Force/velocity curves of fast oxidative and fast glycolytic parts of rat medial gastrocnemius muscle vary for concentric but not eccentric activity.

The purpose of this study was to compare the force exerted by the rat medial gastrocnemius (GM) muscle with either fast oxidative or fast glycolytic parts active during concentric and eccentric contractions at different velocities. The proximal end of the GM contains mainly fast oxidative fibres and the distal end predominantly fast glycolytic fibres. Different parts of GM were activated by selective stimulation of nerve branches. Fast oxidative or fast glycolytic muscle parts of anaesthetised male Wistar rats were activated maximally. After assessment of concentric force/velocity (F/v) relations (n=11), some of the muscles were subjected to a fatiguing series of isometric contractions (n=5). Fast oxidative muscle parts showed a significantly lower mean (+/-SD) maximal power output (P(max) 0.12+/-0.06 W) and fatigability than fast glycolytic muscle parts (P(max) 0.20+/-0.06 W). The remaining muscles performed eccentric contractions. The eccentric F/v curves were not significantly different for fast oxidative and fast glycolytic muscle parts (n=6). Maximum eccentric force relative to the maximum isometric force (157+/-3% and 153+/-6% respectively,P=0.99) was reached at a velocity of 60 mm s(-1). It is concluded that eccentric F/v relations of rat GM with either fast oxidative or fast glycolytic parts active are very similar despite the differences in the concentric F/v relations.