Combined translational and rotational perturbations of standing balance reveal contributions of reduced reciprocal inhibition to balance impairments in children with cerebral palsy

ABSTRACT Balance impairments are common in cerebral palsy (CP). When balance is perturbed by backward support surface translations, children with CP have increased co-activation of the plantar flexors and tibialis anterior muscle as compared to typically developing (TD) children. However, it is unclear whether increased muscle co-activation is used as a compensation strategy to improve balance control or is a consequence of impaired reciprocal inhibition. During translational perturbations, increased joint stiffness due to co-activation might aid standing balance control by resisting movement of the body with respect to the feet. However, during rotational perturbations, increased joint stiffness will hinder balance control as it couples body to platform rotation. Hence, we expect increased muscle co-activation in response to rotational perturbations if co-activation is caused by reduced reciprocal inhibition but not if it is merely a compensation strategy. We perturbed standing balance by combined backward translational and toe-up rotational perturbations in 20 children with CP and 20 TD children. Our perturbation protocol induced a backward movement of the center of mass requiring balance correcting activity in the plantar flexors followed by a forward movement of the center of mass requiring balance correcting activity in the tibialis anterior. We found that the switch from plantar flexor to tibialis anterior activity upon reversal of the center of mass movement was less pronounced in children with CP than in TD children leading to increased co-activation of the plantar flexors and tibialis anterior throughout the response. Therefore, our results suggest that a reduction in reciprocal inhibition causes muscle co-activation in reactive standing balance in CP.