Jones SL^*, Raasch CC^†, Hitt JR^*, Henry SM^*, Bunn JY^**; University of Vermont, Dept. of *Physical Therapy; **Dept. of Mathematics & Statistics, Burlington, VT, USA. †Exponent Failure Analysis Associates, Phoenix, AZ, USA. Stephanie.Jones@uvm.edu.

PURPOSE: Persons with chronic low back pain (LBP) demonstrate altered movement patterns that may be attributed to changes in neuromuscular control. The purpose of this study was to compare the neuromuscular responses of persons with and without LBP following a systematic task designed to elicit automatic postural responses. Net joint torques were interpreted as a reflection of the net neuromusculoskeletal output acting at a given joint. SUBJECTS: Eleven subjects without LBP (NLBP) (38 yrs ± 8 years) and twelve subjects (38 ± 8 yrs) with chronic, recurrent LBP of longer than 6 months were tested. For subjects with LBP vs. NLBP, the McGill Pain Questionnaire word scores were 5.6 vs. 0.04, and the Visual Analog Scores were 3 vs. 0.2, respectively. METHODS: Subjects were instructed to stand with a foot placed on separate force plates, which were mounted on a moveable platform. The platform was translated unexpectedly in one of 12 horizontal directions for a total of 36 trials. Force data and 3-D body kinematics were combined with anthropometric data to calculate net sagittal and frontal plane joint torques for each subject at the ankle, knee, hip and trunk, using inverse dynamics. ANALYSES: Peak torque displacement, time to peak displacement and rate of joint torque development were analyzed with a repeated measures analysis of variance design. RESULTS: In general, anterior perturbations, resulting in backward sway, elicited larger differences between the LBP and NLBP groups than did posterior or lateral perturbations. On average, the LBP group demonstrated joint torque responses that were greater in magnitude (e.g. trunk extension/flexion, p = 0.0009) and that peaked later (e.g. trunk extension/flexion, p = 0.0004) than NLBP subjects in response to perturbations. Further, the rate of joint torque development was greater on average in the subjects with LBP (e.g. trunk extension/flexion, p = 0.0018). CONCLUSIONS: Patients with LBP exhibited altered automatic postural responses compared to NLBP subjects such that the torque responses in persons with LBP were larger, later, and developed more quickly compared to persons without LBP. These results are consistent with a strategy of pre-activation of muscles, and thus stiffening, in the LBP group that could be achieved by co-contraction of agonist and antagonist musculature about the ankle, knee, hip and trunk. Addressing alterations in postural responses should be considered in the treatment of persons with LBP if normalization of movement is a treatment goal. FUNDING SOURCE: Office of Patient-Oriented Research, Fletcher Allen Health Care, Burlington, VT and NIH/NCMRR/KO1 HD01194 - 05

Copyright 2004 by the American Physical Therapy Association

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