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The Journal of Neuroscience, September 2, 2009, 29(35):10820-10826; doi:10.1523/JNEUROSCI.5211-08.2009

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Behavioral/Systems/Cognitive
Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

Kylie Tucker,¹ Jane Butler,³ Thomas Graven-Nielsen,⁴ Stephan Riek,² and Paul Hodges¹

Schools of ¹Health and Rehabilitation Sciences and ²Human Movement Studies, The University of Queensland, St. Lucia, Queensland 4029, Australia, ³Prince of Wales Medical Research Institute, Randwick, New South Wales 2031, Australia, and ⁴Center for Sensory-Motor Interaction, Aalborg University, 9220 Aalborg, Denmark

Correspondence should be addressed to Dr. Paul Hodges, National Health and Medical Research Council Centre of Clinical Research Excellence in Spinal Pain, Injury, and Health, School of Health and Rehabilitation Sciences, The University of Queensland, St. Lucia, QLD 4029, Australia. Email: p.hodges{at}uq.edu.au

Muscle pain is associated with decreased motor unit discharge rate during constant force contractions. As discharge rate is a determinant of force, other adaptations in strategy must explain force maintenance during pain. Our aim was to determine whether motor unit recruitment strategies are altered during pain to maintain force despite reduced discharge rate. Motor unit discharge behavior was recorded in two muscles, one with (quadriceps) and one without [flexor pollicis longus (FPL)] synergists. Motor units were recruited during matched low-force contractions with and without experimentally induced pain, and at higher force without pain. A total of 52 and 34 units were recorded in quadriceps and FPL, respectively, during low-force contractions with and without pain. Of these, 20 quadriceps and 9 FPL units were identified during both trials. The discharge rate of these units reduced during pain in both muscles [quadriceps: 8.7 (1.5) to 7.5 (1.3) Hz, p < 0.001; FPL: 11.9 (1.5) to 10.0 (1.7) Hz, p < 0.001]. All remaining units discharged only with or without pain, but not in both conditions. Only one-third of the additional units recruited during pain (quadriceps n = 7/19, FPL n = 3/15) were those expected given orderly recruitment of the motor unit pool as determined during higher-force contractions. We conclude that reduced motor unit discharge rate with pain is accompanied by changes in the population of units used to maintain force. The recruitment of new units is partly inconsistent with generalized inhibition of the motoneuron pool predicted by the "pain adaptation" theory, and provides the basis for a new mechanism of motor adaptation with pain.

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Received Oct. 28, 2008; revised April 6, 2009; accepted April 16, 2009.

Correspondence should be addressed to Dr. Paul Hodges, National Health and Medical Research Council Centre of Clinical Research Excellence in Spinal Pain, Injury, and Health, School of Health and Rehabilitation Sciences, The University of Queensland, St. Lucia, QLD 4029, Australia. Email: p.hodges{at}uq.edu.au

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