While muscle cells respond in an all-or-nothing manner to a single action potent

There are a number of mechanisms that contribute to alterations in force generation by individual skeletal muscle cells. The first is the frequency of stimulation. A muscle response to frequency contains two components: the treppe phenomena and the summation of contraction. Treppe describes the phenomena where an increase in frequency of stimulation (with complete relaxation between pulses) will progressively increase the force developed by the muscle until force ultimately stabilizes. The explanation for this phenomena involves an increasing concentration of intracellular calcium, due to incomplete removal of calcium during relaxation, which elevates the force developed by the muscle cell. Summation of contraction is observed as the frequency of stimulation increases further, such that the muscle cell does not completely relax between twitches. As the frequency of twitches increases, the first twitch will not completely relax before the second twitch arrives, and so on. Thus, as frequency increases, the force generated by the muscle would continue to increase until a maximum is reached. As the frequency of stimulation increases, the muscle will eventually be able to maintain force with some oscillation around a constant value. This oscillation in force is termed tetanus. As frequency is increased further, force will eventually plateau (the trace flattens and there is no relaxation between twitches) into what is called fused (complete) tetanus or maximal tetanic tension. Prior to this force plateau (fusion), force fluctuates as the muscle cell partially relaxes between twitches, which is called unfused (incomplete) tetanus. Second, the force developed by a muscle fiber is dependent upon the diameter of that muscle fiber. As the diameter of a muscle fiber increases (number of parallel sarcomeres increases), the force generated by that muscle will increase. Finally, skeletal muscle length will affect the extent of tension development by the muscle cell. Typically, the muscle rests at near optimal length for force development. As the muscle is lengthened, the potential for interaction between actin and myosin is reduced as they slide past one another, thereby causing the force generated by the muscle to decrease. As the muscle is shortened, the extent of interaction will also be reduced by the thin filament blocking the binding of myosin and actin. In this case, force is also decreased as the muscle is shortened.