In December of 2019, there was an outbreak of a severe acute respiratory syndrome caused by a new coronavirus (SARS-CoV-2). The virus was first noticed in China and rapidly spread across the country and then across the world [1]. As a consequence, many authorities imposed extreme measures such as quarantines, social distancing, and isolation [2]. Measures included banning sports competition and closing exercise facilities, such as health clubs, gyms, and sport courts ^[3][4][3,4]. These restrictions were accompanied by a reduction in mobility due to public transportation and gathering restrictions, working from home adoption, and school closures. These measures had a negative impact on physical activity levels [5] and decreased the involvement with muscle strengthening exercises [6], which might induce muscle atrophy, loss of muscle strength, and reductions in neuro and mechanical abilities ^[7][8][7,8].

Even when exercise facilities were reopened, many regulations were sustained, such as social distancing, limited gathering, the use of protective masks, and hygiene measures ^[9][10][9,10], which, along with the fear of contamination, might preclude regular exercise performance. Therefore, it is important to propose solutions to stimulate physical activity performance, especially considering that physical inactivity ^{[11][12][13][14][15]}[11,12,13,14,15] and low physical capacity ^[16][17][18][16,17,18] are associated with worse outcomes and an increased mortality risk in cases of infection ^[19][20][21][19,20,21].

Moreover, although COVID-19 is commonly associated with the respiratory system, it is a multisystem disease [22]. Coronaviruses may also induce neurological damage by invading the central nervous system and result in severe muscle pain [23] and loss of muscle strength [24]. Although COVID-19 has relevant morbidity for up to 6 months [25], COVID-19 survivors might develop psychological, physical and cognitive impairments that require rehabilitation and medical care for more than 12 months ^[26][27][26,27]. Among the secondary consequences of the disease and its treatment, physical function is unlikely to recover to normal values spontaneously, even under nutritional and physical exercise counselling [28]. Therefore, specific prescriptions are needed. Considering physical exercise models that might help to prevent and treat the different consequences of COVID-19, resistance training (RT) might be particularly relevant. RT has been consistently used to increase muscle mass and strength in many different populations, being considered an essential part of a physical exercise program aiming to improve or restore physical functioning ^[29][30][31][29,30,31]. Its benefits, largely mediated by strength gains, culminate in reductions in mortality rates in different populations ^{[32][33][34][35][36]}[32,33,34,35,36].

2. Before: Considerations for Preventing COVID-19 Complications

Although it is not possible to attribute a direct cause–effect relationship between RT practice and the mortality risk during the COVID-19 pandemic, current evidence suggests that it might be important to perform RT to improve general health and promote a better prognosis in cases of contamination ^{[11][12][13][14][15][16][17][18][37]}[11,12,13,14,15,16,17,18,37]. Physical inactivity ^{[11][12][13][14][15]}[11,12,13,14,15] and low muscle strength have been associated with an increased risk of hospitalization and death ^{[16][17][18][37]}[16,17,18,37]. The importance of muscle strength should not be underestimated, since it may explain the protective effect of physical activity against COVID-19 hospitalization [17]. Moreover, RT can modulate important risk factors associated with increased morbidity and mortality due to COVID-19, such as high blood glucose, arterial hypertension, obesity, and dyslipidemia ^{[38][39][40][41][42]}[38,39,40,41,42]. In this regard, previous evidence shows that RT can help to control blood pressure ^[43][44][43,44], blood glucose [45], body weight [46], and blood lipids [47]; therefore, it can mitigate complications in cases of contamination. Another possible benefit of RT is its impact on the immune system. Physical exercise has been consistently shown to modulate immune function ^{[48][49][50][51]}[48,49,50,51]. Higher levels of physical activity ^[52][53][54][52,53,54] and fitness [54] decrease the risk of respiratory symptoms and illness. In this regard, people who carry out strength and power activities ^{[55][56][57][58]}[55,56,57,58] usually have a better immunological profile than people who perform long-duration aerobic activities [59], which might be a positive point for RT ^[60][61][60,61]. Strategies for RT prescription for improving or maintaining immune function involve using a low exercise volume (4–6 exercises, with 1–2 sets per exercise), avoiding metabolic stress (perform ≤ 6 repetitions and ≥2 min of rest between sets and exercises) and preferring exercising during the afternoon/evening [60]. If one decides to avoid exercise facilities, RT can be adapted to be performed in many different situations, even with limited space and equipment, and it can easily be adapted to an individual’s characteristics and health status [61]. For example, previous studies have shown that bodyweight exercises ^[62][63][64][66,67,68], stationary bike training ^[65][69], plyometric training ^[66][70], elastic band training ^[67][68][69][71,72,73], and even exercises with no external load ^[70][71][72][74,75,76] promote similar responses to traditional RT. These exercises might be performed as basic multi-joint exercises (i.e., squats, pushups, pullups, rows, etc.) as this has been shown to be sufficient to promote gains in muscle strength and size in most muscles involved ^{[73][74][75][76][77]}[77,78,79,80,81]; the addition of isolated exercises, in general, does not seem to bring benefits ^[76][78][79][80,82,83]. This allows the possibility to exercise at parks, outdoors, and even at home, and still obtain relevant results ^[80][81][63,84].

