Acute Modulation of Lumbar Motor Unit Behavior During Experimentally Induced Low Back Pain: A Motor Unit Decomposition Analysis.

BACKGROUND AND PURPOSE: Acute low back pain is a prevalent musculoskeletal disorder that can disrupt motor control and compromise functional stability. Evidence suggests that pain alters motor unit discharge characteristics and the temporal organization of muscle activation; however, the specific neuromuscular adaptations associated with acute low back pain remain insufficiently understood. This study aimed to investigate the effects of experimentally induced acute low back pain on force steadiness and motor unit discharge characteristics (including recruitment threshold, motor unit firing rate, and action potential amplitude) during a sustained spinal extension task METHODS: Thirty-three healthy participants (aged 18-40 years) performed a sustained spinal extension task at 20% of their maximum voluntary contraction under two experimental conditions: pre- and during pain induced by hypertonic saline injection, and pre- and during-isotonic saline injection into the right lumbar region. Electromyography signals were recorded bilaterally from the longissimus muscles. Signals were decomposed into individual motor unit action potential trains using advanced algorithms and clustered using the K-means method.

RESULTS: The hypertonic saline injection successfully induced moderate pain on the numerical pain rating scale (NPRS) (median = 4.71; interquartile range (IQR) = 1.61), which was significantly higher than the isotonic control condition (NPRS median = 1.50; IQR = 1.00; p < 0.001). During the pain condition, significant lateralized adaptations were observed: motor units within the contralateral (left) longissimus muscle exhibited an increased firing rate (p = 0.020, r = 0.33), with no significant changes in the recruitment threshold or motor unit action potential amplitude. On the ipsilateral side, subtle modulations in recruitment threshold were detected. In contrast, the isotonic condition elicited only localized and nonsystemic neuromuscular changes, without the coordinated bilateral organization observed under acute pain.

DISCUSSION: Acute low back pain induces a dynamic and lateralized modulation of motor unit recruitment, suggesting a compensatory redistribution of neural drive that may help preserve functional stability despite altered motor coordination. These findings provide novel insights into the neuromuscular adaptations underlying acute low back pain and have potential implications for physiotherapy assessment and intervention.