Sag is an unexplained property of fast motor units (MUs) and skeletal muscles, and its presence (in fast) or absence (in slow MUs and muscles) is used in fast/ slow twitch recognition. Several series of experiments aimed to identify in the rat muscle factors that contribute to sag, i.e., an extra force production within first 100‑300 ms of activity in two types of fast MUs: fast fatigable (FF) and fast resistant to fatigue (FR). First, mathematical decomposition of sagging tetanic contractions of FF and FR MUs into twitch-shape responses to consecutive stimuli was performed. This method identified mechanisms of the sag, including a progressive increase in the amplitude of a few initial responses (a process shorter for FF and longer for FR MUs), followed by a decrease in the amplitude 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 sag profiles between FF and FR MUs. In other series of experiments, effects of occlusion of the blood circulation was studied and these experiments revealed that, under ischemic conditions, the sag disappeared, but it reappeared after restoration of the blood supply. Moreover, we have found a very high sensitivity of the sag amplitude to preceding activation, even for single twitches.