Low force production has been documented in children with cerebral palsy (CP) compared to unaffected children, and attributed to either incomplete recruitment or decreased motor unit discharge rates during maximum voluntary contractions.7,8,12,13,31
We are unaware, however, of published studies that have investigated whether children with CP can obtain full muscle activation, and only a few studies have examined motor unit discharge rates at low force levels.19,28
In adults with poststroke hemiparesis, the involved side demonstrates lower initial and maximal motor unit discharge rates.14,15,23
Alternative techniques to single motor unit testing, however, are necessary to look at the overall activation of whole muscle. Superimposing electrical stimulation during maximum voluntary isometric contractions is one such technique,24,30
but it has not been applied to children with CP.
Increased antagonist coactivation could also contribute to the measured deficits in force production in CP. Increased antagonist coactivation occurs in children with CP during ambulation and standing balance.2
We are aware of only two studies, however, that have quantified antagonist coactivation during maximum voluntary isometric contractions in children with and without CP.11,21
At 60° of knee flexion, children with CP had 9.0% activation of their hamstrings during isometric knee extension compared to 4.3% in unaffected children.21
The contribution of coactivation of the hamstrings during isometric knee extension to the weakness of children with CP therefore appears to be modest.
Another source of CP-related weakness may lie within the morphology of single muscle fibers and whole muscle.3,22,29
The most common findings are an increased incidence of muscle-fiber atrophy,3,29
increased intramuscular fat and connective tissue in the most involved muscle groups,3,29
and an increase in the percentage of histochemically identified type I muscle fibers,22,29
all of which may contribute to weakness.
Noninvasive techniques that test the contractile and fatigue properties of muscle using transcutaneous electrical stimulation may be used to identify whole muscle contractile and fatigue properties. Harridge and colleagues18
examined the relationship between contractile/fatigue properties and fiber-type composition of three muscles (triceps surae, quadriceps femoris, and triceps brachii) each within 7 healthy adults and found that the twitch time to peak tension, the rate of force rise to a 50-Hz train, the ratio of 20-Hz:50-Hz forces, and the fatigue index were linearly related to the percentage of type II myosin heavy chain content when the data were collapsed across muscle groups. Noninvasive electrical stimulation techniques, therefore, may be able to provide some insight into whole muscle composition.
Clinically, evidence is accumulating that high-intensity strength training can improve force production, walking velocity, and gross motor function in children with CP.5,6
Knowledge of mechanisms underlying weakness is necessary to design more effective interventions for increasing force production in such children. The purpose of this study was to examine the muscle activation, contractile properties, and fatigability of the quadriceps femoris and triceps surae muscles in children with and without spastic diplegic CP. We hypothesized that the children with spastic diplegic CP would exhibit reduced voluntary muscle activation of the agonists (quadriceps femoris and triceps surae) and increased coactivation of the antagonists (hamstrings and dorsiflexors) during maximum voluntary isometric contractions (MVICs), slower contractile properties in response to electrical stimulation, and less fatigue during an electrically elicited fatigue test.