Abstract Aims: To establish anthropometric-related percentile curves for muscle size and strength in a cohort of typically developing (TD) children and to demonstrate their use through applications in children with cerebral palsy (CP) and Duchenne muscular dystrophy (DMD). Methods: Lower limb muscle size and strength were assessed in a large cross-sectional cohort of TD children with 3D freehand ultrasound (four muscles, n=154, ♂/♀=82/72, age range: 0y7mo-17y10mo) and fixed dynamometry (seven muscle groups, n=153, ♂/♀=108/45, age range: 4y6mo-16y1mo), respectively. Generalized additive models for location, scale and shape were used to estimate anthropometric-related, i.e. body mass and height, TD percentile curves and to convert outcomes of individual patients with CP and DMD into z-scores. Results: Muscle size and strength, as well as their inter-subject variation, increased with increasing anthropometric values. Individual patients exhibited negative z-scores, indicating muscle size and strength deficits in reference to TD peers. Interpretation: The established anthropometric-related percentile curves for muscle size and strength in a cohort of TD children can successfully be used to express patient outcomes in reference to TD. This facilitates the interpretation of muscle size and weakness in children with a motor disability and allows for the evaluation of the disease progression and treatment impact during growth.

The authors have declared no competing interest.

This research was funded by Duchenne Parent Project NL (17.011), by the Research Foundation-Flanders (FWO-Vlaanderen) through an FWO research fellowship to IV (1188923N), an FWO-TBM grant (Applied Biomedical Research with a Primary Social finality) (TAMTA-T005416N) and an FWO research grant number G0B4619N and by the Internal KU Leuven grant 3D- MMAP, Belgium, C24/18/103.

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The local ethics committees (Ethical Committee UZ Leuven/KU Leuven; s59945, s61324, s62187, s62645 and s63340 and Commissie voor Medische Ethiek-Universiteit Gent; EC/2017/0526) approved this study under the Declaration of Helsinki.

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