To come to this finding, researchers gathered 17 active men in their twenties and 15 active men in their sixties. Each participant had one of their legs immobilized for two weeks. After two weeks of inactivity, all the participants lost physical fitness and muscle mass—no shocker there. However, the younger set lost about 17 ounces of muscle and 30 percent of their muscle strength (which is the equivalent of aging about 45 years, according to the study), while the older men only lost about nine ounces of muscle mass and 20 percent of their strength. Simply stated, the fitter and more muscular you are, the more you stand to lose if you slack off.
The amount of bone tissue in the skeleton, known as bone mass, can keep growing until around age 30. At that point, bones have reached their maximum strength and density, known as peak bone mass. Women tend to experience minimal change in total bone mass between age 30 and menopause. But in the first few years after menopause, most women go through rapid bone loss, a “withdrawal” from the bone bank account, which then slows but continues throughout the postmenopausal years. This loss of bone mass can lead to osteoporosis. Given the knowledge that high peak bone density reduces osteoporosis risk later in life, it makes sense to pay more attention to those factors that affect peak bone mass.