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

PMID: 39981303
Journal: Frontiers in physiology
Published: 2025
Authors: Yoder HA, Mulholland AM, MacDonald HV, Wingo JE

## 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.

## Population

- Healthy adults completing lab cycling HIIT in heat
- Sample size: 8
- Age: 25 +/- 7 years; range 18-38
- Sex: 4 women and 4 men
- Fitness level: Mostly recreationally active; one endurance athlete
- Health status: Free of disease

## Methodology

- Counterbalanced repeated-measures laboratory trial
- Five laboratory visits over up to 8 weeks
- Exercise physiology laboratory and environmental chamber
- Randomized and controlled study design.

## Protocol

- RPE-prescribed HIIT.
- Modality: Cycle ergometer.
- Work intervals: 4 min at RPE 17.
- Recovery: 3 min at RPE 12.
- Sets or repetitions: 1 or 5 rounds.
- Intensity: Borg RPE targets in 35 C heat.
- Session duration: 15 or 43 min before GXT.
- Program length: Acute trials.
- Progression: No progression.
- HR-prescribed HIIT.
- Modality: Cycle ergometer.
- Work intervals: 4 min at 90% HRmax.
- Recovery: 3 min at 70% HRmax or 30 W minimum.
- Sets or repetitions: 1 or 5 rounds.
- Intensity: Target heart rate in 35 C heat.
- Session duration: 15 or 43 min before GXT.
- Program length: Acute trials.
- Progression: No progression.

## Outcomes

### Work rate
Status: mixed
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
Status: worse/safety concern
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
Status: no clear change
Rectal temperature increased similarly during HIIT in both conditions.

43HR increase 0.7 C; 43RPE increase 0.8 C during intervals.

### VO2max
Status: worse/safety concern
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.

## Practical 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 And Adherence

- No adverse events were reported in the extracted text.
- Two participants reached or surpassed control HRmax during the submaximal 43RPE HIIT portion.
- Recovery RPE 12 corresponded to vigorous HR percentages in heat.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/39981303/) (pubmed)
- [DOI](https://doi.org/10.3389/fphys.2025.1506325) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11839774/) (full text)

## Agent Guidance

Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.