As we age, bone density decreases as well and can lead to osteoporosis, a condition in which the bones become fragile and weak, and are more prone to fractures. More than 40 million Americans have or are at risk for osteoporosis, and it is more common in women than in men. Exercise can increase bone strength and density. Weight-bearing activity in particular is useful as this causes the bones to work harder. Strength training as well strengthens muscles and helps strengthen bones.