Many motor units in a muscle are stimulated in quick succession. The most likely to happen is that the contractions will produce tetanus. A motor unit is made of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals. A group of these motor neurons work together to coordinate the contraction of a single muscle. During a concentric contraction, a muscle is stimulated to contract according to the sliding filament theory, which occurs throughout the length of the muscle, therefore, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint.
When the motor units in a muscle undergo rapid stimulation without any relaxation in between then the muscle fiber re-stimulate. Since the contractile activity remains with in the muscle fiber, the contractile force sum up.
In case if the stimulation is extremely rapid then muscle do not get enough time to remove all calcium from the sarcoplasm and hence a strong contraction occurs which is some cases can be extremely serious thereby leading to occurrence of sustained contraction called tetanus occurs
