Effect of Vibram FiveFingers Minimalist Shoes on the Abductor Hallucis Muscle

Background: This study investigated the effect of Vibram FiveFingers Bikila minimalist shoes on intrinsic foot musculature. We hypothesized that a gradual transition into minimalist shoes will increase the thickness of the abductor hallucis muscle. Methods: Forty-one individuals were divided into four groups: control (traditional shod) (n = 9), restricted walking in Vibram FiveFingers (n = 11), running in Vibram FiveFingers (n = 10), and unlimited walking in Vibram FiveFingers (n = 11). At baseline, 12 weeks, and 24 weeks, the thickness of the abductor hallucis muscle was determined using ultrasound. Statistical analysis was performed to determine the significance of differences in muscle thickness at the three different time points. Results: The mean thickness of the abductor hallucis muscle at 24 weeks was significantly greater than that at baseline for the restricted walking (P = .005) and running (P < .001) groups. In the unlimited walking group, the mean thickness of the muscle at 12 weeks was significantly greater than that at baseline (P < .05) but not at 24 weeks. There were no significant differences in muscle thickness among the three time points for the control group (P = .432). Conclusions: This study demonstrated that wearing Vibram FiveFinger Bikila footwear over a controlled period of time, an unlimited amount of time, as well as transitioning runners over a 6-month period of time using the 10% philosophy for increasing mileage, significantly increases intrinsic muscle thickness of the abductor hallucis. The abductor hallucis muscle aids in support of the medial longitudinal arch, and an increase in this muscle thickness may help reduce running-related injuries thought to arise from arch weakness.