Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Kinesiology

Supervisor

Rice, Charles L.

Abstract

Corticospinal excitability (CSE) increases prior to a voluntary contraction; however the relative contributions of cortical and spinal mechanisms are not well understood. It is also unknown whether the intended voluntary contractile rate affects the change in CSE. Therefore, the purpose was to assess cortical and spinal contributions to premotor CSE prior to fast (ballistic) and slow (ramp) contractions.

Eighteen young, healthy participants (9F) completed isometric elbow flexion contractions targeting 50% maximal voluntary contraction (MVC) force, at fast (as fast as possible) and slow (25% MVC/s) contractile rates. Participants were cued to contract with warning (red) and “GO” (green) visual signals. Magnetic and electric stimulations were applied to elicit motor evoked potentials (MEPs), cervicomedullary evoked potentials (CMEPs), and M-waves, respectively, in the surface electromyogram (EMG) recorded over the biceps brachii. MEPs and CMEPs were collected at 0, 25, 50 and 75% of premotor reaction time (RT) following the “GO” signal and compared to a resting baseline.

MEP amplitude demonstrated a 45% increase from baseline at 75% RT (p = 0.009). CMEP amplitude was already 49% larger than baseline at 0% RT (p closer to EMG onset of the contraction. However, there were no differences in MEP and CMEP amplitudes when compared between contractile conditions (all p>0.05). Normalized to the CMEP, there was no difference in MEP amplitude from baseline in either contractile condition (all p>0.05).

These results indicate that increases in premotor CSE are predominantly spinally mediated. Furthermore, the increase in premotor CSE is not influenced by the intended voluntary contractile rate.

Summary for Lay Audience

In preparation for a voluntary muscular contraction, there is an increase of excitability in connections made between the brain, spinal cord, and active muscles. However, it is not well understood whether this increase of excitability arises at the level of the brain or spinal cord. Additionally, it is unknown whether the rate of contraction to be performed (fast or slow) influences preparatory activity in the brain and spinal cord. This study investigated the factors influencing increased excitability before voluntary muscle contractions, and whether brain or spinal cord mechanisms play a more significant role when compared between fast and slow contractions.

Eighteen healthy (9 female) young adults participated in the study, completing elbow flexion contractions at both fast (as fast as possible) and slow ramp (2s) rates. Participants received visual cues indicating when to begin their contractions. Magnetic and electrical stimulation were used to elicit responses recorded from surface electrodes placed on the skin over the biceps brachii muscle. Magnetic stimulation was used to excite the motor cortex of the brain resulting in motor evoked potentials (MEP), whereas electrical stimulation was used to excite the spinal cord and musculocutaneous nerve, resulting in cervicomedullary evoked potentials (CMEP) and M-waves in the biceps muscle.

Results showed that the brain evoked response (MEP) increased from baseline before voluntary contractions, however this excitatory response was not different when compared between fast and slow contractions. The CMEP also increased significantly before the onset of contraction, indicating greater excitability at the spinal level, but this response was not different between fast and slow contractions either. When comparing MEP and CMEP responses, these findings revealed that spinal mechanisms play a more significant role to increase excitability in preparation for voluntary contractions. Although, this increase of excitability is not dependent on the rate of voluntary contraction to be performed.

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