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

Abstract

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

Figures

Fig. 1.

Schematic of the directions of surface translation with their induced body sway (left) as well as representative electromyography (EMG; right). The graphs of representative EMG illustrate the directional dependence and time characteristics of EMG burst activity for antagonistic muscles of the trunk and ankle: from top to bottom, the erector spinae at the 3rd lumbar segment (ESP), rectus abdominus (RA), gastrocnemius medialis (GM), and tibialis anterior (TA). Black traces represent responses to backward translations that induce forward body sway; gray traces represent responses to forward translations that induce backward sway. Dashed gray boxes identify the early-phase epochs of the ankle and trunk muscle responses (50–150 ms and 50–120 ms, respectively); solid gray boxes identify the late-phase epochs.

Fig. 2.

Incidence of early-phase EMG burst onsets. Body schematics illustrate the locations of each muscle as black-outlined ellipses; the gray-filled ellipses exhibit significant post hoc differences between groups following a significant group-by-muscle interaction. Approximate to these muscle locations are spoke wheels that represent each direction of support surface translation. Thick, black lines in the spoke wheel identify the directions of translation with significant group-by-muscle interactions.

Fig. 3.

Incidence of late-phase EMG burst onsets. Layout, lines, and fills are all as described for Fig. 2.

Fig. 4.

Group mean baseline integrated EMG amplitudes. The vertical axis presents the integrated EMG amplitudes of the baseline epoch (75 ms before translation onset) as a percentage of each muscle's maximum activation amplitude exhibited within any of the five 75-ms epochs. The horizontal axis at bottom lists each muscle, with a prefix of “l” or “r” representing the left or right side of the body, respectively. Muscles with significant post hoc differences between groups following a significant group-by-muscle interaction are indicated by an asterisk when evident before correction for age or multiple comparisons and outlined by a black box when evident after these corrections. The horizontal axis at right lists the directions of surface translation, ordered counterclockwise from bottom to top and starting from rightward translations (the diagonal directions of translation are not labeled due to space constraints). Black traces represent the grand averages for the group without LBP, and gray traces represent the grand averages for the group with LBP. IO, internal oblique; EO, external oblique; EST, erector spinae at the 1st lumbar segment.