Plantar flexor voluntary activation capacity, strength and function in cerebral palsy

Purpose Distal lower limb motor impairment impacts gait mechanics in individuals with cerebral palsy (CP), however, the contribution of impairments of muscle activation to reduced gross motor function (GMF) is not clear. This study aimed to investigate deficits in plantar flexion voluntary activation capacity in CP compared to typically developed (TD) peers, and evaluate relationships between voluntary activation capacity, strength and GMF. Methods Fifteen ambulant individuals with spastic CP (23 ± 6 years, GMFCS I–III) and 14 TD (22 ± 2 years) people participated. Plantar- and dorsiflexion strength were assessed with a dynamometer. Voluntary activation capacity was assessed using the interpolated twitch technique via single twitch supramaximal tibial nerve stimulation. GMF was assessed using the timed upstairs test, 10 m walk test, muscle power sprint test and six-minute walk test. Results Plantar- and dorsiflexion strength were 55.6% and 60.7% lower in CP than TD ( p < 0.001). Although voluntary activation capacity was 17.9% lower on average for CP than TD ( p = 0.039), 46.7% of individuals with CP achieved a sufficiently high activation to fall within one standard deviation of the TD mean. Plantar flexion voluntary activation capacity did not correlate with strength ( R 2 = 0.092, p = 0.314) or GMF measures in the high functioning CP group (GMFCS I–II). Conclusion In contrast to previous research, plantar flexion activation capacity did not strongly predict weakness or reduced GMF. We propose that muscle size contributes more to weakness than voluntary activation capacity in high functioning individuals with CP and that relationships between muscle activation and functional capacity are complicated by effects at multiple joints.