A review of 31 published studies found that the COVID-19 epidemic has forced many people to stop exercising, which will increase their risk for several diseases that can shorten their lives (Int J Environ Res Publ Health, 2020;17:6265). COVID-19 is caught mainly by breathing indoor air where people congregate, so the epidemic stopped many people from going to gyms or practicing sports indoors (Front Psychol, October 29, 2020;11:590172).

An earlier review of 103 studies found that you lose significant strength within a few weeks after you stop lifting weights. It will take you longer to recover your strength the older you are, the longer you remain inactive, and the less fit you were before you stopped exercising (Scand J Med Sci Sports, 2013 Jun;23(3):e140-9). Even short periods of inactivity can cause dramatic loss of muscle size and strength. After two weeks of stopping exercise, you can lose 5-10 percent of your exercising muscle size (Int J Biochem Cell Biol, 2013 Oct;45(10):2200-8). After just two weeks of having one leg put in a cast, people lost a tremendous amount in all measures of physical fitness, strength and muscle size in the immobilized leg. After six weeks of pedaling a bicycle for rehabilitation, they still did not regain all of the strength that they had lost (Journal of Rehabilitation Medicine, June 2015;47(6)).

Another study showed that a person loses significant strength after stopping exercise for just four days (Exp Gerontol, (2013); 48: 154–161). In that time, you can lose two pounds of muscle or more (Nutr Rev, 2013 Apr;71(4):195-208). Your muscle size can decrease by about 11 percent within ten days after you stop exercising, even when you aren’t bedridden (Scand J Med Sci Sports, 2011 Apr;21(2):215-23). When you take a break from training, most of the loss of muscle weight is due to water loss and glycogen depletion, and this loss can cause your muscles to decrease in size by up to 20 percent in a week (Med Sci Sports Exerc, 1985 Jun;17(3):339-43).

People with the largest muscles to begin with are the ones who will lose the most strength when they stop exercising, are ill, or take a vacation. Younger people lost 30 percent of muscle strength and older people lost 25 percent. The more muscle mass you have, the more you will lose. Twelve days after you stop exercising, you suffer a drop in your ability to get through a workout due to a drop in your VO2 max, your maximal ability to take in and use oxygen. The amount of blood the heart pumps every minute decreases, along with the amount of oxygenated blood available to muscles and other cells (Front Physiol, 2020; 11: 588784). This explains why you lose strength and speed so fast after you stop exercising. Within three weeks after you stop exercising, your VO2 max is markedly reduced (J Appl Physiol, 1992;73(5): 2004-2010).

Rehabilitation After Prolonged Inactivity
Healthy people who had been immobilized for two weeks were started on a program in which they pedaled a bicycle 3-4 times a week for six weeks. They regained some, but not all, of the muscle size and strength that they lost (J of Rehab Med, June 26, 2015). Strength training should be used along with aerobic activity to regain lost muscle strength and size. You need to lift weights or do some other form of strength training for recovery. Other studies show that it usually takes at least three times as long as the period of inactivity to recover full strength (J Am Med Assoc, 2007; 297: 1772–1774).

Why Aerobic Exercise is Not Enough to Regain Lost Strength
To regain the lost muscle strength and size, you need to exercise intensely enough to damage muscle fibers. Aerobic training (such as running or cycling) is not enough. When you use your muscles, you contract the muscle and shorten its fibers. However, you do not contract a muscle fiber equally throughout its length. Muscle fibers are made up of blocks touching end to end to form the long stringy muscle fiber. Each block touches the next block at a point called the Z-line. You have to damage the Z-line to make a muscle grow larger and stronger. If you pedal with great pressure, you will damage the muscle fibers at the Z-lines and when they heal, muscles will become stronger. However, most people do not pedal hard or long enough to cause enough damage to make the muscle larger and stronger when it heals. Adding weight training to the recovery program will help to regain the lost strength and muscle size.

My Recommendations
If you have to stop exercising for even a few days, expect to lose strength and endurance. When you resume exercising, you should do some form of strength training to regain your lost strength. Caution: Pain at the site of an injury means that you are tearing the previously injured muscle fibers and should stop exercising immediately.