Can exercise prevent osteoporosis?

Commonly used definitions of osteoporosis rely upon the measurement of bone mass or bone mineral density and regard the difference between osteopenia and osteoporosis as gradual. An alternative definition has been proposed by Harold Frost, suggesting that osteopenia is the bone's physiological response to disuse. On the contrary, true osteoporoses imply the bone's inability to adapt to the loads imposed on them by their habitual mechanical usage. As a consequence, fractures occur with no or very little trauma in osteoporotic, but not in osteopenic bones. There is now ample evidence that mechanical stimuli can increase strength. Accordingly, exercise, in particular some new forms of it that involve high strain rates, seems to be preventing bone loss and possibly also induces increases in bone mass even at older ages. Hence, exercise may ameliorate osteopenia in the sense of Frost's definition. However, exercise must be feared to facilitate rather than to ameliorate the occurrence of true osteoporoses, e.g., due to microdamage accumulation. This is in sharp contrast to the general 'understanding'.