Lower extremity physical performance and hip bone mineral density in elderly black and white men and women: cross-sectional associations in the Health ABC Study.

BACKGROUND: Aging is characterized by declines in physical capacity and bone mineral density (BMD), both of which contribute to increased risk for fracture. However, it is unclear if these factors coexist or are independent in elderly people, and if a relationship does exist, whether it varies by sex and/or race.
METHODS: Data are from 847 white women, 723 black women, 927 white men, and 544 black men aged 70-79 years participating in the Health Aging and Body Composition (ABC) Study, a cohort of well-functioning community-dwelling elders. BMD (g/cm(2)) of the hip (femoral neck and trochanter) was determined by dual X-ray absorptiometry. Physical capacity measures included knee extensor strength, repeated chair-stands, gait speed (6 m), walking endurance (400 m), and standing balance.
RESULTS: In analyses of BMD as a continuous measure, BMD and physical performance were most consistently related at the trochanter in black women, even after adjusting for body size and several covariates including physical activity. For each performance measure, black women in the bottom quartile(s) had lower BMD than those in the top quartile(s). For the femoral neck, there was a significant association with performance tasks, except for the 400-m walk. In comparison, only chair-rise performance was related to BMD at the femoral neck, and knee extensor strength, chair-rise performance, and balance with the trochanter in white women. For white and black men, trends existed only for lower BMD in those with poorer knee extensor strength.
CONCLUSIONS: In this cohort of well-functioning young-old seniors, physical capacity is only modestly related to BMD at the hip. The association was strongest in black women, who as a group exhibited the poorest functional capacity in the cohort. Our data show that, although both poorer physical performance and low BMD are risk factors for fracture, they remain largely independent in the well-functioning young-old, except in black women. This independence suggests that interventions to reduce fracture incidence should be targeted at improving both physical performance and bone density.