Sliding does occur in isotonic contractions and external work is performed. Motor unit concept The motor nerve and all the fibers it innervates is called the motor unit. The number of fibers is dependent on the necessity for fine control. In general, small muscles that react rapidly with fine control have one nerve and only a few muscle fibers. Therefore, any graded response must come from the number of motor units stimulated at any one time.
Summation is the adding together of individual muscle twitches to make a whole muscle contraction.
These processes almost always occur simultaneously within normal muscle contraction. Usually, individual motor units fire asynchronously. All motor units are not created equal. Therefore one motor unit within a particular muscle may be as much as 50 times as strong as another. Smaller motor units are much more easily excited than larger ones because they are innervated by smaller nerve fibers that have a naturally lower threshold for excitation.