Neuromuscular activation and motor-unit firing characteristics in cerebral palsy

Muscle strength, neuromuscular activation, and motor-unit firing characteristics (firing rate, recruitment, and short-term synchronization) were assessed during voluntary contractions of the medial gastrocnemius (GAS) and tibialis anterior (TA) muscles of 10 participants with spastic diplegic or hemiplegic cerebral palsy (CP). The participants (six females, four males; age range 6 to 37y) walked with equinus gait at Gross Motor Function Classification System levels II to III. These were compared with 10 age-matched controls (five females; age range 7 to 35y). Neuromuscular activation was estimated by the ratio of surface electromyogram amplitude to M-wave amplitude elicited by supramaximal electrical nerve stimulation. Participants with CP produced significantly less torque (normalized by leg length) compared with controls (TA: mean 2.3, SD 1.6 vs mean 8.9, SD 3.4Nm/m; GAS mean 13.7, SD 7.1 vs mean 28.6, SD 5.1Nm/m, p < 0.001). Neuromuscular activation during maximum voluntary contraction was significantly reduced in the participants with CP compared with controls (mean 2.4, SD 1.5 vs mean 9.7, SD 2.7Nm/m for TA; mean 1.04, SD 0.41 vs mean 3.1, SD 1.2Nm/m for GAS, p < 0.001). When compared at the same submaximal level of neuromuscular activation, motor-unit recruitment and firing rates were not different between the groups, although short-term synchronization in TA was reduced in the participants with CP. These data indicate that weakness, known to be an important component of the motor deficit in CP, has a strong central component. Although the relation between recruitment and firing rate remained substantially intact at the low and moderate force contractions tested, results suggest that the participants with CP were unable to recruit higher threshold motor units or to drive lower threshold motor units to higher firing rates.