Test-retest reliability of the phase coordination index: A measure of bilateral coordination

Objective: Synchronized left-right stepping (bilateral coordination) is essential for safe movement and independent mobility function. The phase coordination index (PCI) quantifies this coordination. This study aimed to assess the reliability of PCI when measured multiple times within a study. Methods: Forty-three neurotypical older adults completed a single 2-min walk test, three weeks apart, to assess the reliability and consistency of the PCI. During the 2-min walk test, participants walked along a 15-m hallway at their preferred natural speed while wearing seven inertial monitoring units. PCI was first calculated for each trial using the standard method and then normalized to gait speed using two approaches. The normalization methods included dividing each PCI value by: (1) the mean gait speed of the individual trial, and (2) the mean gait speed averaged across both trials. Results: The reliability (ICC) values were 0.566 for PCI alone, 0.771 when adjusted for gait speed at each trial, and 0.784 when adjusted for gait speed over both trials. Bland-Altman analysis showed no significant bias between trials across analyses. Conclusion: These findings suggest that PCI offers acceptable test-retest reliability. However, the results of this investigation advocate for normalizing PCI to gait speed for enhanced reliability.