Nonlinear summation of contractions in cat muscles. II. Later facilitation and stiffness changes.

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Abstract
The force produced by cat muscles over time with 2 stimuli separated by a short interval .apprx. 3 times that produced by a twitch of cat muscles. This facilitation of force production by a 2nd stimulus involves both increases in magnitude and duration of the contraction. Increased magnitude is relatively more important in the fast-twitch plantaris muscle, whereas increased duration is more important in the slow-twitch soleus muscle. The facilitation decays in an approximately exponential manner with the interval between stimuli, having a time constant between 1 and 2 times the twitch contraction time in different muscles. If a 3rd stimulus is added, the greatest facilitation is seen at intervals longer than the twitch contraction time. The drug Dantrolene, which specifically reduces Ca2+ release from the sarcoplasmic reticulum, eliminates the delayed peak in facilitation with 3 stimuli. Associated with the increases in force with 1 or more stimuli are increases in muscle stiffness, which can be measured with small, brief stretches and releases that do not alter the time-course of contraction. The stiffness of soleus muscle reaches a peak after the peak in force. The increasing stiffness of the muscle can considerably facilitate transmission of force generated internally, in addition to any facilitation arising from Ca2+-release mechanisms.