Corticospinal-Evoked Responses from the Biceps Brachii during Arm Cycling across Multiple Power Outputs

Abstract

Background: We examined corticospinal and spinal excitability across multiple power outputs during arm cycling using a weak and strong stimulus intensity. Methods: We elicited motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) in the biceps brachii using magnetic stimulation over the motor cortex and electrical stimulation of corticospinal axons during arm cycling at six different power outputs (i.e., 25, 50, 100, 150, 200 and 250 W) and two stimulation intensities (i.e., weak vs. strong). Results: In general, biceps brachii MEP and CMEP amplitudes (normalized to maximal M-wave (Mmax)) followed a similar pattern of modulation with increases in cycling intensity at both stimulation strengths. Specifically, MEP and CMEP amplitudes increased up until ~150 W and ~100 W when the weak and strong stimulations were used, respectively. Further increases in cycling intensity revealed no changes on MEP or CMEP amplitudes for either stimulation strength. Conclusions: In general, MEPs and CMEPs changed in a similar manner, suggesting that increases and subsequent plateaus in overall excitability are likely mediated by spinal factors. Interestingly, however, MEP amplitudes were disproportionately larger than CMEP amplitudes as power output increased, despite being initially matched in amplitude, particularly with strong stimulation. This suggests that supraspinal excitability is enhanced to a larger degree than spinal excitability as the power output of arm cycling increases.

Neuromuscular Mechanisms Underlying Changes in Force Production during an Attentional Focus Task

“…The MEP/CMEP would indicate the level of supraspinal or spinal excitability. Levels above 1 are indicative of higher supraspinal excitability compared to spinal excitability [30]. participants completed two blocks of three externally cued condition and two blocks of internally cued condition which were also randomized.…”

“…The MEP/CMEP would indicate the level of supraspinal or spinal excitability. Levels above 1 are indicative of higher supraspinal excitability compared to spinal excitability [30].…”

“…The vertex was located by marking the measured halfway points between the nasion to inion and tragus to tragus. The coil was flipped to ensure the induced current flow was anterior to posterior in the target motor cortex (A side up for right side, B side up for left) to activate the dominant biceps brachii [29][30][31]. The optimum stimulation site was determined by initially placing the coil over the vertex and slightly moving the coil towards the non-dominant hemisphere in three directions, i.e., anteriorly, posteriorly, and laterally.…”

Moving forward: methodological considerations for assessing corticospinal excitability during rhythmic motor output in humans

“…RMT is defined as the lowest stimulus intensity that is required to elicit a discernible response from a resting target muscle in at least 50% of the trials (3,53,54,56). The proposed standard to ensure RMT determination is to evoke a response in at least 5 out of 10 trials (54); however, in practice, this criterion varies and can range, for example, anywhere from 3 out of 5 (55,57,58), to 4 out of 8 (26,33,59,60), to 5 out of 10 trials (29,(61)(62)(63)(64)(65).…”

“…We also know that corticospinal excitability is higher during a tonic muscle contraction than at rest (77) and that corticospinal excitability is different during a locomotor-like task than that observed during an intensity-matched tonic contraction (25,59,69,73). Furthermore, as the intensity of the motor output is increased, corticospinal excitability also increases, regardless of whether the task is tonic (77)(78)(79) or dynamic (26,60,63,69,80). Although many factors may underlie these task-dependent changes in corticospinal excitability, changes in spinal motoneuron excitability are at least partially involved (25,26,69,81).…”