Quantitative Response of Healthy Muscle Following the Induction of Capsaicin

Detailed Summary

Central Sensitization is an aspect of chronic pain and is associated with over-excitability of the central nervous system. Central Sensitization normally presents itself with other chronic syndromes such as Myofascial Pain Syndrome, so it is difficult to separate the characteristics of the two. The purpose of this study is to determine whether sensitizing healthy muscle using capsaicin, a chilli pepper extract, induces a regional change in ultrasound texture features of the targeted muscle, or the muscles in that are in close proximity. This regional change will be accompanied by electromyography (EMG) recordings to confirm the presence of abnormality. Topical capsaicin and injectable capsaicin will be applied at three different concentrations, 0ug(placebo effect),50ug and 100ug. Research Questions: Does sensitization cause a significant regional change in the texture features of the targeted muscle or surrounding muscles? Is this regional change confirmed by abnormality of motor unit firing rates or EMG amplitudes? Is the response dose dependent? Does the induced central sensitization cause a change in force steadiness? Experimental Protocol: The physician on site will apply surface and intramuscular EMG sensors to the trapezius. A goniometer will be placed on the subjects arm to detect position. Following this, an ultrasound image of the trapezius, supraspinatus and infraspinatus will be taken.The participant will perform an increasing and decreasing contraction.The subject will then have capsaicin injected into their trapezius or applied onto their skin. The exercise and measurements will then be repeated. The ultrasound will be acquired by the Sonosite ultrasound system. The EMG data will be acquired by the Delsys Trigno EMG system for the surface EMG and the Cadwell Sierra Wave system for the intramuscular EMG. The data will be processed through Delsys EMGworks software and MATLAB. Texture features of the pre and post injection ultrasound images will be compared. Decomposition of motor units will allow for the analysis of motor unit action potential firing rate and amplitudes. Analysis of force steadiness will also be calculated via the goniometer. Overall, the findings from this study should present preliminary evidence to inform central sensitization's effects on regional muscle structure changes, functional changes and motor unit activity.

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