n. The age-related loss of muscle mass and strength.
Other Forms
Regardless of the causes, losing weight or the will and energy to stay active sets in motion a downward spiral. Sarcopenia — the loss of muscle size and strength that may be age related — worsens as activity slows. As muscles get weaker, sense of balance becomes uncertain, and worry about falling grows. A woman may begin to fear that it's unsafe to exercise or even to go out for a walk. Yet decreased physical activity only increases the odds she will fall.
—“Forestalling frailty,” Harvard Women's Health Watch, March 01, 2003
"Sarcopenia," the name Evans and Rosenberg give to the gradual wasting away of one's body over a period of decades, "is not a necessary or normal component of aging," they insist. Their program, they say, can put off for 20 years or more the period when physical impairments dictate one's activities.
—Larry Weintraub, “Strict regimen puts new life in old bodies,” Chicago Sun-Times, July 02, 1991
1991 (earliest)
We chose the term sarcopenia because "sarco" in Greek, the language from which so much medical terminology emanates, refers to flesh, the body; "penia" means "reduction in amount or need." Hence, we're describing an overall weakening of the body caused by a change in body composition in favor of fat and at the expense of muscle.
—William J. Evans & Irwin Rosenberg, Biomarkers: The 10 Determinants of Aging You Can Control, Simon & Schuser, May 15, 1991
This word introduces the prefix sarco-, flesh or muscle, to the suffix -penia, loss or deficiency. Sarcopenia generally begins in one's early 40s, although some authorities say it can start as early as age 25. By age 50 most people will have lost about 10 percent of their muscle mass, and by age 70 they will have lost about 40 percent. Muscle strength is lost in more or less the same proportions.

What causes sarcopenia? No one is quite sure of the specific causes, although it's suspected that when nerve cells linking the brain and muscles are gradually lost via the aging process, the associated muscle cells are lost as well. Another factor might be diminished levels of hormones such as estrogen and testosterone that would otherwise help stimulate muscle growth. Others have speculated that a weakening immune system causes an increase in substances that break down muscle tissue. But probably the biggest cause of muscle loss is simple inactivity. Muscles that aren't exercised will atrophy much faster than muscles that get moderate, regular workouts. The great news is that it's never too late to start. Study after study has shown that launching a weight training program at any age can significantly improve muscle mass and slow down (but, alas, not stop) sarcopenia.