The relative timing of trunk muscle activation is retained in response to unanticipated postural-perturbations during acute low back pain

Abstract

The purpose of this study was to assess the activation of the erector spinae (ES) and external oblique (EO) in response to unanticipated, bi-directional postural perturbations before and after the induction of acute low back pain (LBP) in healthy individuals. An experimental session consisted of a baseline, control, and an acute LBP condition. For the control and acute LBP condition, isotonic or hypertonic saline (HS), respectively, was injected into the right ES muscle. In each condition, participants stood on a moveable platform during which 32 randomized postural perturbations (8 repetitions of 4 perturbation types: 8 cm anterior slides, 8 cm posterior slides, 10° anterior tilts, and 10° posterior tilts) with varying inter-perturbation time intervals were performed over a period of 4-5 min. Bilateral surface electromyography (EMG) was recorded from the ES and EO in addition to subjective pain records. During the acute LBP condition: (1) the onset time of the ES and EO was delayed for the forward and backward sliding perturbations (P < 0.05); (2) EMG amplitude was reduced bilaterally for all perturbations (P < 0.05); (3) the order of activation and interval between the onset times of the ES and EO were unaltered and (4) ES, but not EO, activity was adjusted to account for the directional differences between the perturbations. This study revealed that re-establishment of posture and balance was a result of the individuals' ability to rapidly modulate ES with respect to EO activity and that the bi-directional postural responses, although shifted in time and amplitude, retained temporal features in the presence of acute LBP.

Fig. 1

Illustration of the four perturbation types with respect to the participant’s stance

Fig. 2

Time course of subjective pain ratings associated with injections of isotonic or hypertonic saline into the right lower back

Fig. 3

Shadings of perceived area of pain associated with the injection of isotonic (a) or hypertonic (b) saline into the right lower back

Fig. 4

Mean of the left- and right EMG amplitude (a, b) and onset time (c, d) of the erector spinae (ES) and external oblique (EO) during the baseline, control, and low back pain conditions for the anterior and posterior sliding perturbations. *Represents significant post hoc SNK results (P < 0.05)

Fig. 5

Mean of the left and right EMG amplitude (a, b) and onset time (c, d) of the erector spinae (ES) and external oblique (EO) during the baseline, control and low back pain conditions for the anterior and posterior tilting perturbations. *Represents significant post hoc SNK results (P < 0.05)

Fig. 6

Mean of the left and right EMG amplitude (a, b) and onset (c, d) of the erector spinae (ES) and external oblique (EO) for each perturbation type (PS posterior slide, AS anterior slide, PT posterior tilt, AT anterior tilt) for the mean of the baseline and control condition (a, b) and the acute low back pain (LBP) condition (c, d). Figures illustrate that balance was maintained from the adjustment of ES amplitude and onset latency for all perturbations type, in comparison with the relatively consistent behaviour of the EO for each perturbation type