A history of low back pain associates with altered electromyographic activation patterns in 1 response to perturbations of standing balance 2

24 People with a history of LBP exhibit altered responses to postural perturbations and the central 25 neural control underlying these changes in postural responses remains unclear. To characterize 26 more thoroughly the change in muscle activation patterns of people with LBP in response to a 27 perturbation of standing balance, and to gain insight into the influence of earlyversus late-phase 28 postural responses (differentiated by estimates of voluntary reaction times), this study evaluated 29 the inter-muscular patterns of electromyographic (EMG) activations from 24 people with and 21 30 people without a history of chronic, recurrent LBP in response to 12 directions of support surface 31 translations. Two-factor general linear models examined differences between the 2 subject 32 groups and 12 recorded muscles of the trunk and lower leg in the percent of trials with bursts of 33 EMG activation as well as the amplitudes of integrated EMG activation for each perturbation 34 direction. The subjects with LBP exhibited (1) higher baseline EMG amplitudes of the erector 35 spinae muscles prior to perturbation onset, (2) fewer early-phase activations at the internal 36 oblique and gastrocnemius muscles, (3) fewer late-phase activations at the erector spinae, 37 internal and external oblique, rectus abdominae, as well as the tibialis anterior muscles, and (4) 38 higher EMG amplitudes of the gastrocnemius muscle following the perturbation. The results 39 indicate that a history of LBP associates with higher baseline muscle activation and that EMG 40 responses are modulated from this activated state rather than exhibiting acute burst activity from 41 a quiescent state, perhaps to circumvent trunk displacements. 42