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.