The tension of the twitch can be measured under conditions in which the muscle is not allowed to shorten (isometric contraction). Because all the sarcomeres are activated, the single-twitch strength of a fiber tends to be the same each time, providing the muscle remains the same length. If a second twitch is elicited before the first has relaxed, the maximum tension achieved is increased. If the muscle is activated at a high enough frequency, the twitches fuse into a continuous smooth contraction (tetanus) of even greater tension.

Muscle Length-Muscle Tension Relationships. The tension developed by a tetanically stimulated muscle depends on the final length the muscle fibers are permitted to reach. Maximum tension is exerted when the length of the sarcomere allows activation of all the crossbridges between the thick and the thin filaments. This occurs at the normal resting length of the muscle fiber. If the muscle is contracted too far, the thin filaments overlap, which interferes with their interactions with the thick filaments, reducing the maximum attainable tension.

On the other hand, if the muscle is stretched, the thin filaments do not have access to all of the available myosin head groups and fewer than the maximum number of crossbridges are formed. If the muscle is greatly stretched, the thick and thin filaments may not overlap at all and no additional tension can develop in response to stimulation.