"A healthy, young person's body composition includes 30 percent muscle, 20 percent fat and 10 percent bone," says Navinchandra Dadhaniya, M.D., a specialist in geriatric medicine at Illini Hospital. "When a person ages, body composition changes. A person age 75 or over may have only 15 percent muscle, 40 percent fat and 8 percent bone."
Though the loss of lean muscle mass is typical, several factors can contribute to an increased amount or faster rate of loss. Sarcopenia is clearly accelerated by physical inactivity. Without adequate exercise, muscle mass may decrease as much as 1 percent each year after age 30. Other triggers can include changes in hormone levels and eating habits.
Acute illness and being bedridden are also factors. A person who is bedridden can lose lean muscle mass at a rate of 1.5 percent each day, says Dr. Dadhaniya. In a small number of cases, deficiency growth hormones or lack of testosterone plays a role in sarcopenia.
The following excerpt was taken from the GenesisHealth website. You may read the entire article at http://www.genesishealth.com/news/2003/muscle_loss.aspx