Identical slow pattern stimulation (20 Hz for 10 s every 30 s) of denervated regenerating slow soleus (SOL) and fast ext. dig. long. (EDL) muscles of adult rats induces markedly different myosin heavy chain (MyHC) expression patterns in SOL and EDL (Kalhovde et al., J. Physiol., 2005, 562: 847). Here we show that MyHC expression patterns in denervated regenerating SOL and EDL, injected with myotoxic bupivacain on day 0 to induce muscle necroses, activation of satellite cells (SCs), and regeneration, are equally different whether the stimulation starts on day 0 or day 7. We also find a close correspondence between relative amounts of fast (2A, 2X, 2B) and slow (1) types of MyHCs and contraction speed. Thus, 20 Hz stimulated EDL muscles contain less type 1 and more type 2X MyHCs and display faster isotonic intrinsic shortening velocities and faster isometric twitches than 20 Hz stimulated SOL muscles. Regenerating EDL differs from regenerating SOL also by greater heterogeneity of fiber types and content of hybrid fibers expressing both fast and slow types of MyHCs. To identify the original type of regenerating fibers, necrosis and regeneration was induced in one end of 20 Hz stimulated EDL muscles, but not in the other end. Analysis of MyHC content along the length of individual fibers then showed that regenerating original type 2B and 2X fibers failed to express type 1 MyHC in contrast to regenerating original type 1 and 2A fibers. We conclude that SCs in normal EDL and SOL muscles of adult rats are already committed to different developmental fates upon activation by injury and that intrinsic differences between SCs in EDL and SOL and within EDL cause regenerating fibers to acquire different phenotypes despite identical electrical activity.