Trunk Compliance Index During Expected and Unexpected External Perturbations in Individuals with and without Chronic Low Back Pain

Many studies have documented changes in motor control among individuals with chronic low back pain (CLBP). Some research indicates that pain causes increased co-contraction of trunk muscles, leading to overall stiffening in various directions. Conversely, other studies suggest that CLBP may decrease muscle co-activation and subsequent instability. Methods for assessing trunk stiffness include biomechanical spine modeling, though this approach has limitations due to generalized anatomical features and complexity. The Trunk Compliance Index (TCI) is a biomarker measuring trunk compliance (i.e., 1/stiffness) by evaluating left shoulder displacements relative to normalized force across the frontal, sagittal, and transverse planes through direct trunk measurements. Our goal was to determine whether TCI varies across six directions during unexpected and maximally resisted multi-directional trunk perturbations in participants with CLBP and healthy (H) controls. Additionally, we hypothesized that sex would influence TCIs in unexpected trials. The study involved 40 participants, including 22 with chronic low back pain (CLBP) (average age 34.27 years; 50% female) and 18 healthy controls (average age 29.55 years; 50% female). A total of 30 sudden, multi-directional perturbations were applied to the trunk using a puller motor system, covering six directions across the sagittal, frontal, and horizontal planes. These perturbations were randomized, with each direction repeated five times. Subsequently, a second set of 30 sudden, expected (participants aware of the timing and direction, and instructed to brace their trunk maximally before each impulse. The data were assessed with six separate linear mixed models, followed by ANOVAs, with interactions examined using pairwise comparisons. The study confirmed main effects of perturbation types (expected, unexpected) across all TCIs for both genders and groups, except for LSB. Interestingly, no main effect of group (LBP, H) on TCIs was observed. Sex differences in TCI measurements across six directions were not significant, except for LSB in CLBP, where females showed higher stiffness than males. No significant TCI differences were found between CLBP and H groups at rest or during maximal stiffness in any direction. While previous findings of differing TCIs at rest between CLBP and H were not supported, the study reaffirmed higher stiffness in females with CLBP compared to males in the LSB direction. Overall, results suggest that CLBP does not increase trunk stiffness and provide valuable insights into spinal stiffness, which could enhance interventions for CLBP.