Relationship among recruitment order, spike amplitude, and twitch tension of single motor units in human masseter muscle

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Abstract
Single motor unit activity was recorded from the jaw elevator masseter muscle of humans during the production of isometric biting force. Data obtained included the threshold force at which motor units were recruited, the amplitude and waveform of their spike potentials and the peak tension and contraction time of their averaged twitches. Positive correlations were found between threshold force of recruitment, spike amplitude and peak twitch tension. Masseter muscle motor units with high recruitment thresholds tended to have larger amplitude spikes and generated twitches of greater peak tension than units recruited at lower forces. The waveforms of the recorded spikes were similar to those identified previously as being generated by the action potentials of single muscle fibers. The amplitude of the recorded potential was a function of muscle fiber size. Apparently, the size of .alpha.- motoneurons is directly related to the size of the muscle fibers they innervate.