Active state of muscle and the second and third derivatives of twitch tension

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Abstract
The electrical signal representing skeletal muscle twitch tension (T) was successively differentiated to obtain dT/dt (velocity), and d2T/dt2 (acceleration). The time from the beginning of the twitch to peak tension acceleration coincided with the value for the duration of the active-state plateau (DASP) obtained by other means. Changes in the peak value of the 3rd derivative of qualitatively paralleled changes in tetanus tension. The 3rd derivative may be usable as a measure of the intensity of the active state (IAS). By the use of the 2nd and 3rd derivatives, the DASP and IAS was empirically determined during the course of a single twitch. In mouse skeletal muscle, the DASP was inversely related to temperature but was unaffected by changes in resting tension. The IAS was linearly related to changes in temperature and resting tension. The first twitch after a tetanus was characterized by decreased twitch height, decreased IAS, and prolongation of the DASP. Staircase, although brief, was accompanied by increased IAS and unchanged DASP. Fatigue due to prolonged periods of stimulation showed decreased twitch tension, decreased IAS, and increased DASP.

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