The effect of low-frequency fatigue on the torque-velocity relationship in human quadriceps.

Low-frequency fatigue (LFF) is usually defined as the decline in low:high-frequency force of electrically evoked isometric muscle contractions. The influence of LFF on dynamic muscle function is not well known. Our aim was to establish a method for assessing LFF using both isometric and concentric muscle contractions using electrical stimulation at different frequencies in humans. Sixteen participants underwent a series of electrically evoked knee extensions in an isokinetic dynamometer to establish torque-velocity relationships at 15 and 50 Hz. Hereafter, fatigue was induced by 10 x 5 maximal voluntary dynamic knee extensions. After 30 minutes of rest, torque-velocity tests were repeated. Maximal contraction velocity (Vmax ) and maximal contraction power (Pmax ) were estimated using Hill's force-velocity equation. 15:50 Hz ratios were calculated for Fmax , Vmax , and Pmax as markers of LFF. Fmax decreased by 40 % at 15 Hz (p=0.001) and by 15 % at 50 Hz (p=0.001) in the fatigued state. 15 and 50 Hz Vmax was unchanged at -2 % (p=0.349) and +3 % (p=0.763), and 15 and 50 Hz Pmax decreased by 30 % (p=0.004) and 10 % (p=0.008). The 15:50 Hz Fmax ratio decreased by 31 % (p<0.001), indicating LFF. The Pmax ratio also decreased by 23 % (p=0.002), while the Vmax ratio was unaffected (p=0.313). Electrically evoked torque-velocity relationships at 15 and 50 Hz yield strong correlations with the Hill equation. Fatigue decreases Fmax and Pmax at both stimulation frequencies, while Vmax is unaffected. LFF influences both isometric torque and power production.