A small step towards understanding impaired walking development in children with cerebral palsy

Malformation or lesions of an immature brain may cause cerebral palsy (CP). Walking patterns of children with CP were previously found to retain some characteristics of immature toddler walking patterns in typically developing (TD) children. At young age, the brain is highly plastic and the corticospinal tract still maturing. Hence, early interventions that target neuromuscular mechanisms appear excellent candidates for improving functional mobility in children with CP. A possibility to unravel neuromuscular mechanisms is muscle synergy analysis. Walking requires refined coordination, and the central nervous system arguably simplifies neuromuscular control by the recruitment of groups of muscles, i.e. muscle synergies. I used this concept to identify mechanisms that underlie impaired walking development in young children with (high risk of) CP. Chapter 1 provides a detailed sketch of this conceptual framework and a brief explanation of the definition of CP. In Chapter 2 I systematically reviewed the literature for the current status of research on muscle synergies during walking in children with CP compared to TD children. This resultant overview showed that the variability was larger in children with CP, yet, it appears that children with CP use a ‘simpler’ motor control strategy compared to TD children. In Chapter 3 I assessed whether neuromuscular control in young children with CP differs from that of TD children with the same walking ability in the early phase of walking development. Twenty children with (high risk of) CP and twenty TD children (6.5-52.4 months) were grouped based on their walking ability, supported or independent walking. Muscle synergy analysis showed that children with (high risk of) CP recruited fewer synergies than TD children in both walking ability groups. The most affected side in children with asymmetric CP walking independently showed fewer synergies than the least affected side. This suggests that early brain lesions result in early alterations of neuromuscular control. Chapter 4 adopted a longitudinal design, employing a combination of kinematic and electrophysiological measures. Three children at high risk of CP with different developmental trajectories were followed during five consecutive sessions covering a period of one to two years. One child did not develop CP, one was diagnosed with unilateral and one with bilateral CP. Our findings indicated that the maturation of locomotor patterns differed between children with divergent developmental trajectories. However, a larger sample should be assessed to verify the current results. Only then one may further speculate about the mechanisms that may underlie impaired walking development in young children at high risk of CP. Muscle network analysis is another approach to assess neuromuscular control and evaluates the synchronization between motor units across a large number of muscles. In Chapter 5 both muscle synergy and muscle network analysis were used to examine whether switches in interlimb coordination in healthy adults are accompanied by changes in functional activity between multiple muscles. Speed changes during treadmill walking served to induce switches in the frequency and phase relationships between arm- and leg-swing. This transition coincided with changes in both muscle synergies and intermuscular coherence. While coherence network changes at low frequencies largely resembled the dynamic pattern of the synergies, at higher frequencies other coherence patterns came to the fore. Both analyses should hence be considered complementary as they potentially capture different aspects of neuromuscular control. These findings in adults are promising for future research in children with CP. Chapter 6 reflects on the findings in this thesis. Muscle synergy analysis, possibly in conjunction with muscle network analysis, is a promising objective method for the detection of impaired neuromuscular control. In the future, this may help in designing early interventions to improve walking ability in children with CP.