Site-specificity of bone mineral density and muscle strength in women: job-related physical activity

We proposed that there are significant correlations between muscle strength and bone mineral density in premenopausal women and that these correlations are site-specific. To test this hypothesis, we examined the relationships among site-specific bone mineral density, physical activity, and muscle strength in a group of 96 healthy premenopausal Caucasian women. Bone mineral density was measured at the lumbar spine and at three sites in the proximal femur (trochanter, femoral neck, and Ward's triangle) with dual-energy x-ray absorptiometry and at the mid-radius with single-photon absorptiometry. The muscle strength of hip and spinal muscle groups was determined with a strain gauge isodynamometer, and grip strength was measured with the JAMAR dynamometer. The strength of shoulder girdle muscle groups was evaluated with the use of free weights. Physical activity was determined by surveying the subjects and by using a standardized scale. Data analysis revealed significant linear correlations of muscle strength with bone mineral density at the mid-radius (r = 0.31; P = 0.002) and at the hip (r = 0.26; P = 0.01). Grip strength was significantly correlated with bone mineral density of both the spine and the femur (r = 0.24, r = 0.34; P < 0.05 for both). Back extensor strength correlated with bone mineral density of the hip (Ward's triangle; r = 0.23; P = 0.023). However, there was no significant positive correlation between the strength of the spinal flexor or extensor muscles and the site-related bone mass (lumbar spine). Only one of the three components of the physical activity score (job) positively correlated with vertebral bone mineral density (r = 0.21; P = 0.04). Physical activity negatively correlated with age (r = 0.24; P = 0.02). We conclude that in premenopausal women, the effect of muscle strength on bone mass is more systemic than site-specific. A positive correlation between vertebral bone mass and components of physical activity demonstrates that even job-related physical activity is an important factor in maintaining adequate bone mass.