3. During: Resistance Training for People with COVID-19

COVID-19 involves an inflammatory response that affects different systems, including the neuromuscular system ^[82][83][84][99,100,101]. Its effects on muscle strength can be detected even in the absence of symptoms, with a strength loss of as much as 30% in 2 weeks of asymptomatic contamination [24]. COVID-19 patients under intensive care can lose 30% of the rectus femoris muscle cross-sectional area in the first 10 days ^[83][100], and 44% of them still have severely limited function for up to one month after weaning ^[85][102]. Previous studies have shown that reduced muscle strength is associated with physical inactivity in pulmonary patients ^[86][103] and is an important predictor of morbidity and mortality independent of the degree of respiratory limitation ^[87][104]. In agreement with this, muscle strength and mass are predictors of length of stay in patients with moderate to severe COVID-19 ^[88][105]. Consequently, it seems important to adopt strategies to maintain or increase muscle strength for all, from asymptomatic patients to those under intensive care, since exercise training during hospitalization due to acute respiratory conditions seems to be well tolerated and have infrequent adverse events ^[89][90][91][106,107,108]. In line with this, previous studies have suggested that rehabilitation programs starting within 30 days seem to bring the most benefits, as early exercise prevents neuromuscular complications; improves functional status in critical illness; and is considered effective, safe, and feasible ^[27][92][27,109]. In this regard, RT has been shown to promote benefits during pulmonary rehabilitation due to improvement in functional capacity, either performed alone or combined with aerobic training ^[93][94][95][110,111,112]. RT can be successfully performed as a stand-alone exercise strategy without increasing adverse events in chronic obstructive pulmonary disease patients under pulmonary rehabilitation ^[95][112]. COVID-19 pathogenesis involves a delayed anti-viral response, which is followed by an excessive proinflammatory state ^[96][114]. The systemic inflammation is associated with disease severity, as shown by the higher serum levels of proinflammatory cytokine in the most affected patients ^[97][115]. This grants importance to interventions with anti-inflammatory properties, which is the case for RT ^{[98][99][100]}[116,117,118].

4. After: Resistance Training after COVID-19 Treatment

Respiratory diseases are associated with impairments in muscle function and the loss of lean body mass ^[101][102][125,126]. Survivors of severe acute respiratory diseases (SARS) might present a functional disability for as long as one year after discharge ^[103][104][127,128], and muscle wasting and weakness are frequent extrapulmonary conditions ^[104][128]. The manifestations include limb muscle weakness, muscle atrophy, and impairments in deep tendon reflexes ^[105][129]. COVID-19 patients show muscular disfunction similar to that observed in chronic obstructive pulmonary disease and IHD patients ^[106][130]. More than 80% of hospitalized COVID-19 patients, all without previous disability, showed reduced quadriceps muscle strength at discharge ^[107][131]. Studies carried out in COVID-19 patients who recovered from mild and moderate disease showed handgrip and quadricep weakness in 39.6% and 35.4% of the participants 12 weeks after discharge, respectively ^[108][132]. This might persist for a longer time, since persistent pulmonary function was impaired in up to 37% of the patients who suffered from SARS one year after discharge; their health status was also significantly decreased in comparison with healthy subjects ^[109][110][133,134], and exercise capacity was also remarkably lower than those found in the normal population for many months ^[110][134]. Moreover, patients admitted to intensive care units commonly present persistent dyspnea, anxiety, depression, impaired physical function, and a poor quality of life for up to 12 months after discharge ^{[111][112][113]}[135,136,137]. Among these, physical function is one of the least likely to recover to normal values over the long term [28]. The high prevalence of impairment in skeletal muscle strength and physical performance in patients recovering from COVID-19 suggests the need for rehabilitation programs after discharge. However, reduced muscle strength has been identified six months after discharge in one in six COVID-19 survivors, even when they were admitted to post-care facilities or received dietary counseling, physical activity guidance, or physiotherapy assistance ^[114][139]. Therefore, it is important to adopt specific strategies aimed at increasing muscle strength, such as working at adequate intensities and using blood flow restriction ^[115][140]. It is important to consider possible risk factors when prescribing RT, as COVID-19 might be associated with cardiac complications that persist after discharge, especially arrhythmias, heart failure, myocardial injury, and increased risk of thromboembolism ^{[116][117][118][119]}[144,145,146,147]. Nevertheless, RT has been shown to be safe and effective for several cardiac patients and has been recommended as a core component of cardiac rehabilitation for many decades ^{[120][121][122]}[148,149,150]. However, it is important to consider proper program design to avoid complications, such as working with a lower number of repetitions, increasing rest intervals and reducing training volume [60].