INTRODUCTION: Separate areas of motor cortex control individual paraspinal muscle fascicles in healthy people and this is consistent with differential control in function. Differential control is lost in people with low back pain (LBP), but the mechanism is unclear. Disorganisation or "smudging" of discrete areas of cortex has been associated with painful conditions that involve loss of discrete finger movement. Here we studied whether LBP is associated with smudging of the motor cortex representation of the paraspinal muscles.
METHODS: Organisation of the motor cortex was mapped with transcranial magnetic stimulation (TMS) using a figure‐of‐eight coil at points on a 5x5 cm grid on the scalp with motor evoked potentials (MEP) recorded in paraspinal muscles using surface electromyography (EMG) electrodes at L4 (to record from multiple paraspinal components simultaneously), and separate fine wire electrodes in short/deep (multifidus) and long/superficial (longissimus) paraspinal muscles. MEPs were evoked during ˜20% of maximum voluntary contraction in people with and without a history of LBP (persistent/recurring episodes over the preceding 18 months).
RESULTS: In pain‐free individuals 2 cortical areas were identified from surface EMG recordings with centres located ˜1‐2 cm and ˜4 cm anterior to vertex (˜2 cm lateral to vertex). Intramuscular EMG recordings revealed single areas of representation; that of multifidus aligned to the posterior site and that of longissimus at the anterior site. In the LBP group the representations of multifidus and longissimus were both located ˜1.2‐1.4 cm anterior to the vertex and did not differ from each other (P=0.14).
DISCUSSION: These data provide novel evidence of loss of discrete organisation (i.e. "smudging") of the motor cortex in people with persistent LBP. This may contribute to loss of differential control of paraspinal muscles and a subsequent compromise to the optimal control of spine motion and position