Triceps surae muscle architecture in ambulant children with cerebral palsy: architectural adaptations vary between children and muscles

Many children with cerebral palsy (CP) have muscles that are smaller than those of their typically developing (TD) peers, but it is largely unknown to what extent this is due to reduced longitudinal growth (i.e., shorter muscle fascicles) or transverse growth (i.e., reduced cross-sectional area of muscle fibres). Here, we use anatomical and diffusion-weighted magnetic resonance imaging to measure physiological cross-sectional areas (PCSA), fascicle lengths and pennation angles of triceps surae muscles of 69 children with CP aged 5 to 18 years. Comparison with reference values from 195 typically developing children showed that triceps surae muscles of children with CP usually had both smaller PCSAs and shorter fascicles, indicating that both longitudinal and transverse growth were impaired. However, the contribution of longitudinal and transverse growth to reduced volumes varied between muscles. In the soleus muscle of children with CP, PCSA was relatively preserved (mean CP/TD ratio 0.88 and 0.97 for age-matched typically developing boys and girls, respectively) while fascicles were substantially shorter (boys: 0.74, girls: 0.76). In the medial gastrocnemius muscle, fascicle lengths were relatively preserved (boys: 0.89, girls: 0.94) while PCSAs were substantially smaller (boys: 0.73, girls: 0.72). These observations suggest the soleus muscle adapts primarily to preserve maximum force-generating capacity at the cost of operating length and velocity range, whilst the medial gastrocnemius muscle adapts to preserve operating range at the cost of force-generating capacity. Thus, architectural adaptations in children with CP typically vary between synergistic muscles.