Force-velocity relationship during isometric and isotonic fatiguing contractions

Fatiguing contractions change the force-velocity relationship, but assessment of this relationship in fatigue has usually been obtained after isometric contractions. We studied fatigue caused by isometric or isotonic contractions, by assessment of the force-velocity relationship while the contractions maintaining fatigue were continued. This approach allowed determination of the force-velocity relationship during a steady condition of fatigue. We used the in situ rat medial gastrocnemius muscle, a physiologically relevant preparation. Intermittent (1/s) stimulation at 170 Hz for 100 ms resulted in decreased isometric force to similar to 35% of initial or decreased peak velocity of shortening in dynamic contractions to similar to 45% of initial. Dynamic contractions resulted in a transient initial increase in velocity, followed by a rapid decline until a reasonably steady level was maintained. Data were fit to the classic Hill equation for determination of the force-velocity relationship. Isometric and dynamic contractions resulted in similar decreases in maximal isometric force and peak power. Only V-max was different between the types of contraction (P < 0.005) with greater decrease in V-max during isotonic contractions to 171.7 +/- 7.3 mm/s than during isometric contractions to 208.8 mm/s. Curvature indicated by a/Po (constants from fit to Hill equation) changed from 0.45 +/- 0.04 to 0.71 +/- 0.11 during isometric contractions and from 0.51 +/- 0.04 to 0.85 +/- 0.18 during isotonic contractions. Recovery was incomplete 45 min after stopping the intermittent contractions. At this time, recovery of low-frequency isometric force was substantially less after isometric contractions, implicating force during intermittent contractions as a determining factor with this measure of fatigue. NEW & NOTEWORTHY The force-velocity relationship was captured while fatigue was maintained at a constant level during isometric and dynamic contractions. The curvature of the force-velocity relationship was less curved during fatigue than prefatigued, but within 45 min this recovered. Low-frequency fatigue persisted with greater depression of low-frequency force after isometric contractions, possibly because of higher force contractions during intermittent contractions.