Meeting Abstract
The spring-mass model for running predicts the leg mechanics of animals during running (Farley et al., 1993). As surface stiffness increases, leg spring stiffness (kleg) of a runner decreases to maintain a constant total stiffness (Ferris et al., 1998), suggesting that different shoes can influence leg stiffness. We hypothesized that runners who forefoot strike (FFS) and are barefoot run with stiffer legs. Healthy runners (N = 21) ran along a 19.8-m runway under four conditions: barefoot, minimalist shoe (Nike Free), traditional shoe (Asics cumulus), and maximalist shoe (Hoka Bondi). We measured foot strike angle using a camera (208 fps), joint kinematics using a motion capture system (220 fps), and 3D ground reaction forces with a force platform (625 Hz). Leg spring stiffness was calculated using three models: (A) slope of the work-loop, (B) peak force divided by peak leg compression, and (C) Farley’s model (1993). With model (A), runners who FFS ran with 23% stiffer legs than those who rearfoot strike (RFS; p<0.05; 3-way ANOVA; R). By contrast, with models (B) and (C), runners who FFS ran with 11% and 21% more compliant legs, respectively, compared with RFS runners. Regardless of model, FFS runners had different leg spring stiffnesses than RFS runners (p<0.05). The three models led to very different outcomes, where (B) resulted in the highest leg stiffnesses and model (C) in the lowest leg stiffnesses. With (A) and (B), barefoot runners stiffened their legs ~30% more than when wearing maximalist shoes. However, with model (C), runners in minimalist shoes had the stiffest legs. Regardless of model, runners adjusted the muscle-tendon-ligament systems of their legs to store elastic energy depending on footstrike pattern and shoe type.
