The effect of experimental and clinical musculoskeletal pain on spinal and supraspinal projections to motoneurons and motor unit properties in humans: a systematic review.

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Numerous studies have examined the influence of pain on spinal reflex excitability, motor unit behaviour and corticospinal excitability. Nevertheless, there are inconsistencies in the conclusions made. This systematic review sought to understand the effect of pain on spinal and supraspinal projections to motoneurons and motor unit properties by examining the influence of clinical or experimental pain on the following three domains: H reflex, corticospinal excitability and motor unit properties.MeSH terms and preselected keywords relating to the H reflex, motor evoked potentials and motor unit decomposition in chronic and experimental pain were used to perform a systematic literature search using CINAHL, EMBASE, Web of Science, Medline, Google Scholar, and Scopus databases. Two independent reviewers screened papers for inclusion and assessed the methodological quality using a modified Downs and Black risk of bias tool; a narrative synthesis and three meta-analyses were performed.Sixty-one studies were included and 17 different outcome variables were assessed across the three domains. Both experimental and clinical pain has no major influence on measures of the H reflex whereas experimental and clinical pain appeared to have differing effects on corticospinal excitability. Experimental pain consistently reduced motor unit discharge rate, a finding which was not consistent with data obtained from patients. The results indicate that when in tonic pain, induced via experimental pain models, inhibitory effects on motoneuron behaviour were evident. However, in chronic clinical pain populations, more varied responses were evident likely reflecting individual adaptations to chronic symptoms.

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Authors: A Sanderson, S F Wang, E Elgueta Cancino, E Martinez Valdes, E Sanchis-Sanchez, B Liew, D Falla