The present study examined the effects of a combined exercise training in the physical and functional capacity in post-COVID patients. The main findings showed that exercise training improved physical and functional capacity, and reduced resting heart rate, systolic and diastolic blood pressure.
An important finding of the present study is the great improvement in functional capacity after supervised combined exercise training, corroborating others with similar protocols and outcomes [27–32]. The handgrip strength increased in our study ~ 32,7% post-COVID rehabilitation. This result presented a higher improvement when compared to Mayer et al. [31] study, that evaluated handgrip strength after 8-week moderate-intensity rehabilitation and observed an improvement of ~ 15,1% and Nambi et al. [32] with post-COVID sarcopenic patients in which observed improvement of ~ 10,9% after 8-week low-intensity rehabilitation. This magnitude difference probably occurred due Mayer et al. [31] performed an ICU recovery previous to outpatient rehabilitation and Nambi et al. [32] analyze community-dwelling elderly with post-COVID, and both studies had higher baseline values than compared to present study. About STS, De Souza et al. [28] showed a significant improvement after 6-week home-based rehabilitation (~ 53,5%). Although De Souza et al. [28] evaluated in non-ICU patients, the magnitude of improvement was similar to observed in our study (~ 44,1%) after a supervised exercise training in severe patients. It is according to Dalbosco-Salas et al. [33] results, which they showed that non-hospitalized, non-ICU and ICU COVID patients had similar improvement in STS after telerehabilitation, but hospitalized patients had lower baseline values. Therefore, rehabilitation with physical exercise is effective to improve STS in post-COVID patients.
The present study also demonstrated that after intervention there was a better performance in flexibility and upper limb strength tests. There are not, to our knowledge, studies that examined effects of exercise rehabilitation in flexibility and upper limb strength. Some of the severe patients develop critical illness polyneuropathy [34], and this adverse outcome could be related to a lower upper limb strength and flexibility after discharge from hospital.
Mayer et al. [31] and Udina et al. [35] demonstrated a faster gait speed after intervention, although Mayer et al. [31] observed an improvement only in supervised rehabilitation, but not in telerehabilitation, and Udina et al. [35] observed that ICU patients had greater change in gait speed test after early rehabilitation. Our findings corroborate these previous studies, which showed a great improvement in gait speed test after supervised rehabilitation with mostly ICU patients. Betschart et al. [27] and Everaerts et al. [29] evaluated 6MWT and showed a changed of 88 m (95% CI, 52–125 m) and 86 m (95% CI, 53–175 m) in 8 and 6 weeks of exercise training, respectively. The present study showed a change of 121.8 ± 69.1 m (~ 49,8%) in 6 weeks of intervention, but baseline values were lower than above-mentioned studies. Hermann et al. [30] and Liu et al. [21] also observed a great change in 6MWT after intervention (baseline = 241.3 ± 154.4 m, change ~ 56%; baseline = 162.7 ± 72.0 m, change ~ 31%, respectively) with post-invasive ventilation patients and elderly patients, respectively, and their baseline values were similar to the present study, however, Liu et al. [21] did not use physical exercise for their pulmonary rehabilitation. These results demonstrated that higher changed occurred in the group of patients with lower baseline 6MWT. Thus, the present study reinforces that physical exercise is a key component in the rehabilitation of post-COVID patients and their benefits are related to whole-body conditioning.
After 6 weeks intervention, resting heart rate, systolic and diastolic blood pressure in post-COVID patients presented lower values when compared to baseline. Resting heart rate declines with exercise training and it is strongly associated with higher cardiorespiratory fitness [36, 37]. In general, some mechanisms could be related to lower resting heart rate after exercise training, and in healthy subjects is due to intrinsic heart rate mechanism [36]. In addition, post-COVID patients may present dysautonomia [38], and mechanisms of heart rate control could be different in these patients, although further studies are necessary to investigate this hypothesis. Angeli et al. [39] observed that blood pressure increased during hospitalization from patients hospitalized by COVID-19, and about 45% persist with higher blood pressure when compared to admission, due a reduction of the angiotensin 1–7, a potent vasodilator, and ACE2 receptor deficiency. In turn, Cornelissen and Smart [40] observed in a meta-analysis that combined exercise training does not reduce systolic blood pressure, but it is effective to reduce diastolic blood pressure. Notwithstanding, Naci et al. [41] showed lower systolic blood pressure after exercise training, but the authors observed that the decrease in systolic blood pressure after exercise training is more pronounced in patients with higher cut-off values. These findings demonstrated that response of blood pressure after exercise intervention is similar to healthy, although the mechanisms could be different.
The percentage of patients with persistent symptoms was not significantly modified after intervention. Nevertheless, in a cohort study, Seeßle et al. [42] showed that frequency of dyspnea and fatigue were higher after 12 months from discharge by COVID when compared to 5 months after discharge, and patients reporting at least 1 symptom presented reduced exercise capacity and reduced physical quality of life. The physiopathology of Long COVID syndrome still needs to be clearly defined, however, there are cardiorespiratory, immune, neurological and hormonal involvement [43]. On the other hand, physical exercise is effective to reduce symptoms and improve quality of life in many chronic diseases [44], and in the context of COVID, exercise could be an inhibitor from spiral related to inflammation [45]. Furthermore, persistent symptoms could be related to sedentary behavior, and engaging in exercise intervention should prevent exacerbation of these symptoms after discharge from COVID.
Some limitations of the present study were: there is not a control group, and it is not possible to distinguish results from rehabilitation to spontaneous physical recovery. Because of COVID-19 is very debilitating condition, all the patients were forwarded to rehabilitation. The sample of the study was formed by severe patients, but not by disability patients, and these results may not be reliable in disability or bed rest patients. Follow-up was not evaluated, and it is not possible to extrapolate these results over time.