Discharge Properties of Motor Units in Relation to Recruitment Order in Voluntary Contraction

Abstract

The discharge pattern and recruitment order of single motor units in voluntary contraction of the normal human anterior tibial muscle was studied with an electrode having a high selectivity and a high positional stability in the muscle. In sustained contractions each motor unit was activated at a characteristic level of tension. The higher the threshold of the motor unit in sustained contraction, the higher was the frequency when the unit attained a discharge at regular intervals and the higher tended the “maximum frequency‘‐of the unit to be. Motor units with low thresholds in sustained contractions exhibited a continuous or tonic discharge in strong sustained contractions, whereas units with high thresholds tended to exhibit a discontinuous or phasic discharge. For some units the threshold of activation remained stable, for others it increased during activity. In twitch contractions the recruitment order of motor units differed considerably from that in sustained contractions.

Axonal conduction velocity and voluntary discharge properties of individual short toe extensor motor units in man.

“…There was a continuum for the voluntary discharge properties (Hannerz, 1974) as well as for the axonal conduction velocities with no obvious signs of bimodality. This agrees with the findings of the axonal conduction velocities of cat hindlimb Burke, 1967).…”

“…Only motor units with potentials that could with safety be identified by their characteristic shape in voluntary contraction and in the proximal and distal electrical stimulations were studied. Bipolar needle or wire electrodes with impedances up to a few MO were used (Hannerz, 1974 2. In about 20 %°o f normal subjects part of the short toe extensor muscle is innervated by an accessory deep peroneal nerve (Bryce, 1896;Lambert, 1969).…”

Recording from a Single Motor Unit During Strong Effort

“…0 dB = maximal value of power spectrum for the potential recorded unipolarly at distance 0 from the fiber surface. [15], [16].…”

Contraction time and voluntary discharge properties of individual short toe extensor motor units in man.

“…Buchthal & Schmalbruch (1970) (Hannerz, 1974a, Grimby & Hannerz, 1977. The aim of the present study was to relate the voluntary firing range to the contraction time of single motor units.…”

“…The minimum rate was not calculated as strictly as in earlier studies (Hannerz, 1974a) and only 'about 10/s', 'about 15/s' and 'about 20/s' were reported. The maximum rate of a motor unit was calculated from the maximum number of discharges in 250 ms of tonic contraction to conform with previous studies (Hannerz, 1974a).…”