Voluntary muscle control in normal and athetoid dysarthric speakers

Time domain and frequency domain analyses were performed on smoothed and averaged electromyographic (IEMG) activity recorded intramuscularly from 6 muscles of the lips, tongue and jaw during speech in normal and athetoid cerebral palsy subjects. The speech IEMG waveforms in both groups were composed of slowly changing tonic activity merging with more rapidly changing phasic bursts. Significant increases both in the durations and average levels of IEMG activity in the athetoid subjects resulted in a 5- to 30-fold increase in the speech muscle energy expended by these subjects. The peak-to-peak amplitudes of the IEMG activity were significantly increased in the athetoid subjects, commensurate with their increased average levels, thus demonstrating that they could vary their muscle contraction levels over a wide range. The velocities (rates of change) of muscle IEMG activity did not differ significantly between the two groups. The velocity of the IEMG activity increased linearly with its amplitude in both subject groups, but the durations of the IEMG bursts nevertheless were highly variable. The slope of the velocity-amplitude relation in the athetoid subjects was less than half that in the normal subjects, suggesting that the frequency bandwidth of muscle activity was reduced in the athetoid subjects, despite a normal range of IEMG velocities. The frequency analysis confirmed this suggestion. The upper limit of the average frequency spectrum of voluntary muscle activity for speech was 7 Hz in the normal subjects, whereas this limit was 4 Hz in the athetoid subjects. In the normal subjects each muscle had a different frequency spectrum, whereas the spectra for the 6 muscles were remarkably uniform in the athetoid subjects, implying an abnormality in the functional organization of their muscles. The findings of this study showed clearly that the temporospatial patterns of voluntary muscle activity in the athetoid subjects were grossly abnormal. Since this voluntary activity was reproducible across multiple repetitions of the same speech sample, the dysarthria in these speakers may be attributed to abnormal control of voluntary activity, not to involuntary movement. The results support the view that the primary disability in cerebral palsy is a disruption of the physiological mechanisms which subserve the acquisition of motor skills.