“Use it or lose it.”  Jimmy Connors

When the weather turns cold, black bears retreat to their dens. During their period of winter sleep, they occasionally rouse themselves and shiver to raise their body temperatures, but for the most part, they engage in little physical activity. In the spring, the bears emerge from their dens, having lost a lot of body fat, but only a little muscular bulk. They are fit enough to begin the season’s hunt for food. A human emerging from an equivalent period of bed rest, however,  would be in terrible shape, in need of a wheelchair to make it to the grocery store because his muscle mass would have declined nearly 80 percent. He would face a long and arduous period of rehabilitation to regain the muscular strength and bulk he lost due to inactivity. 

Human muscle isn’t bear muscle

The difference between the muscle of humans and that of hibernating mammals lies in the genetic makeup of each type of muscle. Human muscle is programmed for almost continuous activity. During times when it is not called into action to contract or resist the force of gravity, it heeds some mysterious signal to begin closing up shop. In medical terms , it begins to atrophy. Anyone who has ever broken a leg or had surgery that required the immobilization of a limb remembersWhen the weather turns cold, black bears retreat to their dens. During their period of winter sleep, they occasionally rouse themselves and shiver to raise their body temperatures, but for the most part, they engage in little physical activity. In the spring, the bears emerge from their dens, having lost a lot of body fat, but only a little muscular bulk. They are fit enough to begin the season’s hunt for food. A human emerging from an equivalent period of bed rest, however,  would be in terrible shape and would be in need of a wheelchair to make it to the grocery store because his muscle mass would have declined nearly 80 percent. He would face a long and arduous period of rehabilitation to regain the muscular strength and bulk he lost due to inactivity.

The difference between the muscle of humans and that of hibernating mammals lies in the genetic makeup of each type of muscle. Human muscle is programmed for almost continuous activity. During times when it is not called into action to contract or to resist the force of gravity , it heeds some mysterious signal to begin closing up shop. In medical terms , it begins to atrophy. Anyone who has ever broken a leg or had surgery that required the immobilization of a limb remembers the sad, shrunken state of the muscles in the limb that emerged after weeks in a cast. The bone has healed but the muscles atrophied. 
.How does muscle shrink?

Atrophy occurs because, in response to inactivity, the normal balance of protein recycling in muscle cells shifts in favor of breaking protein down rather than building it up. During this process, tiny protein-based fibers called myofibrils, the contracting and relaxing elements in all muscles, begin to shrink and disappear. Unless muscles are called into action, atrophy continues and, within two weeks, the loss of muscle mass is visibly apparent.

Other triggers for muscle atrophy

Muscles also shrink in response to some serious illnesses. In these cases, the triggers that set atrophy in motion are things other than inactivity. One trigger  is a substance called tumor necrosis factor, which the body produces in response to some cancers and infections and which contributes to dramatic loss of weight in cancer patients. Another trigger is loss of nerve supply to muscle cells such as happens when the controlling motor nerve cells in the spinal cord die in diseases like polio and amyotrophic lateral sclerosis (also known as Lou Gehrig’s disease) or when nerves that run from those cells to the muscles are severed by trauma or damaged by diseases or toxins (examples are diabetes and lead exposure). Old age, with its lower testosterone levels, gradually robs muscle of some of its bulk. But no matter what the trigger is, once atrophy is set in motion the result is the same. Muscle shrinks. It becomes weak, fatigues easily, and consumes less energy. The body’s metabolism slows down, sensitivity to insulin declines, and fat accumulation becomes easier.

Atrophy conserves energy

Energy demand is the key to understanding muscle atrophy. All living things conserve energy whenever possible. This bedrock survival principle applies not only to the visible animal world , where animals do little but rest when they are not eating or seeking food, but also to the microscopic world of cellular physiology. Those cells that do not get used get put to rest. Muscle is a heavy consumer of the body’s limited energy resources and it has a mysterious ability to measure the time since it has been called upon to work. Once inactivity has exceeded a few days, muscle cells begin to shrink as a means of conserving the body’s precious energy stores. In a gravity-free environment like the International Space Station, for example, astronauts’ muscles atrophy because the work of moving bones is greatly reduced outside the earth’s gravitational field. Weightlessness is rest for muscles. Astronauts use stationary bikes and other exercise equipment in an attempt to counteract muscle atrophy, but these measures do not make up for the normal and continuous activity of resisting gravity.

Muscle loss with age

Muscle atrophy can also occur more subtly over the course of a lifetime and the patterns of muscle loss are related to lifestyles in different cultures. The human foot,  for instance, has more than twenty muscles that control the motion of its intricate, bony structure. In non-shoe wearing cultures around the world, people have remarkably strong and flexible feet because they demand much of the muscles in them. In some of these cultures, people use their feet to grasp the surfaces of trees as they climb in search of things like coconuts. Activities like these, in addition to the lack of the kind of general support that shoes provide, help keep the muscles in their feet strong. Conversely, people in shoe wearing cultures with smooth walking surfaces demand much less of their foot muscles.Not only do their feet lack strength and flexibility, but they frequently develop bony abnormalities like bunions, overlapping toes and hammer toes. The same principles apply to the muscles of the legs and hips. In cultures where people squat instead of sitting in chairs, the ability to squat and rise is retained better than it is in populations where squatting is not required in the daily routine of life. 

Retrieving muscle

Muscle is very forgiving and will respond to resumed calls for action, even in people in their eighties. When they begin to use their muscles in more demanding and repetitive activities, muscle cells once again begin to make more proteins than they break down. Muscle fibers increase in size and tensile strength. As health returns in someone who has been weakened by a debilitating illness, increasing activity improves strength. Following nerve damage, muscle may recover if the nerve supply is re-established, al­though this recovery is usually limited if a long period of time has elapsed between nerve damage and repair of nerve supply. For instance, if muscle has become weak from pressure on spinal nerves and surgical decompression has been delayed, muscle strength may not fully return. Passive electrical stimula­tion of muscles which have no nerve supply helps prevent atrophy but is never as effective as active use in restoring muscle strength.

Maintaining muscle

The lessons to be learned from muscle atrophy are that humans are designed for motion and that healthy muscle is integral to a healthy overall metabolism. Hence the constant refrain from doctors and health writers that exercise is necessary to prevent chronic illnesses like obesity, diabetes, and coronary artery disease. But how much movement is necessary to keep muscles healthy and prevent atrophy? The easiest way to answer this question is to picture the world in which early humans evolved. There were no cars, no chairs, and no grocery stores. Life involved hunting and gathering, squatting around camp­ fires, climbing trees, running from predators and after prey. In such a world, no groups of muscles were neglected. In the modern world, it takes more effort-such as learning Pilates or yoga-to make sure that the muscles of the shoulders, hips, and torso are used regularly. Still, it is easy to find daily opportunities to squat and rise, to walk without shoes, run a few steps, and add a little bounce and speed to  stair climbing.  If you need to sit for prolonged periods of time, be certain to take frequent breaks and stand up and move around. Mindfulness and willfulness about physical activity are keys to healthy muscles in the modern world. Unlike black bears, humans are programmed for short, regular intervals of rest, not for long months of hibernation. Nature will wait only a few days before moving on with her overriding goal of conserving energy. Use it or lose it is her rule. •