Consists of alpha motoneuron together with its axon and extrafusal muscles. The size of the motor unit varies from three fibers for the eye muscles to as many as 500 for muscles in the back, but the size can be altered by training. Muscle contraction is regulated via change in the number of motor units recruited that results in an increased number of muscle fibers that contract. For a given motor unit, multiple action potentials give rise to stronger contractions. In muscles homogeneous for fiber type such as the adductor pollicis (72 to 91% type I fibers), more motor units are activated with greater workloads of up to 50% of maximum force. Motor units are fired more frequently (something termed ‘rate coding’) with workloads greater than 50% of maximum force. As for heterogeneous muscles (combination of Type I and II fibers) such as the biceps brachia and deltoid, they use a different pattern of recruitment and firing frequency. Accordingly, motor units are recruited in order according to their size as voluntary contraction increases from zero to maximal force (100% maximum contraction). The small slow twitch oxidative motor unit is recruited at a low force level, and has a low threshold force for being recruited. Beyond 35% maximum contraction, larger higher threshold fast twitch oxidative glycolytic motor units are recruited. Beyond 65% maximum contraction, even larger higher threshold fast twitch glycolytic motor units are recruited. In the most extreme circumstance, the largest, and an even higher, threshold fast twitch glycolytic motor units are not recruited until the exertion force exceeds 90% of maximum. The motor unit’s firing rate increases as the force of contraction exceeds the its recruitment threshold. Fast twitch units require higher firing rates to attain maximum force due to their faster contractile response. The concept of motor unit was first introduced by Edward G.T. Liddell (1895-1985) and Charles S. Sherrington (1857-1952) in 1925.
See Action potential, Alpha (α) motoneuron, Axon, Degrees of freedom (or Bernstein’s problem), Denervation, Extrafusal muscle fibers, Final common pathway, Force, Motoneuron, Type 1 muscle fibers, Type 2 muscle fibers