The present study indicated that young individuals with mild COVID-19 with preserved lung function have long-term muscle weakness, respiratory and (50% and 66.7% inspiratory and expiratory, respectively) lower extremity muscle weakness, decreased physical activity level, increased cough and perception of dyspnea and fatigue. Also, individuals who survived the disease mildly after COVID-19 had functional limitations even in the long term. The upper extremity muscle strength of patients was in normal ranges but weaker than healthy controls.

Many pathologies, such as chronic inflammation, epithelial destruction, endothelial damage, and hyaline membrane formation, affect the lungs in the long term after COVID-19. It is thought that the long-term consequences of these pathologies will result in a decrease in respiratory muscle strength and a worsening of lung functions [2, 13].

In our study, both inspiratory (50%) and expiratory (66.7%) muscle strength were found as weakened according to the long-term results of young individuals who had mildly recovered from COVID-19. In contrast to the present study, Plaza and Sevilla [21] showed preserved respiratory muscle strength in university students who had recovered from mild COVID-19 at least 6 months later; however, they showed that the inspiratory muscle strength of individuals with COVID-19 decreased compared to healthy individuals, but the expiratory muscle strength did not change. The difference between respiratory muscle strength results may be due to the female gender dominance in this study, unlike the Plaza and Sevilla study [21] because the subgroup results showed that the respiratory muscle strength of women with COVID-19 decreased more [21]. In addition, the inclusion of individuals with asymptomatic COVID-19 in the study [21] may explain the preservation of respiratory muscle strength, as it is thought to reduce the severity and effect of the disease.

In another study, Çelik et al. [35] compared the medium-term results of respiratory muscle strength of female volleyball players with and without COVID-19. Unlike this study, the inspiratory muscle strength of both groups was within normal range, but expiratory muscle strength was decreased [35]. This difference may result from the inclusion of athletes in the study of Çelik et al. [35]. Also, players with COVID-19 had lower inspiratory and expiratory muscle strength than controls [35]. These results are consistent with our study. In addition, our study showed that respiratory muscle weakness persisted in the long term. Respiratory muscle weakness, indicated by decreased MIP and MEP as in our study, may be based on the long-term effects of various factors, such as a myopathy caused by the virus in the respiratory muscles, particularly the diaphragm. In addition, there may be a possible effect of impaired physical activity during the pandemic process, depending on the quarantine conditions [36].

It was previously stated that there are abnormalities in FEV1, FVC, and small airways in the early phase of the COVID-19 disease [2]; however, adult follow-up studies suggest that spirometry results are normal or close to normal [11, 13, 36]. Therefore, the results are inconsistent, and the long-term effects of the COVID-19 disease on individuals still need to be determined. This study showed that the pulmonary function of young individuals with mild COVID-19 was within normal ranges, except for PEF (56%). In addition, pulmonary function was similar to healthy ones. In their study, Lund Berven et al. [18] stated that spirometry results were like healthy and within normal limits in adolescents and young people even in the early period. In another study, Çelik et al. [35] showed that the pulmonary function of young volleyball players with and without COVID-19 was similar in the mid-term; however, a decrease in PEF values of individuals with COVID-19 was observed, similar to our study [35]. PEF is linked to airway size, expiratory muscle strength, and lung tissue compliance [28].

Thus decreased PEF may be associated with a decrease in expiratory muscle strength, as in our study. Also, due to the iron and steel factory in the region where the study was conducted, the gas level causing air pollution is above the limits determined by WHO [37]. This situation is associated with a decrease in FEV1, FVC, and PEF values, even in healthy individuals [38]. Therefore, PEF may also be decreased due to air pollution in the healthy group. Demonstrating long-term preservation of respiratory function in our study is important for investigating the underlying mechanism of persistent respiratory symptoms [18].

