Effects of training in minimalist shoes on the intrinsic and extrinsic foot muscle volume

Background: Minimalist shoes have gained popularity recently because it is speculated to strengthen the foot muscles and foot arches, which may help to resist injuries. However, previous studies provided limited evidence supporting the link between changes in muscle size and footwear transition. Therefore, this study sought to examine the effects of minimalist shoes on the intrinsic and extrinsic foot muscle volume in habitual shod runners. The relationship between participants' compliance with the minimalist shoes and changes in muscle õ-volume was also evaluated. Methods: Twenty habitual shod runners underwent a 6-month self-monitoring training program designed for minimalist shoe transition. Another 18 characteristics-matched shod runners were also introduced with the same program but they maintained running practice with standard shoes. Runners were monitored using an online surveillance platform during the program. We measured overall intrinsic and extrinsic foot muscle volume before and after the program using MRI scans. Findings: Runners in the experimental group exhibited significantly larger leg (P = 0.01, Cohen's d = 0.62) and foot (P < 0.01, Cohen's d = 0.54) muscle after transition. Foot muscle growth was mainly contributed by the forefoot (P < 0.01, Cohen's d = 0.64) but not the rearfoot muscle (P = 0.10, Cohen's d = 0.30). Leg and foot muscle volume of runners in the control group remained similar after the program (P = 0.33–0.95). A significant positive correlation was found between participants' compliance with the minimalist shoes and changes in leg muscle volume (r = 0.51; P = 0.02). Interpretation: Habitual shod runners who transitioned to minimalist shoes demonstrated significant increase in leg and foot muscle volume. Additionally, the increase in leg muscle volume was significantly correlated associated with the compliance of minimalist shoe use