In a typical skeletal muscle fibre, contraction is triggered by an all or nothing action potential lasting for c. 2 ms or so. This causes a more or less fixed pulse of Ca²⁺ in the sarcoplasm, peaking at c. 10 ms and lasting for c. 50 ms. This means that by the time the Ca²⁺ pulse peaks (point of closure of the ryanodine channels) a second AP can be triggered in the fibre, to cause re-opening of the SR Ca²⁺ channels. This will cause the Ca²⁺ concentration in the sarcoplasm to remain high for a longer time, and prolong the contraction. The mechanical twitch resulting from the successive APs therefore, can summate. During a single twitch, the force recorded at the tendon is not equal to the force exerted by the contractile elements, because sufficient time is not allowed for the pull to be transferred through the spring-like tendons (the series elastic component). With summation however, more time is allowed, so that the transmitted force increases. With repetitive stimulation at a low rate successive twitches summate with brief partial relaxations in between, giving an unfused tetanus. At a sufficiently high stimulus rate, no relaxations can be seen between successive stimuli, giving a fused tetanus. The fusion frequency is high for fast muscles and low for slow muscles. The maximum force recorded is proportional to the stimulus frequency, up to a peak level. The peak force during a tetanus is usually about 4X that for a single twitch. This is the Tetanus/Twitch ratio.

Force exerted depends on the number of myosin heads pulling together at the same time. Since the maximum overlap of thick and thin filaments (and hence the maximum number of possible cross-bridges formed) will depend on the length at which the muscle is held (for isometric contractions), force developed should depend on muscle length. A plot of the tension developed in a muscle stretched to different lengths without stimulating, gives a passive tension curve, representing the elastic properties of the parallel elastic component (fascia, blood vessels etc). A plot of peak tetanic tension developed at different lengths (during stimulation) gives the total tension curve. Subtraction of these two curves, gives the active tension curve, which has a characteristic shape: low active tension develops at short lengths, because the thin filaments on either side of the M-line interfere with each other; low tensions develop at long sarcomere lengths when overlap is restricted; and maximal tension is developed at a length which allows maximum overlap, without interference. This relationship helps to verify the hypothesis that it is the linking of cross-bridges between the actin and myosin filaments which generates the contractile tension.