Decreased limits of stability in response to postural perturbations in subjects with low back pain.

BACKGROUND Low back pain is associated with abnormal movement strategies due to changes in neuromuscular control. A plausible contributing factor to low back pain is poor control of trunk muscles, thus understanding motor control alterations in this population can guide rehabilitation. Quantification of postural responses following support surface translations is one way to examine motor control impairments in people with low back pain. METHODS Twenty-four healthy subjects [mean 33 (SD 11) years] who had no low back pain and 26 subjects [mean 39 (SD 13) years] with chronic, recurrent low back pain were instructed to stand with feet placed on separate force plates, which were mounted on a moveable platform. The platform was translated unexpectedly in one of 12 directions for a total of 72 trials. For both the sagittal and frontal planes, the net center of pressure displacement was derived and the total body center of mass was calculated by combining kinematic and anthropometric data. FINDINGS For sagittal plane responses, subjects with low back pain had reduced and delayed sagittal plane center of pressure responses (P<0.01) compared to the subjects without low back pain. In contrast, the sagittal plane center of mass responses were larger in magnitude (P=0.03) yet similarly delayed in onset (P=0.04) for the low back pain group. Frontal plane responses did not differ between groups. INTERPRETATION Subjects with low back pain have altered automatic postural coordination, both in terms of magnitude and timing of responses, indicating alterations in neuromuscular control.

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