Ankle sprains are the most common injury in sport and are often followed by an increased susceptibility to spraining. Several factors have been suspected of influencing susceptibil ity to ankle sprains, but the cause of increased ankle sprain susceptibility has not been identified. Direct examination of ankle sprains in vivo is not possible, and repeated tests to failure are not possible in vitro. Forward dynamics simulation does not have the same lim itations that in vivo and in vitro experiments have, but a model had not yet been developed that was suitable for the investigation of ankle sprains. Therefore, a three dimensional for ward dynamics simulation model of the lower extremity was developed that could be used to simulate ankle sprains.

The model had rigid body segments representing the segments of the leg, was driven by 14 muscle models, and simulated ground contact with an array of contact points. The kine matic response of the model when landing on a wedge during the first half of the stance phase of heel-toe running was compared to the kinematic response of three real subjects to evaluate the model's ability to represent the response of real subjects to interventions. The model was found to respond to these interventions in the same way as the real subjects.

Simulations of ankle sprains were performed using 10 subject-specific side-shuffle move ments on 50 irregular floor shapes. The subtalar joint stiffness and flexibility, peroneus muscle strength, peroneus muscle activity onset time, touchdown plantar/dorsiflexion position and touchdown supination/pronation position of the foot were each varied inde pendently to study their influence on sprain occurrence. Ankle sprain frequency was found to increase with decreased peroneus muscle group strength, delayed onset of activity of the peroneus muscle group and increased foot plantar flexion at touchdown. Therefore, any one of these factors may contribute to increased ankle sprain occurrence following an initial injury. Taping or bracing of the ankle are likely to reduce sprain occurrence by elic iting stronger or earlier peroneal muscle activity, or by preventing excessive plantarflexion at touchdown.