Children with spastic unilateral cerebral palsy (SUCP) have an involved upper limb (IUL) restricted in active range of motion and in velocity when performing elbow extension, due to a combination of muscle impairments. One of them is excessive muscle co-activation (CA). CA usually has a functional role, particularly in joint stabilization. The aim of this study is to identify when pathological CA occurs during active elbow extension of the IUL to highlight its impact on movement restriction and to discriminate impaired muscles.

Thirteen typically developing (TD) children and 13 children with SUCP performed active elbow extension/flexions at 3 externally paced frequencies. Elbow angle and velocity were computed using a subject-specific model tracking the position of 29 upper limb markers. With these data, the extension movement was decomposed into the Extension Acceleration Phase (EAP) (velocity increase) and the Extension Deceleration Phase (EDP) (velocity decrease). The percentage of CA for the brachioradialis (BR)/triceps and biceps/triceps couples for each phase was extracted from the surface electromyographic signals. Statistical analysis was conducted using linear mixed effects models.

During the EAP, excessive and positive frequency-dependent CA was found in the SUCP group, whereas CA in the TD group was low and invariant. These results point to pathological CA in the SUCP group, probably linked to restricted velocity. During the EDP, only excessive BR/triceps CA was found in the SUCP group. CA was positive frequency-dependent in both groups. These results point to mostly functional CA, for joint stabilization at the end of the movement. However, BR seems to stand out in its possible involvement in extension active range of motion restriction.

This study provides insight into pathological CA in children with SUCP. Perspectives include individual clinical interpretation of the results, to assist in each child's therapeutic decision.