AAEM minimonograph #3: Motor unit recruitment

Motor unit recruitment is the process by which different motor units are activated to produce a given level and type of muscle contraction. At minimal levels of muscle contraction (innervation), muscle force is graded by changes in firing rate (rate coding) of individual motoneurons (MNs). At higher levels of innervation, recruitment is accomplished by the addition of different motor units firing at or above physiologic tremor rate. During slowly graded and ballistic increases in force, motor units are recruited in rank order of their size. In addition to MN soma diameter, other factors contribute to the selectivity of MN activation. For la afferent MN activation in the cat, synaptic density and efficacy as well as specific membrane resistance are also rank ordered for slow, fatigue resistant, and fast fatigue motor units with slow motor units recruited first. The central drive for motor unit activation is distributed to all the MNs of the pool serving a given muscle. Size-structure organization of the MN pool determines the order of recruitment and how MNs interact with each other. Disorders of the motor unit affect recruitment. A method for the clinical electromyographic assessment of recruitment is suggested. Assessment is made at three levels of innervation: minimal contraction for onset and recruitment firing rates; moderate contraction required to maintain the limb against gravity for the maximum number of motor units, their firing rates, and motor unit spikes/s; maximal voluntary contraction (MVC) for detection of high threshold enlarged motor units characteristic of reinnervation and completeness of the interference pattern (IP). Loss of muscle fibers results in early and excessive recruitment at minimal and moderate levels of innervation. Loss of motor units can result in both an increased rate and range of single motor unit firing at all levels of innervation. With reinnervation and enlargement of motor units, firing rates increase significantly and the interference pattern during MVC is incomplete.