Biomechanics of the ankle joint complex using a muscle model assisted optimisation model
A seven segment model of the right leg and foot was developed with segments: thigh, lower leg, talus, hindfoot, midfoot and lateral and medial forefoot. Three-dimensional mapping of internal structures was made from CT scans and anatomical photographs (Visible Human Project). Twelve healthy subjects performed level walking and medial walking turns at slow, preferred and fast speed. Equilibrium about the two joints of the ankle complex (ankle and subtalar), was solved using Muscle Model Assisted Optimisation (MMAO). A three component, Hill-type muscle model determined tensions in eight muscles of the lower leg using EMG. Linear optimisation then corrected muscle tensions and solved for ligament tensions and articular surface compression. MMAO was successful in modeling ankle complex equilibrium during walking and walking turn. External forces acting on the right foot were similar for all subjects. Despite similar external loading, subjects employed different muscle tension strategies to produce equilibrium about the ankle and subtalar joints. For all subjects, triceps surae muscle tensions were largest. Peak tension in achilles tendon was 7.9xBW during walking and 8.0xBW during walking turn. The two heads of gastrocnernius behaved as distinct muscles performing different roles during stance. Peroneus brevis produced movement about the subtalar joint while peroneus longus had a stabilising role. The dorsi-flexors were significantly active during stance phase, antagonistic to triceps surae muscle group. This antagonism has not been predicted by previous models. Ligaments acted in an all-or-nothing manner when constraining the ankle complex. Ligaments were either slack or tensed at constant tension. Maximum ligament tension was 1.75xBW in the lateral ligaments of the ankle joint during walking turn. No difference between the walking and walking turn was seen in compressive loading of articular surfaces. Maximum compression of ankle joint was 10.0xBW and of subtalar joint was 8.0xBW.