Growth in bone strength, body size, and muscle size in a juvenile longitudinal sample.

A longitudinal sample of 20 subjects, measured an average of 34 to 35 times each at approximately 6-month intervals from near birth through late adolescence, was used to investigate relationships between body size, muscle size, and bone structural development. The section modulus, an index of bone strength, was calculated from humeral and femoral diaphyseal breadth measurements obtained from serial radiographs. Muscle breadths of the forearm and thigh, also measured radiographically, were used to estimate muscle cross-sectional areas. Body size was assessed as the product of body weight and bone length (humeral or femoral). Stature was also investigated as a surrogate body size measure. Growth velocity in femoral strength was strongly correlated with growth velocity in body weight. femoral length (r2=0.65-0.80), very poorly correlated with growth velocity in stature (r2<0.06), and weakly but significantly correlated with growth velocity in thigh muscle size (r2=0.10-.25). Growth velocity in humeral strength was moderately correlated with that for body weight x humeral length (r2=0.40-0.73), very poorly correlated with that for stature (r2<0.05), and showed a marked sex difference with forearm muscle area velocity, with males having a stronger correlation (r2 approximately 0.65) and females a much weaker correlation (r2 approximately 0.15). Ages at peak adolescent growth velocity were nonsignificantly different between bone strength, body weight x bone length, and muscle area, but significantly earlier for stature. Thus, while there was an early adolescent "lag" between stature and bone strength, there was no such "lag" between a more mechanically appropriate measure of body size and bone strength. "Infancy peaks" in bone strength velocities, earlier in the humerus than in the femur and not paralleled by similar changes in body size, may be the result of the initiation of walking, when mechanical loads relative to body size are changing in both the upper and lower limbs. These results argue strongly for the importance of mechanical factors in the development of the preadult skeleton. Body size is the most important element in the weight-bearing lower limb skeleton, while both body size and muscle strength are important in the upper limb, especially in males.