The number of muscle fibers served by a single axon varies considerably. in muscles that move the fingers, the ratio is small, whereas in muscles that move the legs, the ratio is large. this difference in ratio depends upon the needed to move the muscle.

I believe that the difference in ratio depends upon the Precision needed to move the muscle. A single impulse of a motor neuron produces a single twitch of a muscle fiber. The physical effects for the twitch last longer than the AP, because of the elasticity of the muscle and the time required to rid the cell of Ca2+. The strength of a muscular contraction is determined by the average rate of firing of the various motor units. If, at a given moment, many units are firing, the contraction will be forceful.