The WHO recommends that adults aged 18 years and older should accumulate at least 150 minutes of moderate-intensity aerobic activity, 75 minutes of vigorous-intensity aerobic activity, or a combination of both per week.⁴ People who do not meet these requirements are considered sedentary. The health benefits of physical exercise include lower risks of cardiovascular disease, hypertension, diabetes, obesity, dementia and breast and colon cancer.^{5 - 9} Despite all these benefits, sedentarism is a pandemic, affecting 27.5% of people globally.¹⁰

Sedentarism increases the risk of the world´s major non-communicable diseases. I-Min Lee et al.,¹¹ estimated that physical inactivity is responsible for 6% of the burden of coronary artery disease, 7% of type 2 diabetes, 10% of breast cancer and colon cancers. Thus, inactivity is considered responsible for 9% of premature mortality, globally. Actually, sedentarism kills more people annually than the COVID-19 infection.

Yang et al.,¹² described that, in Chinese patients infected with COVID-19, hypertension was the most prevalent comorbidity (17 ± 7%), followed by diabetes (8 ± 6%) and cardiovascular diseases (5 ± 4%). Another meta-analysis confirmed that these comorbidities were associated with poor prognosis. Data about exercise habits in patients infected by COVID-19 is yet not available. Nevertheless, it is clear that these comorbidities would be less prevalent if minimal WHO´s exercise requirements were globally met.⁴

Exercise training is considered effective in primary prevention and in the treatment of most chronic diseases,^{11 , 13 - 16} especially the most prevalent ones, that confers worst prognosis in COVID infections. Thus, physical training itself may prevent deaths during this pandemic.

Physical training is also known to improve immune response. Despite the concerns of some athletes about the side effects of vaccination, high exercise intensity and frequency enhance vaccine-responses in elite athletes.¹⁷ This is also true in older people where immunoglobulins concentrations after vaccinations are greater in cardiovascular-trained individuals than in control ones.^{17 - 22}

Despite the promising effects of exercise training on the immune system, exercise stress may be associated with an increased risk for upper respiratory tract infection.²³ Acute bouts of heavy exercise and chronic intensive exercise, as usually performed by long-distance runners, compromise host defenses and increase the incidence of upper respiratory tract infections.^{24 , 25} In athletes under heavy training both innate and acquired immunity are often observed to decrease, typically 15-25%.²⁶

It is true that exercise can modulate many immune system components, altering the susceptibility to infections, which means that exercise training may increase or reduce the susceptibility to infections. Although it may seem paradoxical, this assertive is true, and quite straightforward. A large study²⁷ about the Hong Kong flu outbreak in 1998 concluded that mild to moderate exercise, performed three to five times per week reduced the risk of mortality, while people who did not exercise or who exercised too much were at greatest risk of death. Thus, the missing piece of the survival puzzle during pandemic is not exercise itself, but its detailed prescription.

Physicians learn how to prescribe various drugs. Although exercise is beneficial for the treatment of several diseases, exercise prescription is not taught to medicine students or residents.^{28 , 29} As any other medication, exercise training needs the right dosing to achieve the desired effects. The American College of Sports Medicine has introduced the FITT-VP principle as a mnemonic with all the points that must me described in exercise prescription: Frequency, Intensity, Type, Time, Volume and Progression.⁶

Social restrictions, correctly imposed during the COVID-19 pandemic, is certainly disrupting routine daily activities of people globally.³⁰ Still, there are exercises that can be safely done at home. Considering the previous discussion on the minimal training volume⁴ for health, recommended by the WHO, and the embedded risks of immunity reduction due to high intensity training, the following principles can guide home-based exercise while at social isolation.

Government measures that restrict people at home during the COVID-19 pandemic do not need to encourage the wider spread of the sedentarism pandemic. Exercise can fight both public health problems, as long as it is adequately prescribed. Maintaining regular physical activity at home is pivotal for healthy living during and after the COVID-19 global crisis.