# Fidelity, tolerability and safety of acute high-intensity interval training after hospitalisation for COVID-19: a randomised cross-over trial

PMID: 34493958
Journal: BMJ Open Sport & Exercise Medicine
Published: 2021-09-01
Authors: Foged F, Rasmussen IE, Bjørn Budde J, Rasmussen RS, Rasmussen V, Lyngbæk M, Jønck S, Krogh-Madsen R, Lindegaard B, Ried-Larsen M, Berg RMG, Christensen RH

## Question

Whether three widely used supervised HIIT protocols can be performed with acceptable fidelity, tolerability, and safety by individuals recently discharged after hospitalization for severe COVID-19.

## Summary

Ten adults recently discharged after hospitalization for severe COVID-19 completed three supervised cycle-ergometer HIIT protocols in randomized cross-over order: 4 x 4 minutes, 6 x 1 minute, and 10-20-30. Every participant completed all three 38-minute sessions with no adverse events, no angina, and no post-exercise ECG changes. All protocols were rated tolerable, though time spent above 85% maximum heart rate differed between protocols. The findings support feasibility and acute supervised safety in a small, selected post-COVID group, not long-term rehabilitation efficacy.

## Population

- Adults recently discharged after hospitalization for severe COVID-19 requiring oxygen supplementation.
- Sample size: 10
- Age: 61 years (SD 8)
- Sex: 5 males, 5 females
- Fitness level: VO2max 24.9 (8.8) mL/min/kg
- Health status: Recovered from acute severe COVID-19; mildly to slightly affected by persistent symptoms on PFASS

## Methodology

- Randomized cross-over trial comparing three acute supervised HIIT protocols with 1-week washout between protocols.
- Three acute HIIT sessions per participant, each 38 minutes, separated by 1-week washout periods.
- Centre for Physical Activity Research at Rigshospitalet in Copenhagen, Denmark.
- Randomized and controlled study design.

## Protocol

- Supervised acute cycle HIIT protocols: 4 x 4, 6 x 1, and 10-20-30.
- Modality: Bicycle ergometer.
- Work intervals: 4 x 4: four 4-minute intervals; 6 x 1: six 1-minute intervals; 10-20-30: repeated 10-second all-out bouts within 30-20-10 cycles.
- Recovery: 4 x 4: 3-minute active pauses at 50-70% HRmax; 6 x 1: 3-minute active pauses at 30% Wattmax; 10-20-30: 3 minutes at 50-70% HRmax between 5-minute sets plus easy/medium segments within cycles.
- Sets or repetitions: Three protocols completed by each participant in randomized order.
- Intensity: 4 x 4 at intensity inducing >=85% HRmax; 6 x 1 at 100% Wattmax; 10-20-30 with 30 seconds easy, 20 seconds medium, 10 seconds all-out.
- Session duration: 38 minutes each.
- Frequency: Three acute sessions separated by 1-week washout.
- Program length: Acute cross-over trial.
- Progression: No training progression; protocol-specific dose modification allowed based on HR, RPM, or workload criteria.
- Cross-over protocol comparators.
- Modality: Bicycle ergometer.
- Work intervals: Each protocol compared with the other two HIIT protocols.
- Recovery: Protocol-specific active recovery.
- Sets or repetitions: All 10 participants performed all three protocols.
- Intensity: Matched total duration but different HRmax, Wattmax, and subjective effort targets.
- Session duration: 38 minutes.
- Frequency: 1-week washout between sessions.
- Program length: Three-session acute cross-over.

## Outcomes

### Protocol completion
Status: improved
All 10 participants completed all three HIIT protocols with 100% attendance.

Attendance 100%; all randomized participants included in analysis.

### Safety
Status: no clear change
No adverse events were reported, and no angina or ECG changes were seen after HIIT training.

### Time at HR >=85% HRmax
Status: mixed
High intensities were achieved in all protocols, but time above 85% HRmax differed by protocol, with less time for 6 x 1.

4 x 4: 13.7 (6.4) min; 10-20-30: 12.1 (3.8) min; 6 x 1: 6.1 (5.6) min; p=0.03.

### Tolerability
Status: no clear change
All three protocols were well tolerated with similar Likert scores.

4 x 4: 8 (2); 10-20-30: 8 (2); 6 x 1: 9 (2); p=0.72.

### RPE
Status: no clear change
RPE did not differ between protocols.

4 x 4: 5 (2); 6 x 1: 5 (2); 10-20-30: 6 (2); p=0.28.

## Practical Insights

- Acute supervised cycle HIIT may be feasible in selected adults recently hospitalized for COVID-19 after screening and with medical-style safety monitoring.
- Protocol labels do not guarantee equivalent intensity exposure: 6 x 1 produced less time above 85% HRmax than 4 x 4 and 10-20-30 in this sample.
- Post-COVID HIIT safety claims should be limited to supervised, selected, acute settings until larger and longer trials are available.

## Limitations

- Very small sample size of 10 participants.
- Likely selection bias because the main reason for declining participation was feeling too weak to exercise.
- Participants were heterogeneous.
- Only acute HIIT bouts were tested.
- No conclusions can be drawn about extended HIIT rehabilitation programs.
- Clinical supervised cycling setting limits generalization to unsupervised or bodyweight HIIT.

## Safety And Adherence

- No adverse events were reported during the study.
- No angina was reported during HIIT training.
- No ECG changes were seen after HIIT training.
- Safety termination criteria included angina, signs of poor perfusion, request to terminate due to intolerable dyspnea, and severe fatigue manifestations.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/34493958/) (pubmed)
- [DOI](https://doi.org/10.1136/bmjsem-2021-001156) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8413475/) (full text)

## Agent Guidance

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