Voluntary muscle activation, contractile properties, and fatigability in children with and without cerebral palsy

Cerebral palsy (CP) may lead to profound weakness in affected portions of the extremities and trunk. Knowing the mechanisms underlying muscle weakness will help to better design interventions for increasing force production in children with CP. This study quantified voluntary muscle activation, contractile properties, and fatigability of the quadriceps femoris and triceps surae in children with and without CP. Twelve children with CP (7-13 years) and 10 unaffected children (controls, 8-12 years) were assessed for (1) voluntary muscle activation during maximum voluntary isometric contractions (MVICs); (2) antagonist coactivation during agonist MVICs; (3) contractile properties, and (4) fatigability using electrically elicited tests. Children with CP were significantly weaker, had lower agonist voluntary muscle activation, and greater antagonist coactivation. In children with CP, the quadriceps normalized force-frequency relationship (FFR) was shifted upward at low frequencies and was less fatigable than controls. No differences were seen between groups in the normalized FFR and fatigability of the triceps surae. In addition, no differences were seen in the sum of the time to peak tension and half-relaxation times between groups for either muscle. Because children with CP demonstrated large deficits in voluntary muscle activation, using voluntary contractions for strength training may not produce forces sufficient to induce muscle hypertrophy. Techniques such as enhanced feedback and neuromuscular electrical stimulation may be helpful for strengthening muscles that cannot be sufficiently recruited with voluntary effort.