The muscle generates stress during the contraction phase, which is accompanied by the cycling of the cross bridges. During the relaxation phase, the muscle lengthens back to normal.
All of the muscle fibers connected to that motor neuron will contract as an action potential moves down the motor neuron. A muscle twitch is the contraction caused by a single action potential.
A single muscle twitch consists of three parts. the contraction phase, the relaxation phase, and the latent period, often known as the lag phase.
The action potential reaches the muscle, and the latent period is the brief (1-2 msec) interval before tension is seen in the muscle.
This period of time is necessary for calcium to diffuse out of the SR, bind to troponin, transfer tropomyosin away from the active sites, build cross bridges, and pick up any slack in the system.
