Walking Speed and Muscle Mass Estimated by the D₃-Creatine Dilution Method Are Important Components of Sarcopenia Associated With Incident Mobility Disability in Older Men: A Classification and Regression Tree Analysis

Objectives: It is unknown whether muscle mass measured by the D₃-creatine dilution method is a superior predictor of incident mobility disability than traditional components of sarcopenia definitions (including grip strength, walking speed, appendicular lean mass). The objective of this study was to determine the relative importance of strength; physical performance; and lean, fat, and muscle mass in predicting incident mobility disability in older men. Design: Longitudinal cohort study of participants in the Osteoporotic Fractures in Men (MrOS) study. Setting and Participants: Muscle mass was assessed by D₃-creatine dilution in 1098 men (aged 83.7 ± 3.7 years). Participants also completed anthropomorphic measures, 6-m walking speed (m/s), grip strength (kg), chair stands (seconds), and dual x-ray absorptiometry appendicular lean mass (ALM), and total body fat percentage. Men self-reported incident mobility disability defined by the development of an inability to complete at least one of walking 2-3 blocks, climbing 10 steps, or carrying 10 lb over 2.2 ± 0.3 years. Methods: Classification and regression tree analysis was conducted to determine relative variable importance and algorithm cutpoints for predicting incident mobility disability. Given the proximity of walking speed with the primary outcome (mobility disability), analyses were conducted with and without walking speed. Results: Walking speed followed by D₃Cr muscle mass/weight were the most important variables (variable importance: 53% and 12%, respectively) in the prediction of self-reported incident mobility disability. D₃Cr muscle mass was the most important variable in predicting incident mobility disability when walking speed was excluded, followed by chair stands (variable importance: 35% and 33%, respectively). Body fat percentage, ALM, and grip strength were not selected as nodes in either analysis and had low or negligible variable importance. Conclusions and Implications: This study has provided valuable insights into the importance of different variables in predicting incident mobility disability in older men. Muscle mass by D₃Cr may be a key tool for predicting the risk of negative outcomes in older adults in the future, particularly in post-acute and long-term care settings.