Muscle deformity in cerebral palsy: reduced use, overuse, or both?

How and why does muscle deformity develop in children with cerebral palsy (CP)? The study by Moreau et al. in this issue raises new questions and new possibilities.1 The authors used 2D ultrasound imaging to assess muscle thickness, fascicle length, and pennation angle in vastus lateralis (VL) and rectus femoris (RF) in 18 children and adolescents with bilateral spastic CP and in 12 typically developing (TD) children. In the children with CP, RF and VL thickness was reduced, VL fascicle length was maintained, but VL pennation angle was reduced compared to TD children: conversely, RF pennation angle was maintained but RF fascicle length was reduced. The children with CP varied in age and level of function. Only four children with CP had full passive knee extension. The pennation angle of a muscle decreases near the distal musculotendinous junction: shortening of the muscle belly of VL and RF in the group with CP may have lead to an underestimation of pennation angle as this was assessed at a standard point in relation to the skeleton. The architecture of the quadriceps varies through its length and between individual muscles, so that changes at a particular site in one muscle may not represent that muscle or the quadriceps as a whole. These limitations have been discussed by the authors. Why should fascicle length be reduced in one muscle and maintained in another muscle in the same group? The authors note that a reduction in fascicle length has been observed in response to disuse, and suggest that the effects of disuse may have been modulated by the different function and morphology of VL and RF. Although reduced muscle thickness was noted in VL and RF, 15 of the 18 children with CP were ambulators; with the knee fixed flexion deformities noted, their quadriceps may have been subject to overuse rather than disuse. It is difficult to explain the findings on the basis of differences in morphology as muscle length, fascicle length, and pennation angles of the RF and VL are normally similar.2 VL has a greater physiological cross-sectional area, suggesting a greater role as a postural muscle, while the higher ratio between normalized fascicle length and muscle cross-sectional area in RF suggests that it is designed more for velocity of shortening.2 Could these factors be associated with the changes found in fascicle length and pennation angle? Muscles are normally activated in a graded fashion, with smaller motor units activated initially and with larger motor units being progressively recruited as the descending excitatory drive increases. Rose and McGill assessed the activation pattern of the medial gastrocnemius and tibialis anterior in 10 children with CP and in 10 age-matched controls.3 They found that although the relationship between motor unit recruitment and firing rate was similar in both groups at low and moderate levels of contraction, as the level of contraction increased children with CP were unable to recruit higher-threshold motor units or to activate lower-threshold motor units at the same increased rate as their TD peers. Stackhouse et al. superimposed electrical stimulation during maximal isometric voluntary contraction (MIVC) of the quadriceps and triceps surae in 12 children with CP and in 10 TD children. Children with CP produced less than half the knee extension force of the TD children: this was related to reduced voluntary activation of the quadriceps (the children with CP activated one-third less of the available motor units during MIVC compared to the TD group) and to co-contraction of the hamstring muscles.4 Muscle is a dynamic organ that responds to how it is used. The electrical activity caused by a motor neuron can alter the contractile properties of a motor unit, with increased activation leading to a predominance of slow fibre types and reduced activation leading to a fast phenotype.5 It is possible that increased demand on the slow muscle fibres and reduced activation of the fast muscle fibres resulted in the different patterns of deformity in VL and RF described by Moreau et al. The possibility of a shift in fibre-type in VL towards a slower phenotype is supported by the finding that the quadriceps in ambulant children with CP is more resistant to fatigue than the quadriceps of TD children at low frequencies of activation.4 Muscle deformity and weakness in a child with CP may not be related primarily to disuse but may instead be related to a range of factors including muscle fibre-type, muscle morphology, and in particular the activation pattern of the motor units of the muscle. A greater understanding of the mechanisms of muscle deformity in children with CP together with an increasing understanding of normal muscle growth and of the transcription factors which link electrical activity and gene expression in muscle6 may allow us to develop new treatment options to augment our current approach.

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