Factors contributing to sag in unfused tetanic contractions of fast motor units in rat medial gastrocnemius

The sag phenomenon can be observed in fast motor units (MUs) as a transitional decline in force during unfused tetanic contractions; however, its mechanisms are poorly understood. The study aimed to identify in the rat muscle factors that contribute to sag in two types of fast MUs: fast fatigable (FF) and fast resistant to fatigue (FR). First, we performed mathematical decomposition of sagging tetanic contractions of FF and FR MUs into twitch-like responses to consecutive stimuli. This process indicated an increase in the amplitudes of a few initial responses (up to the 2nd–3rd for FF and up to the 2nd–7th for FR MUs), followed by a decrease in the amplitudes of later responses. In comparison to the first twitch, the relative increase in force amplitudes of the several subsequent decomposed responses was smaller, and their contraction and relaxation times were shorter for FF than for FR units, which corresponded to observed differences in their sag profiles. Additionally, after occlusion of the blood circulation, sag disappeared, but it reappeared after restoration of the blood supply. This indicates that the presence of sag depends on the proper circulation in the muscle.