Departures from linearity in the relationship between measures of muscular strength and physical performance of the lower extremities: the Women's Health and Aging Study.

Background: Sarcopenia, an age-related reduction in muscular mass and strength, may cause a decline in physical functioning and subsequent loss of autonomy. It has been suggested that strength is associated with lower extremity function mainly in the lower portion of the range of strength. Identifying the threshold under which strength is most critical to function may help in targeting groups who may benefit most from exercise interventions.

Methods: The study uses data from the Women's Health and Aging Study. The study population, recruited by screening a population-based sample aged 65 years and older, comprised 1,002 women who represent the one-third most disabled women without severe cognitive impairment living in the community. Knee extensor and hip flexor strength were assessed using a hand-held dynamometer. Lower extremity performance was evaluated using tests of walking, standing balance, and rising from a chair.

Results: Among women tested for strength (n = 892), those who could walk (97%), do the side-by-side stand (87%), or complete 5 chair stands (74%) had significantly greater strength. Walking speed was linearly associated with knee extensor strength over the entire range of strength, but its association with hip strength was limited to values below 15 kg. Time for five chair stands was associated with knee extensor and hip flexor strength below 10 and 15 kg, respectively, and no significant association was detected above these values. Stronger women were more likely to hold balance for 10 sec in the side-by-side, semi-tandem, and tandem positions. The percentage of the variance in performance explained by strength alone was always lower than 20%.

Conclusions: In this population, which does not include the strongest older women, there is a departure from linearity in the relationship between muscular strength and some measures of lower extremity performance.