Like nerve cells, individual muscle fibers fire only when stimulated above their threshold intensity and exhibit the nerve cells' all-or-nothing property as well, where the stimulus either causes the fiber to contract all the way or not at all. Yet because muscles are composed of many fibers with different threshold intensities, muscles are capable of producing a varying range of responses in reaction to a varying range of stimuli. However, when a stimulus level is reached that causes all a muscle's fibers to contract, a condition called maximal response, an even stronger stimulus will not elicit a stronger response from the muscle. This will hold true for all muscle stimulations where the muscle has time to relax completely before being stimulated again. The muscle response to such conditions, called a simple twitch, has three phases: 1) the latent period, the lag time of less than 0.004 seconds between the commencement of stimulation and the commencement of contraction, 2) the contraction period, and 3) the relaxation period.
When a muscle is subjected to a series of stimuli that occur too frequently for the muscle to relax in between each stimulation, the muscle contractions elicited will be greater than the contraction that a single stimulus of the same intensity would have produced. This scenario, called summation, results due to the fact that the contractions caused by subsequent stimuli are effectively added together. Under conditions of summation, the muscle can be made to deliver an even stronger response than the maximal response possible under a single stimulus. The increase in the
