Analysis of Lumbo-Pelvic Coordination Variability during a Sit-to-Stand Task in Adults with Low Back Pain Patrick Ippersiel, PhD (c)* a,b , Dr. Shawn Robbins, PhD a,b, Dr. Richard Preuss a,b, PhD a School of Physical and Occupational Therapy, McGill University. b Centre for Interdisciplinary Research in Rehabilitation of Greater Montreal, Constance Lethbridge Rehabilitation Centre *Corresponding Author: Patrick.ippersiel@mail.mcgill.ca

Background and Objectives Low back pain (LBP) is the leading cause of disability in the world, costing our Canadian health-care system an estimated $6-12 billion annually. Pain changes the way people move following injury. These changes are often maintained in the long-term and in-part contribute to ongoing pain and disability. Changes in movement variability, in particular, may indicate reduced adaptability in the motor system. Too little variability is consistent with a guarded approach to movement and is thought to lead to repetitive stress over time Too much variability may indicate error or noise in proprioceptive feedback and/or imprecise motor commands, reducing the robustness of the system Investigating movement variability in common functional tasks may provide insight into the presence of motor control deficits in individuals with LBP OBJECTIVES To compare patterns of lumbo-pelvic coordination and inter-joint coordinative variability, during repeated sit-to-stand-to-sit (STS), in individuals with LBP and healthy adults. A secondary objective was to study the effect of task period (i.e. standing up vs. sitting down) on patterns of lumbo-pelvic coordination and inter-joint variability. We hypothesized that participants with LBP would show more in-phase, less variable patterns of coordination for the lumbo-pelvic complex than healthy controls, consistent with a guarded approach to movement. PARTICIPANTS Participants were adults with LBP (n=16, 11 females) and healthy controls (n=21, 10 females). Participant demographics and baseline scores for pain intensity, disability, and risk-factors for LBP-related disability were collected on intake (Table 1). NPRS: Numerical pain rating scale; ODI: Oswestry disability index; SBST: STarT Back Screening Tool Healthy Group (n=21) LBP Group (n=16) Age (y) 27 (10) 30 (9) Onset (months) 109.9 (113.5) NPRS (/10) 3.4 (1.1) ODI (%) 25.3 (7.4) SBST 4.4 (1.8)

Methods DATA COLLECTION AND PROCEDURES Participants performed a standardized sit-to-stand-to-sit (STS) task as quickly as possible for a total of 10 repetitions. Kinematic data were collected with an electromagnetic system, sampled at 200 Hz, using sensors on T12, L3, S1 and the left thigh. Contact surface reaction forces under the thighs and buttocks were recorded using a force plate sampled at 200 Hz and used to deconstruct the STS task into 4 separate periods (Start/Up/Down/End). DATA PROCESSING Sagittal joint angles for the lower lumbar (L3S1), upper lumbar (T12L3) and left hip (LHip) were extracted by calculating the relative orientation between adjacent segments (e.g. LHip relative to S1 defines hip angle). Continuous Relative Phase (CRP) angle analysis determined inter-joint coordination of adjacent joint pairs (i) LHip-L3S1 and (ii) L3S1-T12L3. Phase angles for each joint were calculated using the Hilbert transform method, and the absolute difference between the two segments quantified the coordination: the CRP. A CRP value of 0 indicates fully in-sync behavior, while 180 suggests fully out-of-phase behavior. The mean CRP quantified degree of coupling of inter-joint coordination across all trials, while the deviation phase (DP) used standard deviations of the 10 CRP curves to measure variability. STATISTICAL ANALYSIS A log10 transformation was first applied to all data, allowing our data set to meet all criteria for normality. T-tests compared coordination (mean CRP) and variability of the full task (DP) between groups, and a mixed ANOVA compared the effects of group (LBP/healthy) and period for the two segments (LHip-L3S1 and T12L3-L3S1).

RESULTS Significant differences (p<.05) between groups for coordination (LHip-L3S1 and T12L3-L3S1) and variability (LHip-L3S1) were found for the full task, showing less in-phase coordination and increased variability in the LBP group (Fig 1 and 2). Variability: For STS periods, at LHip-L3S1, there were significant group (p=.028), period (p<.01) and group-period (p=.023) interaction effects. Pairwise comparison revealed greater variability at the start period (p<.001) in the LBP group. For L3S1-T12L3, there was significant effect of period (p=.022), but no group or interaction effects. Coordination: For STS periods, at LHip-L3S1, significant group (p=.008), period (p<.01) and group-period (p<.01) interaction effects were found. Pairwise comparison revealed greater out-of-phase behavior at the start and end periods (both p<.01) in the LBP group. At L3S1-T12L3, significant group (p=.020) and period (p=.001) effects were found, but with no interaction effect. This suggests more out-of- phase coordination across all periods in the LBP group, while both groups had more out-of- phase patterns during the start and end periods. Fig. 1. Ensemble group means for variability (Deviation Phase, DP) in the LHip-L3S1 segment across the full sit-to-stand (STS) task in adults with LBP (dashed red line) and healthy individuals (black line). Shaded area corresponds to SD. Fig 2. Ensemble group means for coordination (Mean Absolute Relative Phase, MARP) for the LHip-L3S1 segment across the full sit-to-stand (STS) task in adults with LBP (red line) and healthy individuals (black line). Shaded area corresponds to SD. Higher values indicates less in-phase (i.e. less coordinated) behavior. Fig. 1. Ensemble group means of the deviation phase (DP) for the LHip-L3S1 segment across the full sit-to-stand (STS) task in adults with chronic low back pain (CLBP) and healthy individuals (shaded area corresponds to SD. Fig. 1. Ensemble group means of the deviation phase (DP) for the LHip-L3S1 segment across the full sit-to-stand (STS) task in adults with chronic low back pain (CLBP) and healthy individuals (shaded area corresponds to SD.

DISCUSSION AND CONCLUSION Contrary to expectation, the LBP group showed greater out-of-phase and more variable patterns of coordination in the LHip-L3S1 segment, particularly during the start and end periods of a sit-to-stand task. Considering our sample (low pain, disability and psych distress) and their ability to complete the task without exacerbation, perhaps key task-related variables (e.g. center of mass) were controlled while joint configurations were free to vary, allowing for many motor solutions. Excessive variability may also be the result of poor proprioception in the lumbar spine. This may interfere with sensory feedback and result in less accurate tracking of the spine, rendering the system less adaptable in the event of a perturbation. Our findings of increased variability in the start period for CLBP and start/end periods for both groups suggest that the nature of the task should be considered when investigating movement variability. CLINICAL IMPACT Individuals with LBP showed more variable and less coordinated lumbo-pelvic movement patterns, particularly during the start and end portions of a STS task. While a significant difference in motor control between groups was observed, when considering our sample characteristics (i.e. low pain/disability, low-med STarT back scores) and lack of symptom exacerbation during the STS task, it may be excessive to categorize these findings as representative of dysfunctional motor control.