Complex muscular adaptation to perturbations after induction of experimental low back pain in healthy participants.

Background and aims Spine stability is affected in low back (LB) pain and potentially by muscle fatigue and soreness. This study assessed motor control responses to unexpected surface perturbations during stance during experimental LB muscle pain combined with fatigue and muscle soreness. Methods Nineteen healthy participants were examined day 1-3 before and after bilateral injections of hypertonic saline into m. longissimus. Pain intensity was scored on a visual analogue scale (VAS). Day 2 included injections during post-exercise LB muscle fatigue and day 3 during delayed onset back muscle soreness (DOMS). Twenty perturbations were conducted randomly with tilts in sagittal and frontal plane or displacements in the frontal plane. Bilateral electromyography (EMG) was recorded from 12 trunk muscles. The root-mean-square (RMS-EMG) was extracted. Changes (DeltaRMS) and absolute changes (DeltaRMSabs) in the RMS-EMG from the baseline of the day were calculated and averaged among back and abdominal muscles. Results VAS scores during fatigue and DOMS were significantly higher compared with day-1 (P<0.01). Back muscle RMS-EMG decreased and abdominal muscle RMS-EMG increased during DOMS, compared with pain-free conditions (P<0.01). Experimental LB pain increased DeltaRMS in the abdominal muscles day 1 (P<0.01) and in the fatigued back muscles day 2 (P<0.01). Pain increased DeltaRMSabs in day 1-3 in both muscle groups compared to baseline (P<0.01) with a minor effect of pain in the DOMS condition (P<0.01). Conclusions Pain-related reorganization may challenge spine stability during motor tasks in acute pain conditions and this seems to be even further implicated after fatigue and muscle soreness.