the difference was not statistically significant (p=0.08). The increase in rectal temperature over time (1.1±0.4 °C for HR-based and 1.1±0.5 °C for RPE-based) also was not different between treatments (p=0.29). CONCLUSION: These pilot data show that method of exercise prescription did not affect work rate adjustments, cardiovascular strain, or thermal strain during a HIIT workout in hot conditions. Although not statistically significant, work rate had to be lowered ~24 W (65%) more to maintain target HR than to maintain target RPE, which may have practical significance in terms of the metabolic stimulus of the exercise and potentially compromised training adaptations. On the other hand, maintenance of a higher metabolic intensity during the RPE-based trial resulted in 114% greater increase in HR over time which may have implications for reduced maximal oxygen uptake associated with cardiovascular drift.

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