OBJECTIVES: Many patients with COVID-19 suffer from persistent symptoms, many of which may potentially be reversed by high-intensity interval training (HIIT). Yet, the safety and tolerability of HIIT after COVID-19 is controversial. This study aimed to investigate the fidelity, tolerability and safety of three different HIIT protocols in individuals that had recently been hospitalised due to COVID-19. METHODS: The study was a randomised cross-over trial. We compared three supervised HIIT protocols (4x4, 6x1, 10 to 20 to 30) in 10 individuals recently discharged after hospitalisation for severe COVID-19. Each HIIT protocol had a duration of 38 min and was performed with a 1-week washout between them. Outcomes included adverse events, exercise training intensity and tolerability assessed by the Likert scale (1 to 10). RESULTS: All 10 participants aged 61 (mean, SD 8) years (5 males) completed all three HIIT protocols with no adverse events. High intensities were achieved in all three protocols, although they differed in terms of time spent with a heart rate >= 85% of maximum (mean (SD) 4x4 13.7 (6.4) min; 10 to 20 to 30 12.1 (3.8) min; 6x1 6.1 (5.6) min; p = 0.03). The three protocols were all well tolerated with similar Likert scale scores (mean (SD) 4x4 8 (2), 10 to 20 to 30 8 (2), 6x1 9 (2), p = 0.72). CONCLUSION: Our findings indicate that recently hospitalised individuals for severe COVID-19 may safely tolerate acute bouts of supervised HIIT as per protocol. This warrants future studies testing the potential of regular HIIT as a rehabilitation strategy in this context.

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