Within- and Between-Assessor Reliability of Lower-Limb Inter-Joint Coordination During Gait in Individuals With and Without Cerebral Palsy

BackgroundInter-joint coordination plays a key role in walking, particularly in people with cerebral palsy (CP), who experience altered movement patterns. The Continuous Relative Phase (CRP) method quantifies lower-limb coordination by assessing the phase relationships between joints. However, the reliability of CRP measurements during walking in individuals with CP remains unexplored, and may be affected by measurement variability due to marker placement errors, soft tissue artifacts, and natural movement fluctuations. Quantifying this reliability is important for appropriate clinical comparisons. This study aimed to quantify within- and between-assessor reliability of lower-limb CRP measurements in individuals with CP and their non-impaired (NI) peers. MethodsCP (n=19, age=18.4{+/-}7.3 years, GMFCS I-III) and NI (n=19, age=18.3{+/-}11.2 years) individuals completed two gait assessment sessions, each including 3D motion capture of at least 10 walking trials. Two trained assessors independently placed reflective markers and conducted gait analyses. Standard error of measurement (SEM) and minimal detectable change (MDC) were computed for knee-hip and ankle-knee coordination across gait subphases. FindingsThe SEM and MDC were lower for knee-hip than ankle-knee coordination, suggesting higher measurement reliability for proximal joint coupling. For knee-hip coordination, MDC reached 15.1{+/-}0.7{degrees}(CP) and 9.3{+/-}0.6{degrees}(NI) between assessors, and 23.8{+/-}3.0{degrees}(CP) and 9.1{+/-}1.6{degrees}(NI) within assessors. For ankle-knee coordination, MDC reached 29.0{+/-}2.6{degrees}(CP) and 25.0{+/-}3.5{degrees}(NI) between assessors, peaking at 47.3{+/-}10.9{degrees}(CP) and 28.6{+/-}1.5{degrees}(NI) in mid-swing within assessors. InterpretationThis study provides the first metrological reference for reliability of CRP-based inter-joint coordination during gait in CP. Results showed poor reliability, emphazing that such measurements must be interpreted with caution. Highlights- Knee-hip showed greater reliability than ankle-knee coordination across gait phases - Cerebral palsy individuals showed higher variability than non-impaired peers The beginning and the end of gait cycle showed the poorest reliability Pre-post comparisons should account for MDC thresholds to avoid misinterpretation