Intra- and inter-therapist reliability of lower-limb inter-joint coordination during gait in individuals with and without cerebral palsy

PURPOSE: Inter-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 intra- and inter-therapist reliability of lower-limb CRP measurements in CP and non-impaired (NI) individuals. METHODS: CP (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, ten days apart, each including 3D motion capture of at least 10 walking trials. During one session, two trained therapists 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. RESULTS: The 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°(CP) and 9.3 ± 0.6°(NI) inter-therapist, and 23.8 ± 3.0°(CP) and 9.1 ± 1.6°(NI) intra-therapist. For ankle-knee coordination, MDC reached 29.0 ± 2.6°(CP) and 25.0 ± 3.5°(NI) inter-therapist, peaking at 47.3 ± 10.9°(CP) and 28.6 ± 1.5°(NI) in mid-swing intra-therapist. CONCLUSIONS: This 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.