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.
Elbow angle modulates corticospinal excitability to the resting biceps brachii at both spinal and supraspinal levels.Exp Physiol. 2019 Apr;104(4):546-555. doi: 10.1113/EP087472. Epub 2019 Feb 21. Exp Physiol. 2019. PMID: 30690803
Arm posture-dependent changes in corticospinal excitability are largely spinal in origin.J Neurophysiol. 2016 Apr;115(4):2076-82. doi: 10.1152/jn.00885.2015. Epub 2016 Feb 10. J Neurophysiol. 2016. PMID: 26864764 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.Front Hum Neurosci. 2016 Oct 25;10:543. doi: 10.3389/fnhum.2016.00543. eCollection 2016. Front Hum Neurosci. 2016. PMID: 27826236 Free PMC article.
Corticospinal excitability of the biceps brachii is shoulder position dependent.J Neurophysiol. 2017 Dec 1;118(6):3242-3251. doi: 10.1152/jn.00527.2017. Epub 2017 Aug 30. J Neurophysiol. 2017. PMID: 28855295
Phase- and Workload-Dependent Changes in Corticospinal Excitability to the Biceps and Triceps Brachii during Arm Cycling.Brain Sci. 2016 Dec 15;6(4):60. doi: 10.3390/brainsci6040060. Brain Sci. 2016. PMID: 27983685 Free PMC article.