This study showed that an important proportion of individuals with long-Covid have decreased HGS. In addition, dynapenia was associated with worse lung function test scores and lower respiratory muscle strength, which ultimately had a negative impact on the distance walked on the 6MWT after 120 days from hospital discharge. Despite a significant and expected higher age in the dynapenic group, the HGS was associated with lower FEV1, MEP, and 6MWT, even when controlling for age. HGS was correlated significantly with distance walked on the 6MWT and with worse pulmonary function in these individuals.
After 120 days, 22% of the participants still had a loss of muscle strength, which was reflected in decreased muscle functioning. The skeletal muscle’s ability to produce strength (i.e., maximal voluntary force) and power (strength vs speed) affects health outcomes and is associated with increased disability, becoming an important component of health and disease for all ages. The consequences of reduced grip strength are increased risk for physical disability (20, 21), poor physical performance and death from cardiovascular and non-cardiovascular causes, and stroke (22–24).
The current literature shows that most of the symptoms of long-Covid occur in individuals who have been hospitalized, but also in non-hospitalized individuals, or severe and non-severe COVID-19 (25), with the more severe individuals having worse outcomes six months after hospital discharge. Given the number of individuals who recovered from acute COVID-19, regardless of the acute disease phenotype, long-Covid is a problem that is far from being fully grasped. Most sequelae seem to resolve within six months (26), but in some individuals, it may persist for years (27). The individuals from this study demonstrated a worrying functional outcome after four months of acute disease, which probably limit their day-to-day activities. Further studies are needed to see if these alterations persist for longer periods.
A significant proportion of individuals with diabetes, previous smoking habits, lower BMI, older age, and in need for ICU and IMV at hospital admission presented reduced grip strength 120 days after hospital discharge. It is important to emphasize that a 1% loss of lean mass is equivalent to a 3% reduction in muscle strength in the elderly, which may have an impact on physical performance and function (30). To what extent does the interplay between hospitalization (disuse, hypoxaemia, malnutrition, sedation, pharmacological therapy, etc.), past medical history and lifestyle habits, and inflammation and immune activation from SARS-CoV-2 acute infection induces muscle wasting, further reducing muscle function acutely and chronically are not completely understood and merit future investigation.
Studies on respiratory muscle have also shown negative outcomes. Formenti et al. (31) evidenced significantly reduced echogenicity in right and left intercostal and diaphragm muscles at 24h at ICU in deceased COVID-19 patients in comparison to those who survived, indicating loss of muscle mass (31). Regmi and cols. reported persistence of diaphragm muscle weakness 15 months after hospitalization for Covid-19 (32). In this study spirometry parameters, RMS, and 6MWT were all reduced among those with reduced grip-strength several months after hospital discharge, corroborating these previous findings.
Furthermore, chronic fatigue, post-exertional malaise, exercise intolerance, and other functional abnormalities increasingly observed in long-Covid remain poorly understood. A study on the histopathological changes in 16 long-Covid individuals presenting fatigue more than a year after the acute infection showed muscle fiber atrophy and regeneration in 38% and 56%, respectively (33). Also, 62% of these individuals presented loss of cytochrome c oxidase activity and other alterations compatible with mitochondrial changes, together with T lymphocytes and/or muscle fiber human leukocyte antigen ABC expression and capillary basal lamina injury, which, according to the authors, may be involved in reduced energy supply and fatigue (33).
Research on causes for long-Covid has narrowed some hypotheses for such conditions, which include the persistence of SARS-CoV-2 reservoirs, presence of microthrombi, induction, and persistence of autoantibodies, hyperreactive immune activation, reactivation of other latent viral coinfections such as Epstein-Barr Virus (EBV), mitochondrial dysfunction, and gut dysfunction/dysbiosis, that may act individually or collectively, producing this syndrome (34).
Recently, it has been reported that acute inflammation, associated with COVID-19, is a potent detrimental stimulus for the development of reduced muscle strength and mass (35, 36). Of the various deleterious influences of inflammation, elevated c-reactive protein, and interleukin-6 have been strongly correlated with reduced muscle strength and frailty (35, 36). We found that patients with long-Covid with dynapenia had reduced muscle mass, which was reflected in the grip strength, exercise capacity by the 6MWT, and respiratory muscle strength. Therefore, reduced muscle mass and the ability to produce strength, as seen in the dynapenic group, reflected on muscle function systemically, as seen in the reduced RMS and spirometry tests, which may reflect a multifactorial origin to decreased muscle functional features globally, highlighting the possible multifactorial aspect of chronic disability in this population. One of the hypotheses is that these patients present dynapenia at hospitalization and chronically due to an imbalance between muscle protein synthesis and muscle protein breakdown maintained during the follow-up period of 120 days.
The HGS was associated with respiratory function, respiratory muscle pressure, and physical capacity measured in the 6MWT in this study. Several studies have demonstrated the association between the severity of acute COVID-19 and functional outcomes, such as 6MWT, lung diffusion capacity, and spirometry months later (37, 38), which are interesting tools for assessing the detrimental consequences of this disease (39). Nonetheless, the association between HGS and these parameters has important practical implications in long-Covid diagnosis and care once a great number of individuals do not have access to complex functional assessments and remain without proper functional diagnosis and quantitative assessments for exercise prescription on an individual basis and, therefore, being withheld of efficient rehabilitation. Additionally, it would be of great importance to establish cut-off points for such tests, enabling healthcare practitioners to better assess and manage this population, as occurs in other diseases, since HGS is quick, safe, and cheaper (17).
Early evidence of corticosteroid therapy for COVID-19 showed positive effects on mortality outcomes(40), but optimal dosing is still under research(41). However, indistinct use of corticosteroids, acute or chronically, may lead to several adverse events, one of them being myopathy, which could be difficult to distinguish from COVID-19‐related critical illness neuropathy and myopathy (42), or post-ICU syndrome and ICU-acquired weakness, which are well-recognized conditions (43). Corticosteroid therapy is a common cause of non-inflammatory myopathy, ultimately causing muscle weakness, which may last from weeks to months if use is persistent (44). Also, in humans, corticosteroids are known to affect both respiratory and limb muscles, causing loss of muscle cross-sectional area, inducing weakness, and wasting, and ultimately leading to loss of function (44). Nonetheless, these factors altogether may likely explain to some extent acute weakness and fatigue in severe and hospitalized COVID-19 individuals. Their association with persistent long-term outcomes needs further research.
The present study demonstrates the association of a simple tool, such as HGS, with other functional outcomes that demand more trained personnel and robust equipment, such as spirometry and 6MWT, in individuals who had severe COVID-19 in the early 2020s, when the predominant SARS-CoV-2 lineages were B.1.195, B1.1.33 and B.1.1.28, before P.1, Delta and Omicron (28). Also, these individuals were not vaccinated, which likely worsened acute disease and long-term outcomes, as increasing evidence shows the association between prior vaccination and the reduction of long-Covid (29). Nonetheless, the individual characteristics of the population of this study represent an important proportion of the population lingering with symptoms from the COVID-19 infection back then and still suffering from long-Covid.
This study has some limitations. The sample is relatively small and there is a short-term follow-up. There was no assessment of HGS and other functional outcomes at hospitalization, or at hospital discharge, which impair longitudinal comparisons between these variables. This is a single-center study with individuals infected by SARS-CoV-2 in the early 2020s, so the results presented here may not directly be extrapolated to the long-term health functional outcomes of those individuals infected with more recent variants of the virus.