The magnitude of pre-programmed reaction dysfunction in back pain patients: experimental pilot electromyography study.

BACKGROUND Preprogrammed reactions (PPR) appear at a latency of higher than 40 ms, but before the voluntary muscle responds (approximately 120 ms) to postural perturbations. OBJECTIVE To examine the difference in magnitude of preprogrammed reactions in patients with chronic low back pain (CLBP) and without low back pain. METHODS we analyzed electromyographic Root Mean Square (RMS) amplitudes of asymptomatic (n=25) and CLBP patients (n=25) on stable and unstable surfaces during expected and unexpected perturbations for rectus abdominus and erector spinae muscles. The mean PPR and PPR-combined voluntary response RMS amplitudes (VRPPR) were compared between the two groups. To find the presence of PPR in LBP patients, a criteria was set that the obtained PPR RMS amplitude value should exceed 60% mean reflex RMS amplitude that occur within 50 ms after perturbation. RESULTS Fleiss' kappa revealed a good agreement (kappa = 0.7 to 0.9) among raters for absence of PPR in patients with CLBP and presence of PPR in asymptomatic population. The two way ANOVA revealed significantly different mean PPR and VRPPR RMS amplitudes between asymptomatic and LBP population for rectus abdominus and erector spinae muscles (p<0.05). CONCLUSION PPR responses were found absent (<60% of Mean Reflex RMS) in patients with CLBP. Further, patients with CLBP demonstrated lower PPR amplitudes with higher peak voluntary responses compared to asymptomatic population, indicating difficulties in presetting of voluntary responses for regaining postural stability after perturbation.