AIM: Clinical determination of spasticity is confounded by the difficulty in distinguishing reflex from passive contributions to muscle stiffness. There is, therefore, a risk that children with cerebral palsy (CP) receive antispasticity treatment unnecessarily. To investigate this, we aimed to determine the contribution of reflex mechanisms to changes in the passive elastic properties of muscles and tendons in children with CP. METHOD: Biomechanical and electrophysiological measures were used to determine the relative contribution of reflex and passive mechanisms to ankle muscle stiffness in 35 children with spastic CP (21 males, 14 females; mean age 9y, SD 3y 4mo; range 3-15y) and 28 control children without CP (19 males, nine females; mean age 8y 11mo, SD 2y 10mo; range 3-15y). Twenty-seven children were diagnosed as having spastic hemiplegia, six with spastic diplegia, and two with spastic tetraplegia. According to the Gross Motor Function Classification System, 31 children were classified in level I, two in level II, and two in level III. RESULTS: Only seven children with spastic CP showed reflex stiffness outside the range of the control children. In contrast, 20 children with spastic CP showed abnormal passive muscle stiffness (p