# Cardiorespiratory and metabolic stress responses to acute high-intensity interval training anchored to critical power or maximal heart rate

PMID: 41462026
Journal: Scientific reports
Published: 2025 Dec 29
Authors: Bone J, Richards DL, Gibala MJ

## Question

Does critical-power-based HIIT reduce variability in blood lactate response compared with HRmax-based HIIT, and how do cardiorespiratory stress responses differ?

## Summary

Nineteen active adults completed two acute cycling HIIT trials, one anchored just above critical power and one adjusted to elicit 80-90% HRmax. Critical-power anchoring did not reduce lactate-response variability, but it produced higher workload, oxygen uptake, heart rate, ventilation, and perceived exertion. All 19 completed both trials.

## Population

- Nineteen apparently healthy active young adults
- Sample size: 19
- Age: 23 +/- 4 years
- Sex: 9 men, 10 women
- Fitness level: Active, cycling included in habitual exercise; VO2peak 46.2 +/- 7.0 ml/kg/min
- Health status: Apparently healthy

## Methodology

- Randomized counterbalanced crossover acute exercise study
- 9-11 lab visits over 4-5 weeks
- McMaster University exercise laboratory
- Randomized and controlled study design.

## Protocol

- CP_HIIT.
- Modality: Stationary cycling.
- Work intervals: 4 minutes.
- Recovery: 3 minutes at 25 W.
- Sets or repetitions: 4 bouts.
- Intensity: CP + workload to deplete 10% W-prime in 4 minutes.
- Session duration: 4 x 4-min bouts plus warm-up/recovery/cool-down.
- Frequency: Single acute trial.
- Program length: Acute crossover.
- Progression: None.
- HR_HIIT.
- Modality: Stationary cycling.
- Work intervals: 4 minutes.
- Recovery: 3 minutes at 25 W.
- Sets or repetitions: 4 bouts.
- Intensity: Workload adjusted to elicit 80-90% HRmax by end of first bout.
- Session duration: 4 x 4-min bouts plus warm-up/recovery/cool-down.
- Frequency: Single acute trial.
- Program length: Acute crossover.
- Progression: None.

## Outcomes

### Lactate
Status: no clear change
CP anchoring did not reduce lactate-response variability.

Variability comparison p=0.75; net lactate p=0.10.

### Workload
Status: improved
CP_HIIT used a higher workload than HR_HIIT.

192 +/- 39 W vs 180 +/- 43 W; p=0.0013.

### VO2/HR
Status: improved
CP_HIIT produced higher oxygen uptake, ventilation, and heart-rate responses.

Final 30 s VO2 p=0.0019; HRmean and HRpeak higher in CP_HIIT.

### RPE
Status: worse/safety concern
CP_HIIT felt harder than HR_HIIT.

Mean RPE 14 +/- 2 vs 13 +/- 2; p=0.0034.

## Practical Insights

- HR-based HIIT can place users in different metabolic domains even at similar HR targets.
- Threshold-based prescriptions may increase workload and stress but require more testing.
- A familiarization/verification session can help ensure HR-based sessions hit intended responses.

## Limitations

- Acute-only study.
- Small active young adult sample.
- CP + 10% W-prime may have been conservative.
- Critical power estimates had possible error in some participants.
- Point-of-care lactate devices have mixed validity.
- One author reported a relevant exercise-related commercial relationship.

## Safety And Adherence

- Adverse events were not explicitly reported.
- One enrolled participant was lost to follow-up; all 19 analyzed participants completed both CP_HIIT and HR_HIIT trials.
- The CP workload was conservatively set at CP + 10% W-prime depletion to ensure trial completion.
- Sessions were supervised with HR, lactate, metabolic, and RPE monitoring.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/41462026/) (pubmed)
- [DOI](https://doi.org/10.1038/s41598-025-28231-y) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12749765/) (full text)

## Agent Guidance

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