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 submaximal isometric torque makes it difficult to isolate fatigue-related changes to neural excitability because evoked electromyography (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 are 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. Furthermore, participants reported higher ratings of perceived effort scores by 6 min into the matched-torque compared with matched-EMG contraction. These findings indicate that, during a matched-torque contraction, the nervous system enhanced common oscillatory activity to continue the task, but this did not prevent degradation of performance (torque steadiness). In contrast, when motoneuron pool output was maintained, other neural strategies were used to preserve muscle output.NEW & NOTEWORTHY Sustained electromyography (EMG) tasks offer valuable insights into fatigue-related changes to neural excitability, yet nothing is known about communication between the cortex and active muscles. A 10-min matched-EMG contraction reduced maximal torque-generating capacity but did not alter corticomuscular coherence (CMC), intermuscular coherence (IMC), or task performance (EMG steadiness). In contrast, a matched-torque contraction increased CMC and IMC yet further impaired maximal torque and compromised torque steadiness, which reveals unique neural strategies for the two tasks.