INTRODUCTION: Weakness of foot muscles may contribute to a variety of loading-related injuries. Supportive footwear may contribute to intrinsic foot muscle weakness by reducing the muscles' role in locomotion (eg, absorbing forces and controlling motion). Increased stimulus to the foot muscles can be provided through a variety of mechanisms, including minimalist footwear and directed exercise. PURPOSE: To determine the effect of walking in minimalist footwear or performing foot strengthening exercises on foot muscle size and strength. METHODS: Fifty-seven runners were randomly assigned to one of three groups-minimalist shoe walking (MSW), foot strengthening (FS) exercise, or control (C). All groups maintained their prestudy running mileage throughout the study. The MSW group walked in provided footwear, increasing weekly the number of steps per day taken in the shoes. The FS group performed a set of progressive resistance exercises at least 5 days/week. Foot muscle strength (via custom dynamometers) and size (via ultrasound) were measured at the beginning (week 0), middle (week 4), and end (week 8) of the study. Mixed model ANOVA were run to determine if the interventions had differing effects on the groups. RESULTS: There were significant group-time interactions for all muscle size and strength measurements. All muscle sizes and strength increased significantly from weeks 0 to 8 in the FS and MSW groups, whereas there were no changes in the C group. Some muscles increased in size by week 4 in the FS and MSW groups. CONCLUSIONS: Minimalist shoe walking is as effective as foot strengthening exercises in increasing foot muscle size and strength. The convenience of changing footwear rather than performing specific exercises may result in greater compliance.

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