Exercise for people with osteoporosis: translating the science into clinical practice.

The "holy grail" of inquiry regarding exercise and osteoporosis has been identifying a type of exercise that builds bone. Investigation using animal models has provided many insights into how bone responds to mechanical loading, but translating these findings into an exercise prescription for patients with osteoporosis is difficult. Patients expect bone to respond to exercise in a linear fashion, such as they are accustomed to experiencing with muscles in response to progressive strength training or with the cardiopulmonary system in response to endurance training. If the skeleton accrued greater mass in response to increasing intensity and duration of mechanical strain, our bones would weigh so much that we could not move. A unique requirement of bone is that adaptations to loading produce the strongest and the lightest structure. More exercise is not always better, but we are not yet sure exactly what and how much is enough and what and how much is too much. This complexity stymies clear communication, both in the clinic and in public health initiatives.