Objectives: To monitor treatment effects in patients with congenital myopathies and congenital muscular dystrophies, valid outcome measures are necessary. The Motor Function Measure (MFM) was examined for robustness, and changes are proposed for better adequacy.Design: Observational study based on data previously collected from several cohorts.Setting: Nineteen departments of physical medicine or neuromuscular consultation in France, Belgium, and the United States.Participants: Patients (N=289) aged 5 to 77 years.Interventions: None.Main Outcome Measures: A Rasch analysis examined the robustness of the MFM across the disease spectrum. The 3 domains of the scale (standing position and transfers, axial and proximal motor function, and distal motor function) were independently examined with a partial credit model.Results: The original 32-item MFM did not sufficiently fit the Rasch model expectations in either of its domains. Switching from a 4- to a 3-category response scale in 18 items restored response order in 16. Various additional checks suggested the removal of 7 items. The resulting Rasch-scaled Motor Function Measure with 25 items for congenital disorders of the muscle (Rs-MFM25(CDM)) demonstrated a good fit to the Rasch model. Domain 1 was well targeted to the whole severity spectrum close mean locations for items and persons (0 vs 0.316) whereas domains 2 and 3 were better targeted to severe cases. The reliability coefficients of the Rs-MFM25(CDM) suggested sufficient ability for each summed score to distinguish between patient groups (0.9, 0.8, and 0.7 for domains 1, 2, and 3, respectively). A sufficient agreement was found between results of the Rasch analysis and physical therapists' opinions.Conclusions: The Rs-MFM25(CDM) can be considered a clinically relevant linear scale in each of its 3 domains and may be soon reliably used for assessment in congenital disorders of the muscle. (C) 2014 by the American Congress of Rehabilitation Medicine