Without such strength training, the kind of intense aerobic activity that many fitness-conscious women pursue can actually decrease muscular strength, leaving one with a healthy heart and lungs but weak arms, legs, back and stomach muscles, according to new studies of runners.

A growing body of research also strongly suggests that many of the exercises that develop strong muscles also build healthy and dense bones, thus warding off osteoporosis, a thinning of bones that can lead to a stooped-over posture and fractures in later life. Osteoporosis affects an estimated 20 million Americans, mostly women, and causes an estimated 1.3 million fractures a year.

For that reason, those who advocate this type of exercise for women refer to it as strength or resistance training rather than weight lifting and emphasize that women who train for strength and fitness will not develop masculine bodies.