Anthropometric-related percentile curves for muscle size and strength of lower limb muscles of typically developing children.

Muscle size and muscle strength gradually increase during childhood to meet the demands of a growing body. Therefore, the aim of this investigation was to establish anthropometric-related percentile curves for muscle size and strength in a cohort of typically developing (TD) children. Lower limb muscle size and strength were assessed in a large cross-sectional cohort of TD children with 3D freehand ultrasound (four muscles, n = 153 children with in total 156 measurements, male/female = 85/71, age range: 0.6-17.8 years) and fixed dynamometry (seven muscle groups, n = 153 children, male/female = 108/45, age range: 4.5-16.1 years), respectively. Generalized additive models for location, scale, and shape were used to estimate anthropometric-related, that is, body mass and height, TD percentile curves, and to convert absolute outcomes into unit-less z-scores. The results showed that both muscle size and strength, as well as their inter-subject variation, increased with increasing anthropometric values. The mean z-score of the TD children was approximately 0 ± 1 standard deviation (with the largest range from minimum to maximum of approximately -3 to 3) for all investigated muscle outcomes, confirming the fit of the percentile curves to the TD data. The use of the percentile curves was demonstrated through applications in children with cerebral palsy (CP) and Duchenne muscular dystrophy (DMD). The individual patients with CP and DMD exhibited negative z-scores, indicating muscle size and strength deficits in reference to TD peers. The established anthropometric-related percentile curves for muscle size and strength in a cohort of TD children allow for muscle outcomes to be expressed as unit-less z-scores, independent of body size, and relative to TD peers. This approach facilitates the interpretation of muscle size and strength outcomes, enabling the detection of abnormalities or deficits, monitoring of progression, and evaluation of treatment and intervention effectiveness in TD children, as well as in children with genetic, chronic neurological, or muscular disorders.