Final answer:
The size principle says that motor units are recruited progressively from small motor units to large motor units.
Step-by-step explanation:
The size principle refers to the order in which motor units are recruited. It states that motor units are recruited progressively from small motor units to large motor units. When a muscle contraction starts, the smaller, lower-threshold motor units are recruited first. As more strength is needed, larger motor units with bigger, higher-threshold motor neurons are recruited. This recruitment of motor units results in a progressively stronger muscle contraction. Overall, the size principle plays a role in the control and coordination of muscle movement.