<P> If another muscle action potential were to be produced before the complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous twitch, thereby producing a summation . Summation can be achieved in two ways: frequency summation and multiple fiber summation . In frequency summation, the force exerted by the skeletal muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers . Action potentials do not arrive at muscles synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given time . In a typical circumstance, when a human is exerting a muscle as hard as he / she is consciously able, roughly one - third of the fibers in that muscle will be firing at once, though this ratio can be affected by various physiological and psychological factors (including Golgi tendon organs and Renshaw cells). This' low' level of contraction is a protective mechanism to prevent avulsion of the tendon--the force generated by a 95% contraction of all fibers is sufficient to damage the body . In multiple fiber summation, if the central nervous system sends a weak signal to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first . As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones . As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger . A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required . </P> <P> Finally, if the frequency of muscle action potentials increases such that the muscle contraction reaches its peak force and plateaus at this level, then the contraction is a tetanus . </P> <P> Length - tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs . Muscles operate with greatest active tension when close to an ideal length (often their resting length). When stretched or shortened beyond this (whether due to the action of the muscle itself or by an outside force), the maximum active tension generated decreases . This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal . Due to the presence of elastic proteins within a muscle cell (such as titin) and extracellular matrix, as the muscle is stretched beyond a given length, there is an entirely passive tension, which opposes lengthening . Combined together, there is a strong resistance to lengthening an active muscle far beyond the peak of active tension . </P> <P> Force--velocity relationship relates the speed at which a muscle changes its length (usually regulated by external forces, such as load or other muscles) to the amount of force that it generates . Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity . The reverse holds true for when the muscle is stretched--force increases above isometric maximum, until finally reaching an absolute maximum . This has strong implications for the rate at which muscles can perform mechanical work (power). Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). Instead, the optimal shortening velocity for power generation is approximately one - third of maximum shortening velocity . </P>

In relation to contraction of muscle passive force is defined as