Fatigue in phasic and tonic fibers of frog muscle

Single isolated phasic muscle fibers and small bundles of tonic fibers were directly stimulated in one-per-second twitch series by massive electrode shocks. During the stimulation period the isometric tension developed by the phasic fiber continuously decreased, first rapidly and then slowly. The tonic fibers behaved similarly, but showed much less fatigue than the phasic ones. In general, recovery of the fibers after cessation of stimulation also occurred in two phases.