The relationship of muscle and bone loss and activity levels with age in women.

Quadriceps strength and cross-sectional area (CSA), spine bone mineral density (BMD), bone mass in the mid- and distal femur and physical activity levels were measured in 216 healthy British women. Muscle strength, CSA and bone in the distal femur decreased linearly from the third decade. Cortical bone at mid-femur was maintained until the sixth decade. Spine BMD decreased from the fourth decade with an accelerated period of loss in the sixth decade. There were significant positive relationships between quadriceps strength and bone mass at all three skeletal sites. With age kept constant, physical activity levels did not correlate with any of the muscle or bone indices. The force-generating capacity of the quadriceps decreased with age. These results suggest a relationship between the loss of muscle and bone with age which cannot be explained by declining physical activity levels.