Abstract
It has been reported that that discharge of motor units (MU) represented elongating trends in spike interval during voluntary constant-force isometric contraction. To investigate neural mechanisms underlying the elongating trends mechanisms, activity of the single MUs in the vastus medialis muscle during involuntary contraction via tonic vibration reflex was compared with voluntary contractions having almost the same amplitudes and time courses. Subjects performed involuntary or voluntary isometric knee extension with their knee and hip joint angles at 90deg. The action potential of single MU was recorded using non-invasive surface array electrodes. Vibratory stimulation was applied perpendicularly to the patella tendon of knee extensors, which were completely relaxed. The vibration frequencies were 75 and 100Hz, the amplitude was 0.5-0.8mm. It was already reported that MUs were classifiable by their response to vibration with activation based on either a monosynaptic reflex process (MSR) or mono- and polysynaptic process (M-PSR) (Romaigure et al., 1991, Kamo 2007). The MSR MUs discharged at a longer interval without an elongating trend, with similar synchronization between MUs, as compared with that during voluntary contraction. However, M-PSR MUs discharged in much the same way during both types of contraction. These findings suggest that the elongating trend in spike interval during voluntary constant-force contraction is not explainable merely by the decline in muscle spindle discharge.This work was supported by Academic Frontier Project at Japan Women's College of Physical Education,2007 [J Physiol Sci. 2008;58 Suppl:S192]
