The functional role of motor units is largely defined by their properties. Tasks that require prolonged muscle force are carried out by slow, fatigue-resistant motor units, while tasks that require a quick but short increase in muscle force are mostly performed by fast motor units. Many of the postural muscles have a large proportion of S motor units. On the other hand, muscles that participate in quick limb movements such as kicking, hitting, or catching typically have a large proportion of FR and FF motor units. Most muscles, however, have a relatively wide range of the various motor units, reflecting their participation in a variety of motor tasks.
- Motor units of which type would you expect to find in abundance in a marathon runner, in a weightlifter, and in a swimmer?
Sustained rates of firing by motoneurons are commonly high (from about 8 to 35 Hz) so that twitch contractions of individual motor units overlap and lead to tetanus. However, fully fused (smooth) tetanus is rarely observed.
As mentioned, the central nervous system can increase muscle force by recruiting more motor units or by increasing the firing frequency of already recruited motor units (figure 6.5). It uses both methods during natural, voluntary movements. The relative role of recruitment versus increased firing frequency differs across muscles and tasks. For example, hand muscles show full motor unit recruitment at relatively moderate force levels (40% to 50% of the maximal voluntary contraction force), and a further increase in force can only be accomplished by increasing the average firing frequency. In contrast, large leg and trunk muscles recruit new motor units at very high forces.
During most voluntary movements, individual motoneurons do not demonstrate synchronization. However, at very high levels of muscle force, during fatigue, and in some neurological disorders (such as loss of voluntary muscle force following a spinal cord injury), synchronization of motor unit firing becomes a way of achieving higher forces or maintaining force for a considerable time. Motor unit synchronization has both positive and negative features. The gain is obvious: Synchronized discharges sum up to higher total muscle force as compared to asynchronous firing. However, the smoothness of the contraction will suffer, and there is also a possibility of quicker fatigue.
- You have invented a way to induce abrupt synchronization of motor units in human muscles. What groups of athletes would you recommend use this method? What groups of athletes would you suggest not even try it?
Synchronization of motor units can be measured directly through cross-correlation or indirectly by performing spectral analysis of the summed (interferential) electromyogram (see the next section). If you record the activity of two motor units over a long time, the cross-correlation function shows a peak at about zero delay if the motor units are well synchronized.
So far, we have discussed muscles as separate units that the central nervous system controls to produce movements. This is a major simplification. Recently, the notion of muscle compartments has gained prominence (English 1984; Fleckenstein et al. 1992; Serlin and Schieber 1993). Muscle compartments are groups of muscle fibers that show similar behaviors in physiological tests and motor tasks; behaviors may differ considerably across the groups. In a way, muscle compartments are muscles within a muscle. Compartments have been described in both animals and humans. For example, human extrinsic finger flexors have their muscle bellies in the forearm while their distal tendons attach to finger phalanges. Each extrinsic flexor has four distal tendons attached to the different fingers of the hand. Several studies have suggested that motor units of these muscles form groups that act preferentially (or even nearly exclusively) to produce force in only one of the tendons. This allows a person to achieve individual control of finger motion, such as that seen in professional musicians.