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. 2016 Oct 25;10:543.
doi: 10.3389/fnhum.2016.00543. eCollection 2016.

Changes in Corticospinal and Spinal Excitability to the Biceps Brachii With a Neutral vs. Pronated Handgrip Position Differ Between Arm Cycling and Tonic Elbow Flexion

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Free PMC article

Changes in Corticospinal and Spinal Excitability to the Biceps Brachii With a Neutral vs. Pronated Handgrip Position Differ Between Arm Cycling and Tonic Elbow Flexion

Davis A Forman et al. Front Hum Neurosci. .
Free PMC article

Abstract

The purpose of this study was to examine the influence of neutral and pronated handgrip positions on corticospinal excitability to the biceps brachii during arm cycling. Corticospinal and spinal excitability were assessed using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation (TMS) and cervicomedullary-evoked potentials (CMEPs) elicited via transmastoid electrical stimulation (TMES), respectively. Participants were seated upright in front on arm cycle ergometer. Responses were recorded from the biceps brachii at two different crank positions (6 and 12 o'clock positions relative to a clock face) while arm cycling with neutral and pronated handgrip positions. Responses were also elicited during tonic elbow flexion to compare/contrast the results to a non-rhythmic motor output. MEP and CMEP amplitudes were significantly larger at the 6 o'clock position while arm cycling with a neutral handgrip position compared to pronated (45.6 and 29.9%, respectively). There were no differences in MEP and CMEP amplitudes at the 12 o'clock position for either handgrip position. For the tonic contractions, MEPs were significantly larger with a neutral vs. pronated handgrip position (32.6% greater) while there were no difference in CMEPs. Corticospinal excitability was higher with a neutral handgrip position for both arm cycling and tonic elbow flexion. While spinal excitability was also higher with a neutral handgrip position during arm cycling, no difference was observed during tonic elbow flexion. These findings suggest that not only is corticospinal excitability to the biceps brachii modulated at both the supraspinal and spinal level, but that it is influenced differently between rhythmic arm cycling and tonic elbow flexion.

Keywords: CMEP; MEP; cycling; isometric; neutral; pronated; transcranial; transmastoid.

Figures

Figure 1
Figure 1
(A) Example of the experimental setup for Experiment 1. Participants were seated with their shoulders approximately in-line with the crank shaft of the ergometer while arm cycling at a constant cadence of 60 rpm and workload of 5% of their own peak power. Measurements were taken at the 6 o’clock (shown here) and the 12 o’clock position of the dominant arm with both neutral handgrip and pronated handgrip (shown here) positions. (B) Rectified electromyography (EMG) values for the biceps (black, solid trace) and triceps brachii (gray, solid trace) of a single participant throughout a single revolution of arm cycling. In this example, eight frames without stimulations were rectified and averaged over a 1 s window. Cycling cadence was set at a constant pace of 60 rpm (1 s representing one full revolution). The black lines denote the 6 and 12 o’clock positions. (C,D) Example of the experimental setup for Experiment 2. Participants were seated in front of a custom built apparatus and performed tonic, elbow flexions against a stationary handle. The handle could be adjusted to accommodate either a pronated handgrip (shown in C) or neutral handgrip (shown in D).
Figure 2
Figure 2
Average traces of eight motor evoked potentials (MEPs) and eight cervicomedullary-evoked potentials (CMEPs) of the biceps brachii elicited during arm cycling at the two crank positions (6 and 12 o’clock) with a neutral handgrip (solid, black lines) and a pronated handgrip (dashed, gray lines) for a single, representative individual. Average traces are normalized to the average of three Mwaves elicited during the same, experimental conditions.
Figure 3
Figure 3
Group data (mean ± SE, n = 10) during arm cycling for (A) MEP amplitudes of the biceps brachii, (B) biceps brachii pre-stimulus EMG prior to transcranial magnetic stimulation (TMS), and (C) triceps brachii pre-stimulus EMG prior to TMS. Black bars correspond to measures taken during arm cycling with a neutral handgrip while white bars correspond to a pronated handgrip. MEP amplitudes are shown relative to the Mmax taken during the same experimental condition. EMG is normalized to the maximum EMG found during the 10 s, maximal arm-cycling sprint. Asterisks denote a significant difference (P < 0.05) between the two handgrip positions.
Figure 4
Figure 4
Group data (mean ± SE, n = 8) during arm cycling for (A) CMEP amplitudes of the biceps brachii, (B) biceps brachii pre-stimulus EMG prior to transmastoid electrical stimulation (TMES), and (C) triceps brachii pre-stimulus EMG prior to TMS. Black bars correspond to measures taken during arm cycling with a neutral handgrip while white bars correspond to a pronated handgrip. MEP amplitudes are shown relative to the Mmax taken during the same experimental condition. EMG is normalized to the maximum EMG found during the 10 s, maximal arm-cycling sprint. Asterisks denote a significant difference (P < 0.05) between the two handgrip positions.
Figure 5
Figure 5
Average traces of eight MEPs and eight CMEPs of the biceps brachii elicited during tonic elbow flexion with a neutral handgrip (solid, black lines) and a pronated handgrip (dashed, gray lines) for a single, representative individual. Average traces are normalized to the average of three Mwaves elicited during the same, experimental conditions.
Figure 6
Figure 6
Group data (mean ± SE, n = 9) during tonic elbow flexion for (A) MEP amplitudes of the biceps brachii, (B) biceps brachii pre-stimulus EMG prior to TMS, and (C) triceps brachii pre-stimulus EMG prior to TMS, as well as group data (mean ± SE, n = 7) during tonic elbow flexion for (D) CMEP amplitudes of the biceps brachii, (E) biceps brachii pre-stimulus EMG prior to TMES, and (F) triceps brachii pre-stimulus EMG prior to TMES. MEP and CMEP amplitudes of the biceps brachii are expressed relative to the Mmax taken during the same experimental conditions while EMG is normalized to the average EMG found during the 30 s, arm-cycling trial against a constant load of 25W and a constant cadence of 60 rpm. Asterisks denote a significant difference (P < 0.05) between handgrip positions.

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