Background and rationale

Children with cerebral palsy (CP) experience calf muscle weakness and shortening, which affects activities such as walking and running. Interventions aimed to improve these functions include serial casting, botulinum neurotoxin (BoNT) injections, orthopedic surgery, and functional power training. Individual outcomes and long-term effects of these interventions vary tremendously. This could be explained by the unknown effects of the interventions on muscle structure. 3D static ultrasonography (US) provides a detailed picture of muscle morphology and dynamic US can visualize muscle-tendon interaction during activities. This allows an integrated assessment of functionality, muscle morphology, and mechanical muscle properties, providing a complete picture of the pathology and impact of the interventions. Identifying the predictors of the effect of an intervention will help us to individualize treatment and improve its success.

Research questions

1. How do the functional, morphological, and mechanical properties of the calf muscles in CP adapt to interventions for the treatment of neuromuscular impairments?
2. What treatment-induced muscle/tendon changes contribute to the changes in walking and running abilities after treatment?
3. What key factors in pre-treatment functionality, muscle morphology, mechanical muscle properties best predict functional outcome after treatment?

Method

Based on Walhain et al. (2021) (effect size 0.6[9], power 0.95, α 0.05), 10-20 independently ambulant children with spastic CP aged 4-17 are required per intervention to show muscular effects. The interventions are part of regular care and will be performed in Amsterdam UMC and surrounding rehabilitation centers. Five groups of at least six children will participate in the 12-week long functional power training. For this intervention, a double-baseline design will be applied with assessments 12 weeks prior to, and at the start and end of the program (see planning table). For serial casting, BoNT injections, and orthopedic surgery only pre and post variables will be measured. The following variables will be studied in all participants: 1) Functional: muscle strength, functional walking/running tests, and/or 3D gait analysis. 2) Morphological: muscle fiber, belly and tendon lengths, muscle volume, physiological cross-sectional area, pennation angle and echo-intensity. 3) Mechanical: muscle strength, muscle extensibility and tendon strain. Morphological and mechanical variables will be assessed using 3DUS and EMG. In a sub-group of children, we will visualize muscle and tendon interaction during 3D gait analysis (simultaneous recording of kinematics, kinetics, EMG and dynamic US) on an instrumented treadmill as the children perform level and uphill walking and running.