The instantaneous center of rotation (ICR) can be used to investigate movement coordination and control in patients with low back pain (LBP). Tracking of the ICR in LBP patients has not been systematically investigated. This study aimed to (1) determine the within-session measurement error of ICR parameters, and (2) characterize the change in ICR among three groups of participants (no history of LBP = HC; history of LBP = HLBP; and current LBP = LBP).
Ninety-three participants (HC = 31; HLBP = 33; and LBP = 29) were recruited. Participants performed two sets of three repetitions of an active forward bend, while their lumbar and pelvic movements were recorded with an electromagnetic tracking system. Total ICR displacement and the radius of the bounding sphere containing the ICR were derived during the forward bending and the return to upright phases. Intra-class correlation coefficients (ICC3,3) and minimal detectable difference (MDD) were used to determine measurement error and interpret findings of the group analysis. One-way ANOVAs and post hoc Bonferroni comparisons were used to determine differences among groups.
ICC3,3 demonstrated excellent within-session test-retest reliability of the ICR parameters (ICC3,3 = 0.86-0.97). The MDD values were 0.20-3.40 mm. Comparisons between the HC and LBP groups and between the HLBP and LBP groups showed significant differences (p < 0.05) for all ICR parameters, with medium effect sizes (0.51-0.66), except for total displacement during forward bending between the HC and LBP groups. Less ICR displacement and variability in patients with LBP may indicate coping strategies to stiffen the lumbar spine. This could result from patients with LBP adopting a strategy of increased muscle activation to provide spinal stability during functional tasks.