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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 (4×4, 6×1, 10-20-30) 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 (1–10).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); 4×4: 13.7 (6.4) min; 10-20-30: 12.1 (3.8) min; 6×1: 6.1 (5.6) min; p=0.03). The three protocols were all well tolerated with similar Likert scale scores (mean (SD); 4×4: 8 (2), 10-20-30: 8 (2), 6×1: 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.

IntroductionCOVID-19 is associated with a marked systemic inflammatory response with concomitant cardiac injury and remodelling, but it is currently unknown whether the latter is reversible. Given that high-intensity interval training (HIIT) is a powerful stimulus to improve cardiorespiratory fitness while also eliciting marked anti-inflammatory effects, it may be an important countermeasure of reducing cardiopulmonary morbidity following COVID-19.Methods and analysis40 COVID-19 survivors who have been discharged from hospital will be included in this investigator-blinded randomised study with a 12-week HIIT intervention. Patients will be 1:1 block-randomised by sex to either a supervised HIIT exercise group or standard care (control group). The main hypothesis is that a 12-week HIIT scheme is a safe way to improve loss of cardiac mass and associated cardiorespiratory fitness, despite hypothesised limited HIIT-induced changes in conventional lung function indices per se. Ultimately, we hypothesise that the HIIT scheme will reduce post-COVID-19 symptoms and improve quality of life.Ethics and disseminationThis study is approved by the Scientific Ethical Committee at the Capital Region of Denmark (H-20033733, including amendments 75068 and 75799) and registered at ClinicalTrials.gov (NCT04647734, pre-results). The findings will be published in a peer-reviewed journal, including cases of positive, negative and inconclusive results.Trial registration number NCT04549337.

Fitness is essential to military personnel in general, especially in the special operations forces (SOF), where the demanding tasks require a high level of physical fitness and mental robustness. However, little research has been done on SOF to characterise the putative underlying cardiopulmonary adaptations that distinguish them from conventional infantry soldiers (INF). This study aims to evaluate the cardiopulmonary function in SOF compared INF. The study assessed cardiac function and dimension using transthoracic echocardiography obtained at rest in eight soldiers from a SOF unit and in eight INF. was measured by direct calorimetry (secondary outcome) at the same time blood samples were collected to measure lactate levels. Lung function was assessed by spirometry, while the haemoglobin‐corrected pulmonary diffusing capacity for carbon monoxide ( D L,COc ) was examined by the single‐breath technique. SOF had higher stroke volume (mean difference = 21 mL, P  < 0.001) and left ventricular ejection fraction (mean difference = 7%, P  = 0.026) than INF. Furthermore, SOF had higher global constructive myocardial work and global work index compared to INF. as percentage of predicted according to age, weight and sex was higher in SOF, and they also had lower lactate levels during the test than INF ( P  = 0.029). None of the measured lung function metrics differed between groups. In conclusion, when compared to conventional infantry soldiers, SOF soldiers had marked cardiac adaptations with evidence of eccentric LV remodelling. It remains to be determined if this reflects different training regimes or selection. What is the central question of this study? Do special operations forces exhibit superior cardiopulmonary function compared to conventional infantry soldiers? What is the the main finding and its importance? Special operation forces exhibited evidence of eccentric left ventricular remodelling with a concomitant relatively high whole‐body maximal oxygen uptake. It remains to be determined whether this reflects their specific training regime or selection.

In patients previously hospitalised for COVID‐19, a 12‐week high‐intensity interval training (HIIT) intervention has previously been shown to increase left ventricular mass (LVM) immediately after the intervention. In the present study, we examined the effects of the same HIIT scheme on LVM, pulmonary diffusing capacity, symptom severity and functional capacity at 12‐month follow‐up. In this investigator‐blinded, randomised controlled trial, 12 weeks of a supervised HIIT scheme (4 × 4 min, three times a week) was compared to standard care (control) in patients recently discharged from hospital due to COVID‐19. At inclusion and at 12‐month follow‐up, LVM was assessed by cardiac magnetic resonance imaging (cMRI, primary outcome), while pulmonary diffusing capacity for carbon monoxide ( D LCOc , secondary outcome) was examined by the single‐breath method. Symptom severity and functional status were examined by the Post‐COVID‐19 Functional Scale (PCFS) and King's Brief Interstitial Lung Disease (KBILD) questionnaire score. Of the 28 patients assessed at baseline, 22 completed cMRI at 12‐month follow‐up (12.4 ± 0.6 months after inclusion). LVM was maintained in the HIIT but not the standard care group, with a mean between‐group difference of 9.68 [95% CI: 1.72, 17.64] g ( P  = 0.0182). There was no differences in change from baseline to 12‐month follow‐up between groups in D LCOc % predicted (−2.45 [−11.25, 6.34]%; P  = 0.578). PCFS and KBILD improved similarly in the two groups. In individuals previously hospitalised for COVID‐19, a 12‐week supervised HIIT scheme resulted in a preserved LVM at 12‐month follow‐up but did not affect pulmonary diffusing capacity or symptom severity. What is the central question of this study? In patients previously hospitalised for COVID‐19, a 12‐week high‐intensity interval training (HIIT) scheme increases left ventricular mass (LVM) immediately after the intervention: does this persist? What is the main finding and its importance? A 12‐week HIIT intervention led to preserved LVM at 12‐month follow‐up, while LVM decreased by ∼10 g in the standard care group. This indicates that HIIT may be useful as part of post‐COVID rehabilitation strategies for specifically targeting the detrimental effects of COVID‐19 on the heart.