Relative contribution of muscle strength, lean mass, and lower extremity motor function in explaining between-person variance in mobility in older adults

Background: Approximately 35% of individuals > 70 years have mobility limitations. Historically, it was posited lean mass and muscle strength were major contributors to mobility limitations, but recent findings indicate lean mass and muscle strength only moderately explain mobility limitations. One likely reason is that lean mass and muscle strength do not necessarily incorporate measures globally reflective of motor function (defined as the ability to learn, or to demonstrate, the skillful and efficient assumption, maintenance, modification, and control of voluntary postures and movement patterns). In this study we determined the relative contribution of lean mass, muscle strength, and the four square step test, as an index of lower extremity motor function, in explaining between-participant variance in mobility tasks. Methods:In community-dwelling older adults (N = 89; 67% women; mean 74.9 +/- 6.7 years), we quantified grip and leg extension strength, total and regional lean mass, and time to complete the four square step test. Mobility was assessed via 6-min walk gait speed, stair climb power, 5x-chair rise time, and time to complete a complex functional task. Multifactorial linear regression modeling was used to determine the relative contribution (via semi-partial r(2)) for indices of lean mass, indices of muscle strength, and the four square step test. Results: When aggregated by sex, the four square step test explained 17-34% of the variance for all mobility tasks (p < 0.01). Muscle strength explained similar to 12% and similar to 7% of the variance in 6-min walk gait speed and 5x-chair rise time, respectively (p < 0.02). Lean mass explained 32% and similar to 4% of the variance in stair climb power and complex functional task time, respectively (p < 0.02). When disaggregated by sex, lean mass was a stronger predictor of mobility in men. Conclusion: The four square step test is uniquely associated with multiple measures of mobility in older adults, suggesting lower extremity motor function is an important factor for mobility performance.