Dynamic muscle strength as a predictor of bone mineral density in elderly women

Although muscle strength has been shown to predict bone mineral density (BMD) in older adults, the variance explained by isometric and isokinetic testing has been generally low (< 20%) and limited to only a few exercises and muscle groups. To elucidate the relationship of muscle strength to BMD at multiple sites, and to ascertain the most robust predictor of BMD using isotonic strength testing apparatus, we examined dynamic muscle strength and BMD in 40 healthy elderly women aged 65-82 years. BMD of the spine (L2-4), proximal femur (neck, trochanter, Ward's triangle), forearm (midradius), and whole body were measured by dual-energy X-ray absorptiometry. Dynamic strength (1-RM), utilizing isotonic weight-lifting equipment, was assessed for 10 standard upper and lower body exercises. In stepwise multiple regressions, leg press was the only independent predictor of spine (R2 = 0.11), neck and trochanter (R2 = 0.21 and 0.18), forearm (R2 = 0.21), and whole body BMD (R2 = 0.19), while bench press was an independent predictor of Ward's BMD (R2 = 0.12). The most robust predictor of regional and whole body BMD using isotonic equipment was the leg press, which may reflect overall skeletal health in this population. The portion of variance explained by dynamic muscle strength (11-21%) is similar to that reported when strength is assessed by isometric and isokinetic testing. The relationship of dynamic strength to BMD was not generally site-specific.