Now that you’ve had time to assimilate the introduction about The Ignored Risks of Physical Inactivity and Covid-19 (you thought that was it?), let’s get deeper into what brought you here in the first place.
Physical inactivity and muscle mass
The preservation of muscle mass requires a constant supply of mechanical stimuli that directly or indirectly stimulate protein synthesis80. When we stop training, these essential stimuli required for muscle anabolism are removed and the balance between protein synthesis and protein degradation is broken down to degradation. Within a few days, objective signs of muscle atrophy can be found. In fact, significant losses of the quadriceps have been reported after only 2 days of leg immobilization (1.7%) 81 or 5 days of bed rest (2%) 82, which is associated with an even greater loss of muscle strength (8-9%) 82-84. During the following days and weeks, muscle atrophy is able to progress at an inexorable rate, 6% after 10 days 78, 10% after 29 days 85, 13% after 60 days 82, reaching 18% after 90 days 86. This rate of muscle atrophy grows exponentially, predicting a loss of muscle mass of approx. 8% in 30 days and ∼15% in 60 days.
Figure 8: Complications of loss of lean body mass (muscle)87
A recent survey conducted on the impact of sedentarism in 6733 people aged 18 to 98 years showed a clear association between physical activity, lean mass (muscle) and body fat 88. In addition, when comparing the muscle mass and function of sedentary people aged 20-80 with their counterparts in an athletic population, it is clear that maintaining a high level of physical activity preserves muscle mass and function throughout life.89
This benefit translates into a gain of approximately 20 to 25 years in terms of biological age when comparing the muscle mass and performance of 3-age vs. sedentary athletes 89.90. Similarly, people trained over a lifetime show 30% more muscle strength compared to sedentary people of the same age 91. Surprisingly, the benefits of leading an active lifestyle protect not only against loss of muscle mass and strength, but also appear to protect against the progressive muscle denervation that accompanies the aging process and is exacerbated by inactivity 92
Figure 9: Nuclear magnetic resonance showing differences in subcutaneous fat tissue between a 74-year-old sedentary man versus a 70-year-old triathlete93
Muscle loss associated with aging, disuse and certain pathological conditions can lead to adverse health and quality of life deterioration. The loss of muscle mass, strength and function has not only local but also systemic adverse consequences that lead to physical, functional and congenital disability.87
Physical Inactivity and Metabolic Control
The current situation will reduce activity to levels well below the daily recommendation of 7500-10,000 steps per day, exacerbating the health problems resulting from physical inactivity 55.94. It is important to note that the negative health effects can be seen relatively quickly (3-14 days) when the decline in activity is accentuated, as will be the case worldwide in the current pandemic.95
Additional evidence reports that 2 weeks of reduced physical activity (from > 3500 to <1500 steps/day) in healthy older people (> 65 years and typically the most inactive proportion of the population) induced a small increase but measurable in insulin resistance and a reduction in the postprandial rate of muscle protein synthesis.96
Sedentary activities such as desk work, watching television, sitting are associated with increased mortality from all causes and increased morbidity (metabolic syndrome, cardiovascular disease) 97-99. The association is summarised in a recent review which concluded: “Higher levels of total physical activity, at any intensity, and shorter time spent sedentary are associated with a substantially reduced risk of premature mortality, with evidence of a non-linear dose-response pattern in middle-aged adults and older 100″.
The mechanisms through which inactivity results in decreased systemic insulin sensitivity and glucose intolerance are closely related to modifications within skeletal muscle 101-104.
Figure 10: Being habitually active promotes the activation of metabolic pathways related to glucose uptake in skeletal muscle; thus, insulin sensitivity is preserved and less glucose is diverted to metabolic unfavorable stores 105.
Evidence suggests that 7-10 days of bed rest in healthy individuals leads to a 10-34% decrease in insulin sensitivity throughout the body 102,106. However, decreased insulin sensitivity measured by the arteriovenous balance in the forearm 102 or leg 107 shows much greater results (47. -75%). This difference is due to the fact that the forearm and leg consist mainly of skeletal muscle, emphasizing the fundamental role of muscle in inactivity-induced insulin resistance, which appears to be attributable to reduced muscle contraction per se108.
Decreased insulin sensitivity with physical inactivity is not directly related to changes in body composition (loss of muscle mass, increased percentage of body fat) as this develops rapidly (within a few days) and well before muscle atrophy and/or increased body fat (or ectopic fat deposition) is established 109.
In times of restrictions due to the COVID19 pandemic, it is important to realize that a moderate amount of daily exercise of sufficient intensity is necessary 110.
