Explain why the force of muscle twitch can vary?

Force of muscle twitch varies according to the total number of motor units as well as the net amount of contractional force and pull required.

Explanation:

The contraction in the muscle that takes place due to the single action potential travelling down the motor neuron of a muscle fiber which leads to a muscle twitch. A motor neuron (alpha) innervates and branches into the muscle fibers (skeletal) and together forms a motor unit.

Depending upon the size of this motor unit, the force of muscle contractions vary. In areas which undergo fine coordinated movements like eye muscle contraction, finger contraction, etc which encompasses smaller motor units, the force of contraction is lower. In areas like the legs and back, where the motor units are more, the force of contraction is higher.

The force and the tension pull experienced and exerted by the muscle vary depending upon the movement required. The muscle twitches are classified according to the force of contraction - fast (high force-short duration), slow (low force long duration) and intermediate twitch (fast and slow).

Answer

Frequency of stimulus can change the force of muscle twitch.

Explanation

Twitch is tension produced by the contraction of a small muscle or it is muscle fasciculation. Its strength depends on the stimulation frequency. If the frequency is higher then temporal summation takes place and produce higher tension in the muscle. If the frequency is weak then less tension is produced in the muscle. This force also depends on the cross-sectional area of muscle.