Bone tissue and physical activity.

Bones provide more than just a structural framework for the body. In reality, bone is a multifunctional tissue dependent on, and sensitive to, a wide variety of biological, biochemical and biomechanical stimuli. This complex system is highly responsive to the mechanical stresses imposed by gravity and muscular contractions. Animal studies as well as human cross-sectional and longitudinal studies all point to the importance of weight-bearing physical activity and mechanical loading as the prime modalities in the preservation of skeletal integrity. Lack of weight-bearing activity is extremely harmful to the skeleton. Loss of skeletal density in individuals subjected to various types of immobilization or under conditions of weightlessness is well documented. The role of physical activity in the maintenance of skeletal integrity is a topic of considerable current interest when the increasing incidence of skeletal fragility in the elderly is taken into account.