The COVID-19 virus may cause restrictive and obstructive pulmonary function abnormalities in patients by affecting the small airways [39]. This may lead to deterioration in the diffusion capacity of patients [4]. A meta-analysis study showed that patients with COVID-19 who do not have severe disease had 5–23.5% restrictive and 5.5–16.7% obstructive pulmonary function abnormalities in at least a 1-month follow-up [39]. Similarly, in the present study, post-COVID-19 young adults with mild disease were found to have restrictive and obstructive pulmonary function abnormalities in 27.8% and 12.9%, respectively. In addition to the COVID-19 virus, this situation may be caused by respiratory muscle weakness in patients, affecting ventilation [10]. In addition, physical inactivity due to COVID-19 may have led to a decrease in respiratory muscle strength [36], and the regional air pollution may have caused pulmonary function abnormalities in healthy individuals [38].

Previous studies have reported that the most common long-term pulmonary sequelae experienced by non-hospitalized COVID-19 patients are fatigue, dyspnea, and cough [16, 17]. The prevalence of these symptoms varies according to the severity of the acute illness [5]. A study found that in hospitalized and non-hospitalized COVID-19 patients, 92.9% and 93.9% reported fatigue after 3 months, and 89.3% and 87% reported dyspnea, respectively. In addition, most of the COVID-19 patients who were not hospitalized continued to have a cough (68.1%) [40]. In our study, fatigue (79.6%), exertional dyspnea (50%), and cough (20.4%) symptoms were persistent in the long term in young individuals with mild COVID-19. Compared with healthy individuals, a difference was found between dyspnea scores during activity, similar to the literature [19]. Fatigue is a symptom that causes exhaustion in individuals and is caused by various factors [2]. Healthy individuals also experienced similar levels of fatigue to those with COVID-19. As a result, healthy individuals were also evaluated during the pandemic period. Studies confirmed that inactivity caused by the pandemic and quarantine also affected healthy individuals [41]. Therefore, the underlying mechanism needs to be investigated.

It is claimed that skeletal muscle may be vulnerable to the COVID-19 virus via angiotensin-converting enzyme 2. Therefore, metabolic homeostasis is impaired, and muscle loss occurs [42]. Paneroni et al. [43] showed that most COVID-19 patients had quadriceps muscle weakness at discharge. A study showed that more than one third of adult post-COVID-19 patients had mid-term handgrip and quadriceps muscle weakness. In addition, hand grip and quadriceps muscle strength were weaker in the moderate disease group than in the mild disease group [22].

This study showed that the quadriceps femoris muscle was similarly weakened in individuals with COVID-19 and healthy controls. In addition, upper extremity muscle strength was preserved in individuals with COVID-19 and weaker than in healthy controls. The decrease in peripheral muscle strength can be explained by the mechanism described above; however, various factors may have caused a decrease in lower extremity muscle strength in almost all healthy individuals. Exposure to air pollution and during the pandemic, less exercise and physical activity [36] due to the quarantine may explain peripheral muscle weakness by causing insufficient oxygen use and transport. To avoid COVID-19 and control the spread of the disease, quarantine, and social distancing have led to restrictions on ordinary daily activities and social interaction, reducing physical activity. It has drastically changed the lifestyle of the young in particular [41].

In our study, individuals with COVID-19 were more inactive in the long term than healthy individuals. Similarly, a study showed that adults who recovered from mild to moderate COVID-19 decreased physical activity in the medium term [22]. These results showed that the decrease in physical activity after COVID-19 is a long-term problem. Various strategies to increase physical activity should be applied to improve general health and reduce the effects on pulmonary function, musculoskeletal system, cardiovascular, and psychological factors in individuals after COVID-19 [44]. Recent studies have shown that enduring physical activity and exercise training improve cardiovascular fitness, respiratory parameters, and general health in post-COVID-19 [45,46,47].

This study has limitations. First of all, post-COVID-19 patients were recruited from a single center. Another limitation is the lack of data on healthy controls because all controls were evaluated during a pandemic and those may all possibly be affected during a pandemic. In addition, the assessment of exercise capacity and physical activity with objective methods, such as pedometers and accelerometers would provide more detailed and accurate information.