This systematic review includes five randomized clinical trials that meet the inclusion and exclusion criteria outlined earlier, aiming to evaluate the effectiveness of pulmonary rehabilitation programmes and/or respiratory muscle training in patients with post-COVID conditions.

In the first study [13], four variables of relevance to our study were identified (6-minute walk test distance, dyspnea, fatigue, and PImax). In the first three variables, both groups improved after treatment, but in the 6-MWTD distance and fatigue, there was a statistically significant difference in favour of the intervention group. Regarding dyspnea, the improvement was also greater in the intervention group. As for PImax, significant improvement was only identified in the intervention group [13]. Hence, it could be concluded that while both groups showed improvement in most of the examined variables, the treatment combining diaphragm release with inspiratory muscle training was more effective than the treatment consisting solely of inspiratory muscle training [13].

In the second of the articles included in this review [14], two important variables for our study were assessed (6-MWTD and dyspnea). Regarding the first variable, there was improvement in both groups without a significant difference. However, concerning dyspnea, although there was improvement in both groups, the control group showed a greater improvement. Although not as clear in this case, it could be argued that the traditional pulmonary rehabilitation programme (including cycle ergometer exercises, breathing exercises, general fitness exercises, resistance training, and relaxation) was more effective than pulmonary rehabilitation using virtual reality [14].

The third study [15], examined two variables, the 6-MWTD and fatigue. Both variables improved after treatment in both groups, with group B showing better results in the 6-MWTD. Regarding fatigue, group B also demonstrated a statistically significant improvement compared to group A [15]. Taking into account these variables, it could be said that the group receiving the traditional physiotherapy programme (aerobic exercise, muscle strengthening exercises, and respiratory exercises) along with the Active Breathing Cycle technique (based on a cycle for controlling breathing, including chest expansion exercises and forced expiration techniques to clear bronchial secretions and promote increased lung volume) was more effective than the group that only received the traditional physiotherapy programme [15].

The fourth article [16], identified four relevant variables for this study (dyspnea, fatigue, lung function, and quality of life). In the last three variables, the intervention group experienced a significant improvement. Regarding dyspnea, both groups benefited, with the intervention group showing a more pronounced improvement [16]. Hence, it is evident that a multicomponent exercise programme (combining resistance training with aerobic training) was more effective than following the WHO guidelines alone [16].

To conclude with, the last study [17], assessed four variables dyspnea, fatigue, lung function, and quality of life. The first two showed significant positive changes in two of the groups (CT and CTRM). Regarding lung function, both of the aforementioned groups did not exhibit a significant overall improvement, but significant individual improvements were identified. Lastly, quality of life did not show statistically significant changes in any of the four study groups. Therefore, it is clear that a treatment that includes a multicomponent exercise programme or this programme combined with inspiratory muscle training was more effective than inspiratory muscle training alone or following WHO recommendations [17].

In conclusion, considering the results obtained from the various included studies, it becomes evident that the most effective treatment approach involves combining a personalized and supervised pulmonary rehabilitation programme (aerobic training and strength training) along with inspiratory muscle exercises, as separately they have not achieved such significant results.

Other systematic reviews are in line with the results obtained in this study [19].

Rehabilitation programmes consisting of aerobic exercise, anaerobic exercise, and respiratory training could be the key treatment to alleviate post-COVID symptoms such as fatigue, dyspnea, reduced respiratory function, physical condition, and quality of life [19]. A prospective study [20], assessed the effects of a treatment programme comprising interval training, muscle strength exercises, and individualized respiratory exercises in 39 individuals with post-COVID sequelae. The study concluded that a personalized treatment programme containing the aforementioned elements demonstrated positive effects on dyspnea, aerobic endurance, and cardiorespiratory performance [20]. Furthermore, it is important to highlight that after the two-year follow-up, a reduction in dyspnea was achieved in all participants in the study. At the two-year mark, none of the participants exhibited any pre- or post-treatment side effects or adverse effects [20]. In an observational cohort study, 58 patients with respiratory sequelae underwent a 6-week individualized rehabilitation programme, which included resistance training, strength training, and inspiratory muscle training. The study supported that a comprehensive and personalized rehabilitation programme improved the fatigue and functional limitations experienced by the participants [21]. Another systematic review, which included 20 articles, also concluded that aerobic training, along with muscle strengthening exercises and inspiratory muscle training techniques, could be an effective treatment option for patients with post-COVID symptoms [22].

Firstly, regarding the dyspnea variable, of the 4 included studies that assess this variable, 3 measure dyspnea using the Modified Medical Research Council scale (mMRC) [13, 16, 17]. The other study measured this variable using the Borg scales [14]. Although there is no clear guideline on which scale to use for patients with post-COVID conditions, most studies utilise these two scales. Another article that was found also used the Borg scales [23], but a greater number of studies employ the mMRC scale [20, 24,25,26,27].

Regarding fatigue, of the 4 articles that examine this variable, three of them use the Fatigue Severity Scale (FSS) [13, 16, 17]. The other article used the Fatigue Assessment Scale (FAS) [15]. Due to the lack of consensus on a specific scale for assessing fatigue in post-COVID patients, there is a variety of scales chosen by different studies to measure this variable. Two of the studies found use the FAS [21, 28], In contrast, another study uses the FSS [29]. However, another study found uses a different scale than the ones mentioned previously (FACIT-Fatigue) [30].

Regarding quality of life, the two articles that studied this variable used the 12-item Short Form Survey (SF-12) [16, 17]. There is also no consensus on which quality of life scale is most suitable to use in this population. Each article employs different scales. For instance, one study also uses the SF-12 [31]. However, other studies use various questionnaires, for example, the Short-Form 36 Questionnaire (SF-36) [20], EuroQol visual analogue scale (VAS) [32], the EuroQol Group 5-dimension 5-level (EQ-5D-5 L) questionnaire [21], the Euro-QoL-5D (EQ-5D) questionnaire [33].

One thing that is clear is that a significant percentage of articles studying different treatments in this population use the 6-MWTD to assess physical capacity [20, 21, 23, 33,34,35,36,37,38,39].

Taking into account the aforementioned, we can observe that there is no clear consensus regarding which scales or tests to use for the different variables evaluated in patients with post-COVID conditions, although there are some that are more commonly used than others.

As the main limitation of the article, there was a limited number of clinical trials that met the inclusion and exclusion criteria. Many of the articles found were excluded because they were conducted remotely or through virtual reality. Therefore, further research is needed in individuals with post-COVID condition to evaluate the effectiveness of an in-person, individualized program that includes both aerobic and muscular training, as well as inspiratory muscle training.