Oscillatory drive is increased and steadiness is impaired when torque but not EMG is matched during a fatiguing contraction.

Abstract

The increasing descending drive needed to sustain a submaximal isometric torque makes it difficult to isolate fatigue-related changes to neural excitability because evoked electromyographic (EMG) responses are influenced by the relative activation of the motoneuron pool. Hence, it is becoming increasingly common to investigate fatigue using a sustained contraction with maintained output from the motoneuron pool; i.e., matched surface EMG. Although this approach controls motoneuron pool output, it is unknown how cortical contributions to ongoing muscle activity or common modulation between muscles is altered during a matched-EMG contraction. During separate visits, 16 participants performed a sustained 10-min isometric elbow flexion contraction at 20% maximal voluntary contraction (MVC) torque or the level of integrated biceps brachii EMG recorded at 20% MVC torque. Electroencephalographic and surface EMG recordings were obtained from the sensorimotor area and biceps and triceps brachii, respectively. The matched-torque contraction caused increased corticomuscular coherence for biceps brachii (~75%) and intermuscular coherence (~97%), but reduced MVC torque (~33%), voluntary activation (~9%), and torque steadiness (~83%). In contrast, the matched-EMG contraction caused reduced MVC torque (~21%), with no change in coherence, voluntary activation, or EMG steadiness. Further, participants reported higher ratings of perceived effort scores by 6min into the matched-torque compared to matched-EMG contraction. These findings indicate that, during a matched-torque contraction, the nervous system enhances common oscillatory activity to continue the task, but this does not prevent degradation of performance (torque steadiness). In contrast, when motoneuron pool output was clamped, other neural strategies are used to maintain muscle output.