Fatigue in high- versus low-force voluntary and evoked contractions.

Low-frequency fatigue (LFF) is defined as a greater loss of force that occurs in during low versus high frequencies of stimulation. In order to determine which types of fatigue protocols are most likely to induce LFF, ten individuals participated in four different fatigue experiments which induced similar reductions in maximal force output as following: (1) 2-min intermittent high-frequency stimulation (40 Hz), (2) 4-min intermittent low-frequency stimulation (20 Hz), (3) sustained 100% maximal voluntary contraction (MVC), and (4) low-force voluntary contractions (20% MVC). Short (5s) trains of 10, 20, 30, 40, and 80 Hz were used to determine the force-versus-frequency relationships before and after the fatigue tasks. LFF was higher following the low-force voluntary contractions compared to the high-force voluntary and evoked contractions. The degree of LFF during the low-force voluntary contractions was most highly correlated to the duration of the fatigue task and to a lesser extent, to the decrease in maximal force output and the force-time integrals during the fatigue task.