Knee extension strength cutpoints for maintaining mobility

OBJECTIVES: To identify levels of knee extensor strength that are associated with high and low risk of incident severe mobility limitation (SML) in initially well-functioning older adults. DESIGN: Prospective cohort study. SETTING: University clinic center. PARTICIPANTS: One thousand three hundred fifty-five men and 1,429 women (aged 73.6 +/- 2.85) who reported no mobility limitation. MEASUREMENTS: Unilateral knee extensor isokinetic strength of participants was obtained. Participants were followed over a median of 5.90 years for the onset of SML, defined as two consecutive reports of a lot of difficulty or inability to walk one-quarter of a mile or climb 10 steps. Deciles of knee extension strength relative to body weight were evaluated to identify cutpoints most predictive of incident SML. Cutpoints were then compared with prevalence of having slow gait speed (< 1.22 m/s) and mortality. RESULTS: Two sex-specific knee extension strength cutpoints were found. High and low risk of SML corresponded to less than 1.13 newton-meters (Nm)/kg (1st decile) and more than 1.71 Nm/kg (6th decile) in men and less than 1.01 Nm/kg (3rd decile) and more than 1.34 Nm/kg (7th decile) in women, respectively. Moderate risk was defined as being between the low- and high-risk cutpoints. Individuals with knee extension strength in the high- and moderate-risk categories were more likely to have a gait speed less than 1.22 m/s (hazard ratio (HR)=7.00, 95% confidence interval (CI)=5.47-8.96 and HR=2.14 7.00, 95% CI=1.73-2.64, respectively) and had a higher risk of death (HR=1.77, 95% CI=1.41-2.23 and HR=1.51, 95% CI=1.24-1.84, respectively) than individuals in the low-risk category. Adjustment for demographic factors, health behaviors, and medical conditions did not alter these associations. CONCLUSION: Knee extensor strength cutpoints provide objective markers to identify initially well-functioning older adults at high and low risk of future mobility limitation.