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.