title: Between-muscle differences in the adaptation to experimental pain.
abstract: This study aimed to determine whether muscle stress (force per unit area) can be redistributed between individual heads of the quadriceps muscle when pain is induced into one of these heads. Elastography was used to measure muscle shear elastic modulus (an index of muscle stress). Electromyography (EMG) was recorded from vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF). In experiment I (n = 20), participants matched a knee extension force, and thus any reduction of stress within the painful muscle would require compensation by other muscles. In experiment II (n = 13), participants matched VL EMG amplitude and were free to vary external force such that intermuscle compensation would be unnecessary to maintain the experimental task. In experiments I and II, pain was induced by injection of hypertonic saline into VM or RF. Experiment III aimed to establish whether voluntary drive to the individual muscles could be controlled independently. Participants (n = 13) were asked to voluntarily reduce activation of VM or RF while maintaining knee extension force. During VM pain, there was no change in shear elastic modulus (experiments I and II) or EMG amplitude of VM (experiment II). In contrast, RF pain was associated with a reduction in RF elastic modulus (experiments I and II: -8 to -17%) and EMG amplitude (experiment II). Participants could voluntarily reduce EMG amplitude of RF (-26%; P = 0.003) but not VM (experiment III). These results highlight between-muscle differences in adaptation to pain that might be explained by their function (monoarticular vs. biarticular) and/or the neurophysiological constraints associated to their activation.
authors.affiliation: The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia; University of Nantes, Laboratory "Motricité, Interactions, Performance" (EA 4334), UFR STAPS, F-44000, Nantes, France; and francois.hug@univ_nantes.fr.
authors.forename: François
authors.identifier: http://orcid.org/0000-0002-6432-558X
authors.initials: F
authors.lastname: Hug
authors.affiliation: The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia;
authors.forename: Paul W
authors.initials: PW
authors.lastname: Hodges
authors.affiliation: The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia;
authors.forename: Wolbert
authors.initials: W
authors.lastname: van den Hoorn
authors.affiliation: The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia; The University of Queensland, School of Biomedical Sciences, Brisbane, Australia.
authors.forename: Kylie
authors.initials: K
authors.lastname: Tucker
mesh_terms: D000222:Adaptation, Physiological; D000223:Adaptation, Psychological; D000328:Adult; D001696:Biomechanical Phenomena; D055119:Elastic Modulus; D054459:Elasticity Imaging Techniques; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D053580:Muscle Strength; D063806:Myalgia / Q000175:diagnosis / Q000503:physiopathology* / Q000523:psychology; D058765:Neurofeedback; D010147:Pain Measurement; D058748:Pain Perception; D017288:Pain Threshold; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000503:physiopathology*; D014836:Volition; D055815:Young Adult
keywords: elastography; electromyography; noxious stimulation; quadriceps; supersonic shear imaging