Single-motor-unit discharge characteristics in lumbar multifidus muscle of acute low back pain patients

Acute low back pain (ALBP) causes rapid deterioration of paraspinal muscle function. The underlying neurophysiology is poorly understood. We therefore carried out this observational study in patients with ALBP to characterize motor unit (MU) activity in deep lumbar multifidus (LM) muscle and compare with our previous findings from pain-free subjects. Nine subjects (1 woman; age 26–59 yr) with ALBP duration of 1–21 days were recruited from outpatient clinics. Fine wire electromyography (EMG) electrodes were implanted bilaterally at the painful spinal level under computer tomography guidance. EMG was recorded during spontaneous sitting and standing, and during voluntary force production. Linear mixed models were utilized to test or control for the effects of a number of predefined variables. Compared with sitting, standing increased total duration of EMG activity, median MU discharge rate, interspike interval variability, and common drive measured as common drive coefficients (CDC) derived from concurrently active MU pairs. Median discharge rate in 73 MUs was 5.5 and 6.6 pulses per second (pps) during spontaneous sitting and standing, and 7.2 pps during voluntary force production. Interspike interval variability was lower during voluntary tasks than during spontaneous force production. Common drive was less pronounced in bilateral vs. unilateral unit pairs, also in spontaneous standing. This difference was not seen in our previous pain-free subjects, suggesting altered bilateral control of the spine in ALBP. The distribution of CDC values was not a homogeneous continuum but could be seen as two partially overlapping populations of CDC distributions. NEW & NOTEWORTHY We implanted fine-wire electrodes in the deepest part of axial postural muscles in patients with acute low back pain and characterized their motor unit activity. We found less pronounced common drive to the two sides of the spine compared with pain-free subjects, suggesting a different postural control strategy in patients with acute low back pain. An unexpected finding was that common drive coefficient values appeared to consist of two partially overlapping populations of normal distributions.