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First published online December 15, 2004
Journal of Experimental Biology 208, 55-63 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01329

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

J. M. Rijkelijkhuizen^1,*, C. J. de Ruiter¹, P. A. Huijing^1,2 and A. de Haan^1,3

¹ Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands
² Integrated Biomedical Engineering for Restoration of Human Function, Instituut voor Biomedische Technologie, Faculteit Construerende Wetenschappen, Universiteit Twente, Postbus 217, 7500 AE Enschede, The Netherlands
³ Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Crewe & Alsager Faculty, Cheshire ST7 2HL, UK

^* Author for correspondence (e-mail: j.rijkelijkhuizen{at}vumc.nl)

Accepted 11 October 2004

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_0,200Hz, (17.0±1.4%, mean ± S.E.M.), force loss at 60 Hz-stimulation was maximal near L_0,200Hz (55.1±4.3% at L_0,200Hz-1 mm). When the muscle was potentiated, force loss at 60 Hz-stimulation was maximal at short muscle length: L_0,200Hz-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_0,200Hz; the 60:200 Hz force ratio had decreased to 54.6±5.9% of the pre-exercise ratio at L_0,200Hz-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.

Key words: force-length characteristics, maximal stimulation, submaximal stimulation.

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