Increased trunk muscle activity during gait after bilateral experimental pain induction in recurrent low back pain patients during a pain-free period

Introduction: Trunk muscle control is consistently altered in persistent low back pain patients during a variety of functions but the effect of experimental pain induction in recurrent low back pain patients during a pain-free period remains unknown. The aim of this study was to investigate the consequence of unilateral versus bilateral experimental low back muscle pain in recurrent low back pain patients and a group of healthy control participants during gait. Methods: In 17 healthy control participants (10 females, age 26.1±4.2 years) and 16 recurrent low back pain patients (10 females, age 26.5±6.6 years) the trunk muscle activity was recorded with surface electromyography (EMG) before and during unilateral versus bilateral experimental low back pain, during gait on an instrumented treadmill (Zebris FDM-T, Zebris, Germany). The velocity was self-selected according to the comfortable pace during baseline gait and the same velocity maintained during gait trials after pain induction. Pain was induced by unilateral or bilateral injections of hypertonic saline into m. longissimus at L2 level in randomized order. The pain intensity was rated verbally by the Numeric Rank Scale ((NRS) 0 = no pain and 10 = worst imaginable pain) and mean of peak NRS was calculated. Bilateral surface EMG was recorded from 12 muscles (bilateral iliocostalis, longissimus, multifidus lumbalis, rectus abdominalis, obl. int. and ext. abdominalis muscles). The muscle amplitude was quantified as the mean root-mean-square EMG (RMS-EMG) for each of four gait phases extracted from treadmill pressure data in continuous cycles defined by left and right initial contact and toe-off that defined the phases through a full stride (2 double stance phases and left and right swing phases). The pain-evoked RMS-EMG difference from the baseline RMS-EMG (Delta-RMS-EMG) was calculated and compared between the two groups for each pain condition. Results: The mean peak NRS scores were higher in patients compared with control participants during unilateral (P<0.001) and bilateral experimental pain (P<0.04). Bilateral pain increased the Delta-RMS-EMG in all muscles in the patients compared with controls during the left (P<0.05) and right (P<0.04) swing phase. No statistical differences were observed during stance phases during bilateral pain or in any phases during unilateral pain. Conclusions: Higher pain intensity and bilateral pain-evoked trunk muscle activity during swing phases in patients compared with control participants indicated persistent sensorimotor changes in recurrent low back patients during a pain-free period. Acknowledgement: The study was supported by Center for Neuroplasticity and Pain (CNAP), SMI, Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Denmark and University College of Northern Denmark, Department of Physiotherapy.