Is Fatigue-induced Loss Of Power Associated With a Change In The Curvature Of The Force-velocity Relation?

Abstract

In skeletal muscles, the ability to generate power is reduced during fatigue. Maximal power (Pmax) is determined by the force-velocity relationship, which can be closely approximated by the Hill equation, containing three key parameters: 1) Maximal isometric force (Fmax) 2) Maximum contraction velocity (Vmax) and 3) Force-velocity curvature. PURPOSE: To investigate the possible association between the fatigue-induced loss of power and changes in curvature of the force-velocity relation in muscles with different fiber type composition. METHODS: The force-velocity relationship was measured before and during development of fatigue in 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 for 0.75 s (soleus) or at 150 Hz for 0.2 s (EDL) to obtain serial concentric contractions leading to fatigue. The Force-velocity relationship was measured at different stages of fatigue and after 1 hour of recovery. 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). RESULTS: At the end of the fatiguing protocol, maximal power had decreased by 58 ± 5% (soleus) and 69 ± 4% (EDL) compared to initial values in non-fatigued muscles. Curvature increased in both muscle types as judged from the decrease in a/F0 of 81 ± 20% (soleus) and by 31 ± 12% (EDL). At the end of the fatiguing protocol, the calculated contributions to the total loss of Pmax was 25 ± 10% from Fmax, 17 ± 3% from Vmax and 58 ± 14% from a/F0 (soleus) and 64 ± 6% from Fmax, 16 ± 3% from Vmax and 20 ± 9% from a/F0 (EDL). Complete recovery of a/F0 was observed following one hour of rest in both muscle types. CONCLUSIONS: Increased curvature of the force-velocity relationship occurs during fatigue in both slow and fast twitch muscles. Particularly in slow-twitch muscles, this increase in curvature is strongly associated with fatigue-induced loss of power.