Introduction: Individuals with chronic low back pain (LBP) report impaired somatosensory function and balance. However, there is a lack of investigation on kinematic similarities during gait. The aim of this study was to compare gait parameters as well as combined limb motions for the kinematic similarity index (KSI) between subjects with and without LBP.

Methods: Twenty-two subjects with LBP and 19 age- and body mass index-matched control subjects participated in this study. The combined limb motions in the gait cycle were compared between subjects with LBP and those of a prototype derived from healthy subjects. The KSI was computed as the normalized inner product or the cosine of the solid angle between the vector representing the distribution of activity generated by subjects without LBP (prototype response vector: PRV) and that representing the distribution in the subjects with LBP (RV). Thus, the index is constrained to lie between 0 and 1The calculations resulted in response vectors that were analyzed to controls-derived prototype response vectors for the normalized index at 5 % increments in the gait cycle (Figure 1).

Results: The overall index in the gait cycle was significantly different between groups (t = 6.52, p = 0.001, Cohen’s d = 1.43) and the standard deviation of KSI (t = -7.62, p = 0.001, Cohen’s d = 1.53). There was also a significant difference for the KSI of the stance phase (t = 6.26, p = 0.001, Cohen’s d = 1.40) and the KSI of the swing phase (t = 4.23, p = 0.001, Cohen’s d = 1.11). The results of our study indicated that the LBP group demonstrated greater variability based on the standard deviation than the control group in the GC (Figure 2). The LBP group may have modified their walking patterns during these specific phases.

Discussion: The KSI is useful for clinical outcome measures to differentiate kinematic changes and to demonstrate quantified similarities in the gait cycle. It is warranted to validate the KSI along with a larger sample size for physiological gait asymmetry in future studies.

Keywords: kinematic similarity index; normalization; gait cycle; low back pain; and asymmetry.