Muscle Strength and Body Weight Mediate the Relationship Between Physical Activity and Usual Gait Speed

Abstract Background Usual gait speed (UGS) is recognized as a good indicator of a variety of health conditions as well as a predictor of future disability and mortality. There is evidence that UGS increases with physical activity. Muscle function and body composition changes may mediate these changes. However, we do not know exactly which of these factors contribute to the effect of physical activity on gait speed or the magnitude of their contribution. Methods Data are from 1378 women aged 75 years and older recruited in the Toulouse EPIDemiologie de l'OSteoporose cohort. Body weight and composition (dual energy X-ray absorptiometry), knee extension strength, and physical activity habits were assessed. The INDIRECT macro for SPSS (SPSS Inc, Chicago, IL) was used to determine total and specific indirect effects of physical activity on UGS through body weight (BW) and composition as well as muscle mass and strength. Results Knee extension strength [95% confidence interval (CI) 0.0091, 0.0231] and BW (95% CI 0.0037, 0.0412), but not fat mass or muscle mass, were mediators of the relationship between physical activity and gait speed. However, muscle mass significantly mediated the relationship between physical activity and muscle strength (95% CI −3.0722, −0.6265). Conclusions Results suggest that muscle strength and BW are the most significant mediators of the relationship between physical activity and UGS. Although the specific effect of muscle mass appears negligible at first sight, its role should not be overlooked given its impact on muscle strength.