Objective: To compare single muscle fiber contractile properties in elderly persons “at risk for mobility disability” and age-matched healthy controls. Design: Cross-sectional study. Setting: Muscle physiology lab. Participants: 18 subjects (12 men, 6 women; mean age, 74y), were classified by Short Physical Performance Battery score, an indicator of future mobility disability. A score of 9 or lower was considered “at risk” (n=6) and of 10 or higher was considered healthy (n=12). Interventions: Percutaneous biopsies were obtained from the vastus lateralis muscle. The cross-sectional area (CSA) of 339 chemically skinned single muscle fibers was measured followed by activation in Ca+2 solution. The slack test was used to determine maximal force, specific tension, and unloaded shortening velocity. A series of isotonic contractions was performed to generate a power curve for each fiber, allowing measurement of peak and specific power. Myosin heavy chain (MHC) composition was determined using sodium dodecyl sulfate–polyacrylamide gel electrophoresis. Main Outcome Measures: CSA, maximal force, specific tension, unloaded shortening velocity, peak power, specific power, and MHC composition. Results: Type I muscle fibers from “at risk” subjects had a lower unloaded shortening velocity compared with healthy subjects (0.62FL/s vs 0.57FL/s; P=.016). There were no between-group differences in CSA, maximal force, specific tension, peak power, or specific power. Type IIa muscle fibers did not show significant differences between groups. Conclusions: Elderly people who are “at risk” for mobility disability have reduced single muscle fiber velocity. This may contribute to a decrease in whole muscle contractile velocity and power generation, which in turn may decrease ability to correct balance perturbations and increase the risk of falls.