Physical Inactivity and Cardiorespiratory Impact
VO2max is the maximum capacity to take oxygen from the environment in the lungs, transport it in the blood and use it by the body, mainly in the muscles, and therefore it is considered a variable that evaluates the maximum performance of the cardiorespiratory system and the skeletal muscles in the transport and use of O2111. Besides being one of the main determinants of exercise tolerance, VO2max is considered an index of “cardiorespiratory fitness”. In both healthy subjects and patients with cardiovascular disease, “exercise capacity is one of the strongest predictors of mortality, even more so than other established risk factors for cardiovascular disease “112. According to the same authors, for every 1 MET, the decrease in mortality due to cardiorespiratory fitness increases by 12% 112.
Saltin et al, one of the fathers of exercise physiology, in 1968, showed that individuals lose an average of 28% of maximum oxygen consumption (VO2max) and 11% of cardiac volume after a period of 20 days in bed113 .
Saltin et al, one of the fathers of exercise physiology, in 1968, showed that individuals lose an average of 28% of maximum oxygen consumption (VO2max) and 11% of heart volume after a period of 20 days in bed113 .
In line with these data, a recent study “anticipated” in much the same way the context to which hundreds of millions of people worldwide are now exposed as a result of confinement to the home. In that study, a group of young, healthy men sharply reduced the number of steps per day, from a baseline of ∼10,000 to ∼1350, and maintained this level of activity for 2 weeks.114 The effect, devastating, after 2 weeks, subjects had a ~ 7% decrease in VO2max114. Interestingly, the rate of decrease in VO2max reported was remarkably similar to the average rate of decrease in VO2max observed in the bed rest studies115 . If we assume that the rate of decrease in VO2max is also linear after forced inactivity not associated with bed rest (like the confinement by COVID-19), over a period of 2 months, the VO2max would decrease by approximately 30%, a not very real and speculative number but enough to try to prevent the worst possible scenario from becoming a reality.
As for older subjects, the percentage decreased of VO2max during 2 weeks of bed rest was twice as high in subjects aged 60 (-15%) compared to that observed in younger controls116 . Furthermore, during a 2-week post-bed rest period, young subjects would appear to recover the baseline VO2max, while the elderly recovery is less or even incomplete 116 complicating the picture for this population.
Therefore, in a hypothetical 70-year-old sedentary subject with a VO2max of ∼25 ml/kg/min-1, 4 weeks of forced inactivity would probably translate into a decrease of ∼15% in VO2max, corresponding to a decrease of ∼3.75 ml/kg/min-1, which is equivalent to ∼1 METs: Which, in turn, would translate into an increase of ∼12% in mortality.
Figure 11: Physical exercise improves the health of older people by acting on different systems and organs117.
Finally, there would appear to be a direct dose-response relationship between the “volume” of exercise (duration x intensity) and cardiorespiratory fitness. Approximately 50% of the protective effects of physical activity are explained by a reduction in traditional cardiovascular risk factors, such as high blood pressure and blood lipids55. Other protective effects are presumably related to a decrease in low-grade inflammation of visceral fat tissue and a decrease in insulin resistance55.
The evidence that exercise is critical to preserve health and quality of life is indisputable as is the damaging impact of the COVID-19 pandemic on sedentary and inactive people 117,118. It is important to note that we may also be at risk of a vicious cycle where current and potentially accelerated patterns of inactivity may magnify the impact of current and future pandemics. Not surprisingly, people infected with COVID-19 are much more likely to be hospitalized and have a worse prognosis if they have underlying medical conditions, such as chronic diseases, that affect the immune response. In addition, the evidence associating with a significantly increased risk of chronic disease in physically inactive people is strongly evidenced6,8,117,118. Ergo, the relationship between physical inactivity and risks of health complications and mortality rates associated with 19-COVID can no longer be ignored. If the prevalence of chronic conditions caused by unhealthy lifestyles were lower, would the catastrophic effects of the COVID-2019 pandemic be reduced?
Figure 12: Physical inactivity and sedentary behavior can be detrimental to health, the cardiovascular risk profile, physical capacity and function, and mental health, resulting in a poor quality of life, while maintaining more optimal levels of physical activity can help improve these detrimental effects.119
Therefore, during quarantine, being physically active is essential for mental and physical health. Fortunately, a wide range of exercises, such as aerobic exercise or strength training with or without equipment, taking advantage of new technologies that allow for guidance by appropriate professionals through video or application, can be performed in the home and should be encouraged. Along these lines, we encourage national, federal and regional governments around the world to include exceptions for physical activity in activity closures at the national level. These should also allow for outdoor physical activities (e.g., walking, running, or other individual sports), and thus prevent the COVID-19 pandemic from generating unfavorable consequences beyond the virus120.
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