For the average person, age 25 is the high watermark for muscle mass. After that, muscles begin to waste away, shrinking at least 10 percent before age 50, after which the reduction quietly picks up speed. In the years after 50, adults lose an average of one-half to one pound of muscle each year—but often don’t notice the loss because fat slides in smoothly to replace it. The biological term for this phenomenon is sarcopenia. If ignored, it can leave older people unable to carry out daily activities and make them defenseless against injury. An otherwise minor accident—like a fall in the bath—becomes more likely, more damaging, and more difficult to recover from.
Many of the factors that cause sarcopenia are biological. As the body ages, it becomes more efficient in reclaiming unused muscle, and slower to recover after a bout of exercise.
The body’s levels of testosterone and human growth hormone plummet. Muscle growth slows, and some muscle fibers may die off completely. However, an equally important factor is the changing lifestyle of midlife and old age. Without continuous activity, muscles shrink, much as they do for astronauts after mere weeks of a weightless space flight.
Fortunately, there’s no need to go gently into that good night. Studies consistently show that older adults can use strength training to maintain their muscles and stimulate new growth in more or less the same way that young people can. In fact, two strength-training sessions a week is enough to keep your muscles in tune and your body in good health for years to come. (However, as you age, your body becomes less forgiving of injuries and strain. So if you embark on a new workout after 60, it’s a good idea to get your doctor’s clearance, join a workout group, and get the advice of a personal trainer.)
Source of Information : Oreilly - Your Body Missing Manual