Responses to multi-directional surface translations involve redistribution of proximal versus distal strategies to maintain upright posture.

Evaluation of postural control in multiple planes is necessary to determine the movement strategies used to respond to unexpected perturbations. The present study quantified net joint torques of the lower limbs and trunk in the sagittal and frontal planes following multi-directional surface translations. Twenty-one healthy subjects stood with feet on separate force plates mounted on a moveable platform, translated unexpectedly in one of 12 directions. Peak net torque magnitudes and latencies following perturbation onset were determined as were the relative contributions of each joint to total torque production. Magnitude of net torque generated by each leg varied by perturbation direction, with the largest individual joint magnitude elicited in directions of limb loading. Relative contributions of individual joint torques to the total response were dependent upon perturbation direction. Results suggest that a redistribution of the relative contributions of hip/trunk versus ankle strategies occurs dependent on perturbation direction, with a significant contribution by the knee joint in response to forward perturbations. Direction-specific redistribution of proximal versus distal strategies appears to depend upon the biomechanical constraints imposed by a given perturbation direction. Thus, it appears that sagittal and frontal plane posture-righting responses may not be uniquely controlled, and may instead be governed similarly, with modulation of relative torque contributions among joints when necessary, given direction-specific anatomical constraints.

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

There is increasing evidence that individuals with non-specific low back pain (LBP) have altered movement coordination. However, the relationship of this neuromotor impairment to recurrent pain episodes is unknown. To assess coordination while minimizing the confounding influences of pain we characterized automatic postural responses to multi-directional support surface translations in individuals with a history of LBP who were not in an active episode of their pain. Twenty subjects with and 21 subjects without non-specific LBP stood on a platform that was translated unexpectedly in 12 directions. Net joint torques of the ankles, knees, hips, and trunk in the frontal and sagittal planes as well as surface electromyographs of 12 lower leg and trunk muscles were compared across perturbation directions to determine if individuals with LBP responded using a trunk stiffening strategy. Individuals with LBP demonstrated reduced peak trunk torques, and enhanced activation of the trunk and ankle muscle responses following perturbations. These results suggest that individuals with LBP use a strategy of trunk stiffening achieved through co-activation of trunk musculature, aided by enhanced distal responses, to respond to unexpected support surface perturbations. Notably, these neuromotor alterations persisted between active pain periods and could represent either movement patterns that have developed in response to pain or could reflect underlying impairments that may contribute to recurrent episodes of LBP.