Individuals with non-specific low back pain in an active episode demonstrate temporally altered torque responses and direction-specific enhanced muscle activity following unexpected balance perturbations

Abstract

Individuals with a history of non-specific low back pain (LBP) while in a quiescent pain period demonstrate altered automatic postural responses (APRs) characterized by reduced trunk torque contributions and increased co-activation of trunk musculature. However, it is unknown whether these changes preceded or resulted from pain. To further delineate the relationship between cyclic pain recurrence and APRs, we quantified postural responses following multi-directional support surface translations, in individuals with non-specific LBP, following an active pain episode. Sixteen subjects with and 16 without LBP stood on two force plates that were translated unexpectedly in 12 directions. Net joint torques of the ankles, knees (sagittal only), hips, and trunk, in the frontal and sagittal planes, were quantified and the activation of 12 muscles of the lower limb unilaterally and the dorsal and ventral trunk, bilaterally, were recorded using surface electromyography (EMG). Peaks and latencies to peak joint torques, rates of torque development (slopes), and integrated EMGs characterizing baseline and active muscle contributions were analyzed for group by perturbation direction (torques) and group by perturbation by epoch interaction (EMG) effects. In general, the LBP cohort demonstrated APRs that were of similar torque magnitude and rate but peaked earlier compared to individuals without LBP. Individuals with LBP also demonstrated increased muscle activity following perturbation directions in which the muscle was acting as a prime mover and reduced muscle activity in opposing directions, proximally and distally, with some proximal asymmetries. These altered postural responses may reflect increased muscle spindle sensitivity. Given that these motor alterations are demonstrated proximally and distally, they likely reflect the influence of central nervous system processing in this cohort.

Fig. 1

Experimental setup for support surface translations. Panel a depicts the kinematic marker setup, black dots represent reflective marker placement and the grey bars represent two force plates. Panel b depicts a displacement (cm) vs. time (ms) trace for platform movement during a rightward (0°) perturbation, as determined by the force plate kinematic markers. Panel c depicts the directions of platform perturbations with the induced body sway resulting from perturbations in the cardinal directions (i.e. left, forward, right and backward perturbations). Schematic stick figures are depicted with the subject facing to the right for the sagittal plane views and are viewed from the back for the frontal plane views.

Fig. 2

Peak joint torque latencies in the sagittal plane for trunk (early and late responses), and left and right hips, knees and ankles. Polar plots depict group means comparing individuals with LBP (ALBP; black circles) to those without LBP (NLBP; grey circles), with the radial axis reporting latency in ms following perturbation onset. Significant group effects (P < 0.050) are denoted by #.

Fig. 3

Peak joint torque latencies in the frontal plane for trunk (early and late responses), and left and right hips and ankles. Polar plots depict group means comparing individuals with LBP (ALBP; black circles) to those without LBP (NLBP; grey circles), with the radial axis reporting latency in ms following perturbation onset. Significant group effects (P < 0.020) are denoted by #, while significant group by direction interactions (P < 0.040) are denoted by &. Post hoc group differences are denoted by * at the directions that achieved significance.

Fig. 4

Average normalized muscle activities for the left proximal and distal muscles for the 100-175 ms epoch. Polar plots depict group means of the left External and Internal oblique muscles, left Erector Spinae at L1 and L3, Tibialis Anterior and Gastrocnemius muscles comparing individuals with LBP (ALBP; black circles) and those without LBP (NLBP; grey circles), with the radial axis reporting the % normalized muscle activity. Significant group by epoch by direction interactions (P < 0.050) are denoted by ampersand, significant group by epoch interactions are denoted by hash symbol, with post hoc group differences denoted by asterisks at the directions that reached significance.

Fig. 5

Average normalized muscle activities for the left proximal and distal muscles averaged across direction depicting group by epoch interactions. Plots depict group means ± S.E. comparing individuals with LBP (ALBP; black circles) to those without LBP (NLBP; grey circles) across four epochs, Epoch 1 (baseline: 250 to 50 ms prior to perturbation), Epoch 2 (25 to 100 ms after perturbation), Epoch 3 (100 to 175 ms prior to perturbation), Epoch 4 (175 to 250 ms after perturbation). Significant group by epoch by direction interactions (P<0.050) are denoted by #. Significant post hoc group differences (P<0.050) are denoted by *.

Fig. 6

Average normalized muscle activities for the left (left panel) and right (right panel) trunk muscles for the 100-175 ms epoch, depicting side-specific group differences. Polar plots depict group means comparing individuals with LBP (ALBP; black circles) to those without LBP (NLBP; grey circles). Significant group by direction interactions (P < 0.050) are denoted by #. Post hoc group differences (P<0.050) are denoted by * at the directions that reached significance.