Force-limiting factors in triceps surae muscle of children and adolescents with spastic cerebral palsy.

BACKGROUND: Children with cerebral palsy (CP) experience functional impairments, including reduced muscle force. Although previous research has demonstrated reduced plantarflexion force and the feasibility of assessing muscle function in children with CP, little is known about contraction-related muscle mechanics that limit force generation. This study aimed to identify key mechanical and neural factors contributing to reduced muscle performance in CP. METHODS: Twelve children and adolescents with spastic CP and twelve typically developing controls participated in this cross-sectional study. Participants performed isometric ramp contractions on a dynamometer while plantarflexion force, electromyography (EMG), and ultrasound images of the triceps surae were recorded simultaneously. Analyses of nine participants of the CP and eleven of the control group focused on force output, muscle activation patterns including co-contraction, and muscle architecture parameters such as fascicle length, pennation angle, curvature, and muscle thickness. FINDINGS: Children with CP showed significantly thinner muscles and shorter fascicles compared to controls, as well as decreased fascicle rotation per strain. Both the soleus and lateral gastrocnemius exhibited steeper increases in muscle activation relative to force output in the CP group. While co contraction decreased with increasing force in controls, it increased in children with CP, with large interindividual variability observed within the CP group. INTERPRETATION: These results suggest three main force-limiting factors in CP: increased antagonist co-contraction, muscle atrophy, and reduced effective fascicle rotation. The latter may reflect stiffer intramuscular connective tissue that restricts normal fiber movement. The applied measurement approach shows promise for evaluating the effects of therapeutic interventions on muscle mechanics in children with CP.