Fatiguing stimulation increases curvature of the force-velocity relation in isolated fast-twitch and slow-twitch rat muscles

In skeletal muscles the ability to generate power is reduced during fatigue. Maximal power can in isolated muscles be calculated from the force-velocity relationship. This relationship is well described by the Hill equation, which contains three parameters: 1) Maximal isometric force, 2) maximum contraction velocity, and 3) curvature. Here, we investigated the hypothesis that a fatigue-induced loss of power will be associated with changes in curvature of the force-velocity curve in slow-twitch muscles but not in fast-twitch muscles during the development of fatigue. Isolated rat soleus (slow-twitch) and EDL (fast-twitch) muscles were incubated in Krebs-Ringer solution at 30°C and stimulated electrically at 60 Hz (soleus) and 150 Hz (EDL) to perform a series of concentric contractions to fatigue. Force-velocity data were fitted to the Hill equation, and curvature was determined as the ratio of the curve parameters a/F0 (inversely related to curvature). At the end of the fatiguing protocol, maximal power decreased by 58±5% (soleus) and 69±4% (EDL) compared to initial values in non-fatigued muscles. At the end of the fatiguing sequence, curvature increased as judged from the decrease in a/F0 by 81±20% in soleus and by 31±12% in EDL. However, during the initial phases of fatiguing stimulation we observed a small decrease in curvature in EDL muscles, but not soleus, which may be a result of post-activation potentiation. In conclusion, fatigue-induced loss of power is strongly associated with an increased curvature of the force-velocity relationship, particularly in slow-twitch muscles.