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AIM: To explore the psychometric properties of the general movements optimality score (GMOS) by examining its dimensionality, rating scale functioning, and item hierarchies using Rasch measurement. METHODS: Secondary data analysis of the GMOS data for video-recording of 383 infants with uni-, multidimensional, and mixed Rasch partial credit models. Videos were scored based on the global General Movement Assessment categories, and on the amplitude, speed, spatial range, proximal and distal rotations, onset and offset, tremulous and cramped components of the upper and lower extremities (21 items), resulting in the GMOS. RESULTS: The GMOS data fits best to a unidimensional mixed Rasch model with three different classes of infants, with all but two items contributing to the infants' separation. Rating scales functioned well for 19 items. Item difficulty hierarchies varied depending on infants' class. No floor effect and no substantive gaps between item difficulty estimates were found. CONCLUSION: The GMOS has strong psychometric properties to distinguish infants with different functional motor performance and provides a quantitative measure of quality of movement. INTERPRETATION: The GMOS can be confidently used to assist with early diagnosis, grade motor performance, and provide a solid base to study individual general movement developmental trajectories. [ABSTRACT FROM AUTHOR]