Mechanical force on bone is essential for modeling and remodeling,
processes that increase bone strength and mass (47).
Whereas body weight and weight-bearing exercises provide a
direct mechanical force on bones, the largest voluntary loads on
bone are proposed to come from muscle contractions (47). Correlations
between grip strength and bone area, bone mineral
content, and bone mineral density in both healthy athletes (48)
and stroke patients (49) support the notion that muscle contractions
play a significant role in bone strength and mass. Even the
correlation between body weight and bone mass (47) can be
explained on the basis of the force exerted on bone by muscle
contractions, in that it takes more force per unit area to move
heavier bodies. Furthermore, changes in bone mass and muscle
strength track together over the life span (47). Although it is
debatable whether it is muscle strength or simply muscle mass
that is important in determining bone strength and mass, it is
significant that skeletal muscle mass was correlated positively
with bone mineral content and bone mineral density in MINOS
(Mediterranean Intensive Oxidant Study), a prospective study of
osteoporosis and its determinants in men (50). Men with the least
skeletal muscle mass also had increased risks of falls due to
impaired static and dynamic balance, presumably at least in part
because of a decrease in muscle strength (50).
Thus, maintenance of adequate bone strength and density with
aging is highly dependent on the maintenance of adequate muscle
mass and function. The relative importance of muscle compared
with normal hormonal and nutritional effects on bone may be
argued. Because some of the factors, such as dietary protein,
insulin growth factor, and testosterone (51), that are proposed to
affect bone directly also affect muscle, it is impossible to distinguish
in vivo whether these factors directly affect bone if their
effects on bone are the consequence of increased muscle
strength, which putts greater mechanical force on bone. Regardless,
the importance of muscle in prevention of osteoporosis is
clear.