Answer
Threshold stimulus is the minimum voltage necessary to generate an action potential (AP) in a muscle fiber.
A muscle twitch is a quick cycle of contraction and relaxation caused by a stimulus at threshold or higher
voltage.
Work Step by Step
Latent period.
This is the approximately 2 ms delay between the onset/application of a threshold stimulus and onset of the muscle twitch. During the latent period excitation develops, excitation -contraction coupling and tensing of the elastic components of the muscle fibers take place.
The force generated within the muscle is the internal tension. External begins to develop only after the elastic components of the muscle are stretched taut. By dint of external tension the muscle is able to move a load, lift a weight or resist tension in an object.
The muscle contraction phase is short-lived because the sarcoplasmic reticulum quickly reabsorbs Ca++ from the cytoplasm before the muscle can develop maximal force. As the calcium level (Ca++) in the cytoplasm decreases, myosin releases actin filaments, and muscle tension falls. This is the relaxation phase of the muscle twitch.The average twitch length in skeletal muscles is about 90-100 ms (milliseconds).
