The recruitment of motor units is studied with an electromyographic technique for secure identification of single motor unit potentials. It has been shown in previous studies that the recruitment order is different in tonic and in phasic activities; in tonic activity the recruitment order is stable and low frequency units are always recruited before high frequency units; in phasic activity, however, the recruitment order is unstable and units with a higher frequency range may be recruited before units with lower frequency range. In this investigation the shifts between tonic and phasic recruitment order in voluntary contraction were compared in normal subjects and in patients with severe bradykinesia of Parkinsonism. Upon initiation of a voluntary contraction in a normal subject, phasic recruitment order may be used for a few 100 msec but tonic recruitment order then takes over. In bradykinetic patients, however, this shift from phasic to tonic recruitment order is delayed. After termination of tonic voluntary contraction in a normal subject, phasic recruitment order can again be used after a few seconds. In bradykinetic patients, however, the shift back from tonic to phasic recruitment order is also delayed. In favourable experimental situations the shift from phasic to tonic recruitment order can be normalized by passive stretch of the muscle and the shift from tonic to phasic recruitment pattern by unloading the muscle. It is discussed whether the pathological recruitment in bradykinesia might be due to disturbed gamma loop function.
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