Increased coactivation during clinical tests of spasticity is associated with increased coactivation during reactive standing balance control in cerebral palsy

Joint hyper-resistance is a common symptom in cerebral palsy (CP). It is assessed by rotating the joint of a relaxed patient. Joint rotations also occur when perturbing functional movements. Therefore, joint hyper-resistance might contribute to reactive balance impairments in CP. Our aim was to investigate relationships between altered muscle responses to isolated joint rotations and perturbations of standing balance in children with CP. Twenty children with CP and 20 typically developing children participated in the study. During an instrumented spasticity assessment, the ankle was rotated as fast as possible from maximal plantarflexion toward maximal dorsiflexion. Standing balance was perturbed by backward support-surface translations and toe-up support-surface rotations. Gastrocnemius, soleus, and tibialis anterior electromyography were measured. We evaluated alterations in reciprocal pathways by plantarflexor-dorsiflexor coactivation and the neural response to stretch by average muscle activity. We evaluated the relation between muscle responses to ankle rotation and balance perturbations using linear mixed models. Coactivation during isolated joint rotations and perturbations of standing balance was correlated in CP but not in typically developing children. The neural response to stretch during isolated joint rotations and balance perturbations was not correlated. Our results suggest that increased coactivation, possibly due to reduced reciprocal inhibition, during isolated joint rotations might be a predictor of altered reactive balance control strategies in CP.<b>NEW & NOTEWORTHY</b> It has been challenging to relate altered muscle coordination during functional movements to altered muscle responses to isolated joint rotations in children with cerebral palsy. We performed more comprehensive assessments by not only considering mean muscle activity but also agonist-antagonist coactivation. Our results indicate that muscle coactivation during balance control might partly result from altered reciprocal pathways, for example, reduced reciprocal inhibition, in the spinal cord. These insights could improve clinical assessments of balance impairments.