Brain Advance Access originally published online on July 18, 2008
Brain 2008 131(8):2161-2171; doi:10.1093/brain/awn154
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Reorganization of the motor cortex is associated with postural control deficits in recurrent low back pain
1NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane and 2School of Physiotherapy, The University of Melbourne, Melbourne, Australia
Correspondence to: Dr Paul Hodges, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Qld 4072, Australia E-mail: p.hodges{at}uq.edu.au
Many people with recurrent low back pain (LBP) have deficits in postural control of the trunk muscles and this may contribute to the recurrence of pain episodes. However, the neural changes that underlie these motor deficits remain unclear. As the motor cortex contributes to control of postural adjustments, the current study investigated the excitability and organization of the motor cortical inputs to the trunk muscles in 11 individuals with and without recurrent LBP. EMG activity of the deep abdominal muscle, transversus abdominis (TrA), was recorded bilaterally using intramuscular fine-wire electrodes. Postural control was assessed as onset of TrA EMG during single rapid arm flexion and extension tasks. Motor thresholds (MTs) for transcranial magnetic stimulation (TMS) were determined for responses contralateral and ipsilateral to the stimulated cortex. In addition, responses of TrA to TMS over the contralateral cortex were mapped during voluntary contractions at 10% of maximum. MTs and map parameters [centre of gravity (CoG) and volume] were compared between healthy and LBP groups. The CoG of the motor cortical map of TrA in the healthy group was 
2 cm anterior and lateral to the vertex, but was more posterior and lateral in the LBP group. The location of the CoG and the map volume were correlated with onset of TrA EMG during rapid arm movements. Furthermore, the MT needed to evoke ipsilateral responses was lower in the LBP group, but only on the less excitable hemisphere. These findings provide preliminary evidence of reorganization of trunk muscle representation at the motor cortex in individuals with recurrent LBP, and suggest this reorganization is associated with deficits in postural control.
Key Words: motor cortex; postural control; transcranial magnetic stimulation; abdominal muscles
Abbreviations: LBP, = low back pain; MVC, = maximum voluntary contraction; MT, = motor thresholds; TMS, = transcranial magnetic stimulation; TrA, = transversus abdominis
Received April 23, 2008. Revised June 6, 2008. Accepted June 23, 2008.