Our nervous systems have two ways of controlling the amount of force we produce during a contraction – motor unit recruitment and rate coding. A combination of these two methods produce optimal patterns of neural discharge to facilitate the huge variations in force required for different human movements.
Motor Unit Recruitment
Motor unit recruitment refers to the process of recruiting more motor units to increase the force elicited by the muscle.
Via the central nervous system (CNS), we are able to adjust the number of motor units we recruit for a given muscle action.
Clearly, not every muscle action requires the recruitment of all motor units to fire at the same time. If every muscle action recruited every motor unit then we would not be able to control the force output of a muscle.
Imagine your car is stuck in the mud and the wheels are spinning on the spot; you need to apply force to the rear of the car to move it so you call in a friend to help push. It still doesn’t budge so you grab another friend, and another, and so on and so forth until you can move the car.
Well that’s effectively what motor unit recruitment is – grabbing more and more units until there are enough of them active to apply the amount of force that the central nervous system is demanding.
In short, if a low force muscle action is required then slow twitch (fatigue-resistant) muscle fibers are recruited first and deactivated last. As more force is required, larger (less fatigue-resistant) motor units that generate more force are recruited until the force demands are met. Therefore, the larger motor units are recruited last and deactivated first. This is known as the size principle (5).
The precise recruitment order minimises fatigue by recruiting fatigue-resistant fibers first, and saving the fatiguing fast twitch fibers for high force generation. The nervous system will only switch on as much muscle as it needs to overcome the resistance.
Aside from motor unit recruitment, we are able to vary contractile force in the muscle in one other way – rate coding.
Rate coding refers to the adjustment in firing rate of the neural impulses which travel down the motor neuron. As the intensity of the stimulus increase, the firing rates of motor units increase and more force can be produced.
The relative contribution of recruitment and rate coding to increase muscle force depends on the function of the muscle. For example, the muscles in the hand require precise control of incremental force so motor units in these particular muscles largely employ the rate coding method to increase force production, with new motor units only being recruited up until 50-75% of maximum voluntary contraction (2, 3, 4).
In contrast, the elbow flexors (think of a bicep curl) require more forceful contractions without the need for precision, so these muscles rely on the recruitment method to a greater extent, with recruitment contributions of up to 88% of maximal voluntary contraction (2).Follow MuayThaiScholar