The influence of shoulder position on corticospinal excitability of the biceps brachii

Collins, Brandon (2017) The influence of shoulder position on corticospinal excitability of the biceps brachii. Masters thesis, Memorial University of Newfoundland.

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Abstract

The objective of this thesis was to examine corticospinal excitability (CSE) of the elbow flexors during two shoulder positions (0° shoulder flexion and 90° shoulder flexion) during rest and during an active state (10% maximum voluntary contraction (MVC)). Ten recreationally active males participated in two randomized experimental sessions (Day 1: n=10, Day 2: n=8) with 4 experimental conditions; 1) 0° shoulder flexion with biceps brachii at rest, 2) 0° shoulder flexion with biceps brachii at 10% MVC, 3) 90° shoulder flexion with biceps brachii at rest and 4) 90° shoulder flexion with biceps brachii at 10% MVC. Transcranial magnetic, transmastoid and Erb’s point stimulations were used to induce motor evoked potentials (MEPs), cervicomedullary MEP (CMEPs) and maximal muscle action potential (Mmax). All MEPs and CMEPs were normalized to Mmax. Mmax and CMEP amplitudes were position-dependent (0° versus 90°), whereas MEP amplitude was position dependent but differed (higher at rest and lower at 10% MVC) between the state at which it was recorded (rest versus active). At 0° compared to 90°, MEP/Mmax ratio was higher at rest but lower at 10% MVC. Whereas, CMEP/Mmax experienced no change at rest but lower at 10% MVC. Finally, MEP/CMEP ratios showed that starting in the 90° position ratios were lower at rest while higher at 10% MVC. On the other hand, starting in the 0° position MEP/CMEP ratios only changed at 10% MVC with the ratio being higher with the change in position. Results showed participants could produce more elbow flexor force in 0° shoulder flexion compared to 90° shoulder flexion. RMS EMG of the biceps brachii was higher and lower at rest and 10% MVC, respectively, at the 90° compared to 0° position. In conclusion, CSE of the biceps brachii is dependent on a change in shoulder position and the state it is recorded. In addition, it seems that there are several factors that play a role in the change of CSE such as: large changes in Mmax amplitudes, differences in biceps brachii RMS EMG and changes in cortical excitability.

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