Perception of effort changes following an isometric fatiguing exercise of elbow flexors

The way psychometric and neurophysiological measurements of fatigue are connected is not well understood. Thus, the time course of perceived effort changes due to fatigue, as well as the peripheral and central neurophysiological changes accompanying fatigue, were evaluated. Twelve healthy participants (35 ± 9 years old) undertook 10 min intermittent isometric fatiguing exercise of elbow flexors at 50% of maximum voluntary contraction (MVC). Perceived effort ratings, using the 0-10 numeric rating scale (NRS), were recorded at midrange of MVC. Single pulse TMS of the left motor cortex and electrical stimulation over the biceps muscle was used for the assessment of voluntary activation and peripheral fatigue. The fatiguing exercise caused a 44% reduction in the MVC (p < .001) accompanied by an 18% nonsignificant reduction of the biceps MEP amplitude. The resting twitch force decreased (p < .001) while the superimposed twitches increased (p < .001) causing a decrease (19%) of the voluntary activation (p < .001). The perceived effort ratings increased by 1 point at 30%, by 2 points at 50% MVC respectively on the NRS (p < .001) and were accompanied by an increase in mean biceps EMG. A substantial role of the perceived effort in the voluntary motor control system was revealed.