Low-frequency fatigue, post-tetanic potentiation and their interaction at different muscle lengths following eccentric exercise.

Low-frequency fatigue (LFF) and post-tetanic potentiation (PTP) were quantified at different muscle lengths in rat medial gastrocnemius (GM) muscle. In situ experiments were performed on GM muscle-tendon complexes of anaesthetised (urethane, 1.5 g kg(-1) i.p.) Wistar rats (N=8). Force-length characteristics were determined at maximal (200 Hz) and submaximal (60 Hz) stimulation. Data for submaximally stimulated muscle were obtained in a non-potentiated and in a potentiated condition. LFF was induced by a series of 40 eccentric contractions. Post-exercise (40-80 min), data for the force-length relationships were obtained once more. Whereas force loss at 200 Hz-stimulation was least at optimum muscle length, L(0,200 Hz), (17.0+/-1.4%, mean +/-S.E.M.), force loss at 60 Hz-stimulation was maximal near L(0,200 Hz) (55.1+/-4.3% at L(0,200 Hz)-1 mm). When the muscle was potentiated, force loss at 60 Hz-stimulation was maximal at short muscle length: L(0,200 Hz)-4 mm (53.5+/-3.8%). The extent of LFF, quantified by a decrease in the 60:200 Hz force ratio, varied with muscle length: LFF increased with decreasing muscle lengths when muscles were potentiated. However, in the non-potentiated condition, LFF was maximal at a length just below L(0,200 Hz); the 60:200 Hz force ratio had decreased to 54.6+/-5.9% of the pre-exercise ratio at L(0,200 Hz)-1 mm. Compared with the non-potentiated condition, LFF was less pronounced in the potentiated condition. PTP counteracted LFF particularly at long muscle lengths. However, at short muscle lengths, LFF was still observed in potentiated muscles.