Within- and Between-Assessor Reliability of Biarticular Muscle-Tendon Unit Lengths: Application of a Conventional Gait Model 2-based musculoskeletal model

Cerebral palsy (CP) is characterized by neuromusculoskeletal impairments, including reduced muscle fiber lengths, which alter muscle-tendon unit (MTU) lengths and contribute to gait deviations. Estimation of MTU length reliability using musculoskeletal modeling is essential for guiding interventions such as muscle lengthening. This study aims to assess within-assessor (WA) and between-assessor (BA) reliability of MTU length estimation during gait in CP and non-impaired (NI) individuals. 38 individuals (19CP,19TD) participated in 3 3DGA sessions (2 by the same assessor). Normalized MTU length, MTU lengthening, and maximum lengthening range of the rectus femoris, semitendinosus, and gastrocnemius medialis were reported. Reliability was quantified through the standard error of measurement (SEM) and minimal detectable change (MDC). The mean SEM (MDC) during the gait ranged from 1.0-2.1% (2.5-5.7%) for normalized MTU length, from 3.6-8.7 mm (10.1-24.1 mm) for MTU lengthening, and from 2.9-8.0 mm (8.0-22.1 mm) for MTU lengthening range in individuals with CP. In NI individuals, the mean SEM (MDC) during the cycle ranged from 0.6-1.3% (1.8-3.7%) for normalized MTU length, from 2.6-5.0 mm (7.3-13.9 mm) for MTU lengthening, and from 2.7-5.0 mm (7.5-12.4 mm) for MTU lengthening range. Results suggest reliable estimations by the same assessor, supporting therapeutic decision-making and patient progress monitoring.