Muscle length and joint angle influence spinal but not corticospinal excitability to the biceps brachii across forearm postures

Abstract

Forearm rotation (supination/pronation) alters corticospinal excitability to the biceps brachii, but it is unclear whether corticospinal excitability is influenced by joint angle, muscle length, or both. Thus the purpose of this study was to separately examine elbow joint angle and muscle length on corticospinal excitability. Corticospinal excitability to the biceps and triceps brachii was measured using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation. Spinal excitability was measured using cervicomedullary motor evoked potentials (CMEPs) elicited via transmastoid electrical stimulation. Elbow angles were manipulated with a fixed biceps brachii muscle length (and vice versa) across five unique postures: 1) forearm neutral, elbow flexion 90°; 2) forearm supinated, elbow flexion 90°; 3) forearm pronated, elbow flexion 90°; 4) forearm supinated, elbow flexion 78°; and 5) forearm pronated, elbow flexion 113°. A musculoskeletal model determined biceps brachii muscle length for postures 1-3, and elbow joint angles (postures 4-5) were selected to maintain biceps length across forearm orientations. MEPs and CMEPs were elicited at rest and during an isometric contraction of 10% of maximal biceps muscle activity. At rest, MEP amplitudes to the biceps were largest during supination, which was independent of elbow joint angle. CMEP amplitudes were not different when the elbow was fixed at 90° but were largest in pronation when muscle length was controlled. During an isometric contraction, there were no significant differences across forearm postures for either MEP or CMEP amplitudes. These results highlight that elbow joint angle and biceps brachii muscle length can each independently influence spinal excitability. NEW & NOTEWORTHY Changes in upper limb posture can influence the responsiveness of the central nervous system to artificial stimulations. We established a novel approach integrating neurophysiology techniques with biomechanical modeling. Through this approach, the effects of elbow joint angle and biceps brachii muscle length on corticospinal and spinal excitability were assessed. We demonstrate that spinal excitability is uniquely influenced by joint angle and muscle length, and this highlights the importance of accounting for muscle length in neurophysiological studies.

Keywords: biceps brachii; forearm posture; joint angle; muscle length; spinal excitability.

Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Figures

Fig. 1.

A: experimental setup. B: musculoskeletal model and corresponding elbow angles used to produce a constant biceps brachii muscle length across forearm rotations.

Fig. 2.

Average of 10 motor evoked potentials (MEPs) and 10 cervicomedullary motor evoked potentials (CMEPs) of the biceps and triceps brachii during rest of a representative individual. Black lines, dark gray lines, and light gray lines correspond to supinated, neutral, and pronated measurements, respectively. Traces are normalized to the average of 3 M waves elicited during the same conditions. Mmax, maximum M wave.

Fig. 3.

Group data (means ± SE, n = 10) during rest for motor evoked potential (MEP) amplitudes of the biceps (A) and triceps brachii (B) and group data (means ± SE, n = 5) for cervicomedullary motor evoked potential (CMEP) amplitudes of the biceps (C) and triceps brachii (D). Black, gray, and white bars correspond to supinated, neutral, and pronated forearm postures, respectively. MEP and CMEP amplitudes are shown relative to the maximum M wave (Mmax) taken during the same conditions. *P < 0.05.

Fig. 4.

Average of 10 motor evoked potentials (MEPs) and 10 cervicomedullary motor evoked potentials (CMEPs) of the biceps and triceps brachii during isometric elbow flexion of a representative individual. Black lines, dark gray lines, and light gray lines correspond to supinated, neutral, and pronated measurements, respectively. Traces are normalized to the average of 3 M waves elicited during the same conditions. Mmax, maximum M wave.

Fig. 5.

Group data (means ± SE, n = 10) during isometric elbow flexion for motor evoked potential (MEP) amplitudes of the biceps (A) and triceps brachii (B) and prestimulus (Pre-stim) electromyography (EMG) before transcranial magnetic stimulation for the biceps (C) and triceps brachii (D). Black, gray, and white bars correspond to supinated, neutral, and pronated forearm postures, respectively. MEP amplitudes are shown relative to the maximum M wave (Mmax) taken during the same conditions. EMG is normalized to the maximum EMG found during muscle-specific maximal voluntary contractions. *P < 0.05 between forearm postures.

Fig. 6.

Group data (means ± SE, n = 5) during isometric elbow flexion for cervicomedullary motor evoked potential (CMEP) amplitudes of the biceps (A) and triceps brachii (B) and prestimulus (Pre-stim) electromyography (EMG) before transmastoid electrical stimulation for C) the biceps and D) triceps brachii. Black, gray, and white bars correspond to supinated, neutral, and pronated forearm postures, respectively. CMEP amplitudes are shown relative to the maximum M wave (Mmax) taken during the same conditions. EMG is normalized to the maximum EMG found during muscle-specific maximal voluntary contractions.