Individuals with non-specific low back pain use a trunk stiffening strategy to maintain upright posture

Time series of joint torque responses to perturbations across all perturbation directions for two exemplar subjects; an individual with LBP (black) and an individual without a history of LBP (gray). Sagittal plane torques of the trunk and left hip, knee and ankle are presented in response to predominately forward or backward perturbations (unshaded). Frontal plane torques of the trunk, and left hip and ankle are presented in response to primarily lateral perturbations (shaded). Traces range from 250ms prior to platform perturbation to 1000ms post-perturbation onset on the horizontal axis (vertical line represents the onset of platform movement) and ±50nm on the vertical axis. Schematic stick figures (depicted with the figure facing to the right for the sagittal plane views and viewed from the back for the frontal plane views) demonstrate the resulting direction of body sway and the dominant torque demonstrated at each joint for each of the cardinal directions. The boxed inset contains a displacement (cm) vs. time (ms) trace for platform movement during a rightward (0°) perturbation, as determined by the force plate kinematic markers.

Peak joint torque responses in the sagittal and frontal planes at the trunk (a), and left lower limb (b, hip; c, knee; and d, ankle). Polar plots depict group means of the absolute values of normalized peak torque magnitudes comparing individuals with LBP (black circles) and without LBP (NLBP; gray circles). All joints demonstrated significant main effects of direction (P<0.01). Trends or significant main effects for group differences (^∗) are indicated by superscripts.

Average normalized trunk muscle activity for the 100–175ms epoch following platform perturbations. Polar plots depict group means of the muscles of the left ventral (left panel) and left dorsal (right panel) trunk comparing individuals with LBP (black circles) and without LBP (NLBP; gray circles). Significant group effects (P<0.05) are denoted by #. Inset depicts normalized, rectified, filtered EMG traces for two exemplar participants (LBP, black; NLBP, gray) in response to a forward (90°) perturbation; shaded area corresponds to 100–175ms epoch of interest.

Average normalized Tibialis Anterior and Gastrocnemius muscle activity for the 100–175ms epoch following platform perturbations. Polar plots depict group means comparing individuals with LBP (black circles) and without LBP (NLBP; gray circles). Both muscles demonstrated significant direction and group effects (P<0.05).