Individuals with chronic low back pain show impaired adaptations of lumbar extensor muscle reflex amplitude during unexpected perturbations.

Purpose: This study investigated the adaptability of trunk muscle responses to a series of unexpected external perturbations in patients with chronic low back pain (LBP).

Methods: Thirty-seven adult participants, including 19 without LBP (control group) and 18 with chronic LBP, were submitted to 15 repetitions of trunk perturbations applied in a posterior-to-anterior direction, inducing trunk flexion. High-density surface electromyography (HDsEMG) was used to analyze lumbar muscle reflex amplitude. A two-way repeated measures ANOVA (2 × 2) was conducted to compare group differences and the effect of trial repetition over time (first five trials vs. last five trials of perturbations).

Results: Significant interaction effects were found on both sides (Left: p = 0.038; Right: p = 0.007). Post hoc comparisons revealed a decrease in response amplitude only in the control group between the first and last five perturbations, with reductions of 5.0% on the left side (p = 0.026, Bonferroni corrected) and 5.7% on the right side (p = 0.030, Bonferroni corrected). In contrast, individuals with chronic LBP showed no significant adaptation through repetition in the reflex response amplitude of the lumbar extensor muscles (post hoc both sides: p > 0.05).

Conclusion: Individuals with chronic LBP fail to adapt reflex amplitudes to repeated perturbations, possibly due to impaired proprioception, reduced motor variability and neuroplastic changes observed in individuals with chronic LBP. These changes might limit their ability to optimize responses to repeated perturbations, potentially compromising spinal stability and increasing functional cost.