Balanced Foot Dorsiflexion Requires a Coordinated Activity of the Tibialis Anterior and the Extensor Digitorum Longus: A Musculoskeletal Modelling Study

Abstract

Equinus and equinovarus foot deviations (EVFD) are the most frequent lower limb acquired deformities in stroke survivors. We analysed the contribution that the tibialis anterior (TA), extensor digitorum longus (EDL) and plantarflexor muscles play in EVFD via a biomechanical musculoskeletal model of the ankle–foot complex. Our model was composed of 28 bones (connected by either revolute joints or bone surface contacts), 15 ligaments (modelled as non-linear springs), and 10 muscles, modelled as force actuators. Different combinations of muscle contractions were also simulated. Our results demonstrate that, compared to the condition when the foot is suspended off the ground, the contraction of the TA alone produces dorsiflexion (from −18° to 0°) and a greater supination/inversion (from 12° to 30°). The EDL alone produces dorsiflexion (from −18° to −6°), forefoot pronation (25°) and calcaneal eversion (5.6°). Only TA and EDL synergistic action can lead the foot to dorsiflexion suitable for most daily life activities (≥20°) without any deviation in the frontal plane. When pathological contractures of the plantarflexor muscles were simulated, foot deformities reproducing EVFD were obtained. These results can be relevant for clinical applications, highlighting the importance of EDL assessment, which may help to design appropriate functional surgery and plan targeted rehabilitation treatments.