Work rate adjustments needed to maintain heart rate and RPE during high-intensity interval training in the heat
Question
When cycling HIIT is performed in the heat, how do target heart rate and target RPE prescriptions differ in required work-rate adjustments, thermal/cardiovascular strain, and post-exercise VO2max?
Summary
Eight adults completed cycling HIIT trials in 35 C heat using either target heart rate or target RPE. Both methods required substantial reductions in work rate over a longer 43-minute session; target RPE preserved more recovery work but produced greater cardiovascular strain and a larger temporary drop in VO2max.
Methodology
- Healthy adults completing lab cycling HIIT in heat
- 8 participants.
- Cycle ergometer.
- Work intervals: 4 min at RPE 17.
- Recovery: 3 min at RPE 12.
- Intensity: Borg RPE targets in 35 C heat.
- 15 or 43 min before GXT.
- Acute trials.
- Counterbalanced repeated-measures laboratory trial
- Work rate, Heart rate drift, Thermal strain, and VO2max decline were tracked.
Outcomes
Work rate
Both prescriptions required work-rate reductions; HR-based reductions were moderately larger.
43HR -46 +/- 29 W vs 43RPE -30 +/- 28 W; mean difference 16 +/- 36 W; ES=0.53; not statistically significant.
Heart rate
RPE prescription allowed HR drift during work and recovery intervals.
43RPE work HR increased by 12 bpm from first to fifth work interval; recovery final-minute HR reached 85% HRmax.
Temperature
Rectal temperature increased similarly during HIIT in both conditions.
43HR increase 0.7 C; 43RPE increase 0.8 C during intervals.
VO2max
VO2max declined more over time in the RPE-based condition.
15.6% decrease between 15RPE and 43RPE, p=0.005 in abstract; HR-based decline was not significant after adjustment.
Insights
- In heat, interval power often must decrease even when the workout feels or looks controlled.
- Pair RPE with HR/symptom monitoring during hot HIIT sessions.
- HR targets can protect cardiovascular strain but may reduce training stimulus.
Limitations
- Very small sample
- Acute laboratory physiology only
- Cycling-specific
- Heat-chamber setting
- RPE interpretation varied
Safety
- No adverse events were reported in the the paper.
- Two participants reached or surpassed control HRmax during the submaximal 43RPE HIIT portion.
- Recovery RPE 12 corresponded to vigorous HR percentages in heat.