The skeletal muscles

Reduction in muscle strength and muscle mass
Proceeds differently in various muscle groups, but most markedly in the underarm flexors and the muscles that hold up the body (Ufland in Theimer 1973, 87)
Decrease in muscle mass as a reduction of muscle strength (Bugyi in Platt 1976, 87)
Close correlation between decrease in muscular performance and reduction of the sex hormones (Hettinger 1966, 28/103)
Weight training can slow down the decrease of muscle mass considerably!