A sagittal-plane mathematical model of joint mobility, including the mechanical effect of the extensor retinacula, was used to predict the lever arm lengths of the main flexor and extensor muscles of the human ankle over the range of movement. In plantarflexion, the centre of rotation lies posteriorly and distally, maximising the lever arm of the tibialis anterior. The action of the gastrocnemius and soleus is maximised in dorsiflexion. Traditional calculation of ankle joint moment based on a fixed centre of rotation is acceptable only in exercises such as level walking with a limited range of motion about the neutral position. The present model with a moving centre is particularly advised in exercises which take the joint nearer to the extremes of sagittal motion.