Frederik Foged, Iben Elmerdahl Rasmussen, Josephine Bjørn Budde, Rasmus Syberg Rasmussen, Villads Rasmussen, Mark Lyngbæk, Simon Jønck, Rikke Krogh-Madsen, Birgitte Lindegaard, Mathias Ried-Larsen, Ronan Martin Griffin Berg, Regitse Højgaard Christensen. Fidelity, tolerability and safety of acute high-intensity interval training after hospitalisation for COVID-19: a randomised cross-over trial. 


The full paper can be found here.






Tell us more about yourself and the author team.

We are both physicians and practising clinicians. Regitse holds a PhD from 2019 and is currently specialising in cardiology while serving as a junior group leader at the Centre for Physical Activity Research, where she studies cardiac adaptations to exercise training in various patient groups, including those with metabolic syndrome and rheumatoid arthritis, both in terms of structural changes and cardiac fat, as well as the mechanistic involvement of cyto/myokines. Ronan holds a PhD from 2014 and a DMSc from 2020 and is a board-certified specialist in clinical physiology and nuclear medicine, subspecialising in respiratory physiology/nuclear pulmonology. He is an Associate Professor of Physiology at the University of Copenhagen. Also, He serves as a group leader at the Centre for Physical Activity Research. He studies ventilation-perfusion relationships focusing on pulmonary vascular function, including pulmonary adaptations to exercise training, in various disease states, including COPD.


What is the story behind your study?

The global COVID-19 pandemic came as quite a shock for everyone, including health professionals such as ourselves, when it first struck back at the beginning of 2020. We both contributed to the initial handling of several severely ill cases of COVID-19 during the first wave of the pandemic in Denmark. While our colleagues and we were worried mainly by the high mortality rates associated with the acute infection, it soon became clear that the effects of the disease would reach far beyond the acute infection.

The hospitalised patients we encountered were severely hypoxaemic and entirely bedridden for extended periods. We discussed this on numerous occasions and soon concluded that this prolonged bedrest would expectedly lead to severe deconditioning. So, it became clear that the clinical consequences of COVID-19 would reach way beyond the immediate pandemic and potentially pose a health problem for the next many years to come. As more and more of the patients we encountered, as well as acquaintances and colleagues (including several of the co-authors of this paper!), developed persistent breathlessness and fatigue after the acute COVID-19 infection. In contrast, more knowledge was gained about the fundamental mechanisms of disease. We inferred that many of these symptoms could be alleviated by targeted rehabilitation with exercise training – both by reversing the severe deconditioning due to prolonged bed rest and through the well-established anti-inflammatory effects of exercise. There were no rehabilitation-focused studies at the time, and we thus decided to apply an ‘Exercise as Medicine’ approach and test high-intensity interval training (HIIT) as a rehabilitation intervention in previously hospitalised COVID-19 patients. HIIT was chosen because it is usually well-tolerated in patients with symptoms of breathlessness. This may seem counterintuitive, but it is actually established in many lung diseases such as COPD. Higher workloads can be achieved than conventional continuous aerobic training; thus, HIIT permits a strong ‘exercise stimulus’.

We thus wanted to conduct a formal randomised clinical trial on HIIT in previously hospitalised COVID-19 patients. Still, before we could do this, we needed to establish whether HIIT was at all feasible in this context, specifically 1) whether previously hospitalised COVID-19 patients could complete a HIIT session at all, 2) whether any of the three most widely used HIIT protocols was superior to the other two, both in terms of patient performance, tolerability and safety.


In your own words, what did you find?

High-intensity interval training is for several reasons, including safety concerns and a somewhat controversial rehabilitation modality in patients suffering from COVID-19. In the present study, we found that previously hospitalised patients with mild to moderate persisting symptoms were fully capable of safely completing an acute bout of exercise based on all three HIIT protocols.


What was the main challenge you faced in your study?

The study was a great experience both for our research team and the patients. Indeed, patients generally found it empowering and were both surprised and proud that they could complete a HIIT session, despite suffering from varying degrees of breathlessness and related symptoms in their daily lives. While we cannot conclude on the feasibility of using HIIT as an intervention in a prospective clinical trial, the results of such efforts must be available before it can be concluded whether HIIT should be actually be implemented in clinical rehabilitation programmes. Specifically, in which subsets of patients, this is indicated. Moreover, our findings are restricted to the fidelity, tolerability and safety of an acute HIIT-based exercise bout in a selected population of previously hospitalised patients with mild to moderate persisting symptoms. This is notably clear because the main reason for not participating among patients was feeling too weak to exercise. Our findings thus strictly apply to the less severely affected population of previously hospitalised COVID-19 patients. After this study, other studies made it clear that many patients, both hospitalised and non-hospitalised, suffer from severe persisting symptoms, including those of the chronic fatigue syndrome with post-exertional malaise. While this was not a formal exclusion criterion because their association with COVID-19 was unknown when we designed and conducted the present study, no such patients were included. Our findings thus do not add to the evidence base for rehabilitation in this large patient population.


If there is one take-home message from your study, what would that be?

Patients who have recently been hospitalised due to COVID-19 with mild to moderate persisting symptoms can perform an acute bout of HIIT – and they find it both enjoyable and empowering!

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