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.
