Evaluation of Motor Unit Abnormalities After Experimentally Induced Sensitization Using Capsaicin: A Randomized, Double-Blinded, Placebo-Controlled Study

Central sensitization describes a state of neuronal hyper-excitability in the central nervous system that may occur due to malfunction of spinal and supraspinal pain facilitatory and inhibitory circuits resulting in amplification of somatosensorial responses. Beyond somatosensorial changes, alteration in motor function can also be present with pain, and may be a reflex of neuromuscular function impairment. A normal afferent input and normal central processing circuitry is essential to deliver normal efferent output. However, the influence of the changes that occur within the dorsal horn on the ventral horn remain largely ill defined.

Motor unit assessment is crucial in evaluating diseases and abnormalities within the ventral horn. Activity of the ventral horn, where anterior horn cells reside, is very important for motor unit activation. Surface EMG (sEMG) and intramuscular EMG (iEMG) can be used to assess the neural drive to muscles, by recording motor units to understand the effects of central sensitization on motor control and the ventral horn. Based on Henneman's size principle, motor units should be recruited in the same order, with smaller units being recruited first. This principle presents an opportunity to investigate if central sensitization creates abnormalities on the motor unit level.

Previously, central sensitization has been induced in healthy subjects to examine its neurophysiological effects via capsaicin. Capsaicin, a chilli pepper extract, can be used to effectively induce experimental transient states of central sensitization. The presence of expanded sensorial responses and the involvement of the spinal nociceptive system post capsaicin have been largely tested by means of quantitative sensory testing methods and electromyography (EMG).

Despite the usefulness of experimental capsaicin to better understand the sensorial abnormalities, its impact on motor function and motor unit recruitment are lesser studied. Evidence suggests that nociceptive input by peripheral capsaicin exerts a centrally-mediated inhibitory effect on motor function. A decrease in root mean squared (RMS) amplitude during exercise at the time of peak sensitization was measured by needle EMG. However, the effect that capsaicin-induced sensitization has on individual motor units or on their recruitment patterns has not been previously examined.

The purpose of this study was to determine whether topical capsaicin-induced sensitization has any influence on ventral horn activity. We hypothesize that capsaicin induces a change in individual motor unit activity, as well as the recruitment pattern of many motor units, and may affect motor unit activity at different segmental levels from the level of capsaicin application.