Relationship between motor unit activity and muscle unit length dining voluntary contractions

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To elucidate the relationship between force development and activation pattern in motor units, I simultaneously observed the motor unit spike interval and muscle unit length during human voluntary contraction in vastus medialis muscle using surface electrode methods.1)The motor unit action potentials recorded >2 cm from the motor-end plate at the distal end of muscle fibers facilitated accurate estimation of muscle unit length in the vastus medialis muscle.2)The muscle unit length shortened >5% to 15% of maximal voluntary contraction (%MVC). The relationship between spike interval and muscle unit length was not linear.3)Regardless of spike interval elongation, the muscle unit length gradually shortened 5% during the sustained contraction below 5%MVC for 3min.4)The muscle unit length gradually shortened during the evoked contraction from tonic vibration, but the amount of shortening was smaller than that of voluntary contraction.These findings suggested a shortening of muscle unit length during voluntary contraction at a low-level might be caused by both motor unit activity and the activities of other adjacent motor units.