The interaction effects of rocker angle and apex location in rocker shoe design on foot biomechanics and Achilles tendon loading

Abstract

The influences of rocker shoes on foot biomechanics were controversial because the interaction between two design factors—rocker angle and apex location, was usually omitted. This study investigated the interaction effects of rocker angle and apex location on plantar foot pressure, metatarsophalangeal/ankle angle, and Achilles tendon force during walking. Ten participants performed walking trials under six rocker shoe conditions: 2 rocker angles (mild and severe) ​× ​3 apex locations (distal, standard, and proximal), wherein the plantar foot pressure was measured and the movement data were processed by musculoskeletal modeling to report joint angle and Achilles tendon force. A two-way ANOVA repeated measures was used for statistics. Significant interaction effects were reported in examinations of forefoot pressure, midfoot pressure, and metatarsophalangeal dorsiflexion. The standard apex significantly reduced peak forefoot and midfoot pressures (p ​= ​0.008–0.034, Hedges'g ​= ​0.75–0.84), which was further decreased by a severe rocker angle (p ​= ​0.006, Hedges'g ​= ​0.51–0.81). Moving the apex proximally reduced Achilles tendon forces (p ​< ​0.001, Hedges' g ​= ​0.80) and facilitated both metatarsophalangeal dorsiflexion and ankle plantarflexion during push-off (p ​= ​0.003–0.006, Hedges' g ​= ​0.03–0.82). Rocker angle seemed to have fewer effects on ankle joint angle and Achilles tendon force. We concluded that apex location was likely the dominant design factor of the rocker sole in influencing foot biomechanics, yet its interactions with rocker angle should be considered. The configuration of the two features could be varied to possess different therapeutic merits and adapt to specific application purposes.