Modelling the effect of soleus and gastrocnemius contractions against the floor resistance in a forward dynamics simulation revealed that hip flexion, internal rotation and adduction together with external pelvic rotation could be attributed to a direct, but distant effect of triceps surae contraction. Knee flexion smoothed out the effect. To validate this clinically relevant biomechanical observation, ankle plantar flexion was correlated with hip and pelvic rotation retrospectively in children with spastic cerebral palsy. In 49 children with spastic hemiplegia, plantar flexion showed a significant correlation with increased pelvic retraction and hip internal rotation. In contrast, in 47 children with spastic diplegia no significant effect of the triceps surae on hip and pelvis kinematics was found. Bilateral hip and knee flexion in diplegia appeared to prevent the proximal effect of the triceps surae seen in the hemiplegics. In diplegia triceps surae overactivity did not appear to be a significant cause of internal rotation gait.