The fatigue of voluntary contraction and the peripheral electrical propagation of single motor units in man

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Abstract
Single human motor units from the short extensors of the toes were classified by their voluntary threshold and axonal conduction velocity. The voluntary discharge intervals and the shortest discharge intervals at which action potentials were conducted to the muscle fibers were studied during maximal voluntary effort maintained for 1 min and were compared with the intervals calculated to be necessary for full fusion. Electromyographic techniques were used. High selectivity of the recordings was obtained after blocking of the main motor nerve with lidocaine in subjects with an accessory innervation of just a few motor units or 6 mo. after lesions to intramuscular nerve twigs, when the muscle-fiber density within the motor units was increased. For a whole minute of maximal voluntary effort, motor units with low threshold and low conduction velocity fired tonically at a rate that should have been sufficient for maximal tension. These intervals were longer than the intervals at which blocking occurred. In ordinarily motivated and untrained subjects, motor units with high threshold and high conduction velocity fired tonically at intervals sufficient for full tension for some seconds. Under these circumstances the voluntary intervals were longer than the critical ones. Through extraordinary motivation and training, motor units with high threshold and high conduction velocity could also be driven tonically for 20-60 s at the high rates required for full tension. The critical intervals then approached the voluntary ones. Blockings occurred and increased in frequency as the contraction continued. When motor units with high threshold and high conduction velocity were tetanized electrically at 20 Hz, which was the lowest discharge rate of these units in tonic voluntary contraction, blockings occurred within 60 s. The role of the peripheral block is discussed in relation to loss of tension in prolonged voluntary contraction.