Hyperextension of the knee in stance (knee recurvatum) is a common disorder in patients with spastic cerebral palsy (CP). A group 35 children with CP (47 lower limbs) was divided into two subgroups according to the timing of maximum knee extension during the stance phase of gait. Gait analysis and musculoskeletal modelling data were compared with a control group of 12 normally developing children. We observed no difference in kinematics between the CP groups who showed an equinus position of the foot at initial contact. Both groups showed increased external extensor moments across the knee. The muscle–tendon lengths of the hamstrings were abnormally long at initial contact, and in both recurvatum groups, contracted faster compared with the control group. Surface electromyography revealed prolonged activity of the hamstrings in stance and early activation in swing. Abnormally long hamstrings at initial contact together with equinus position of the foot are the main causes of genu recurvatum in children with CP.