Childhood muscle growth: Reference curves for lower leg muscle volumes and their clinical application in cerebral palsy

Skeletal muscles grow substantially during childhood. However, quantitative information about the size of typically developing children's muscles is sparse. Here, the objective was to construct muscle-specific reference curves for lower leg muscle volumes in children aged 5 to 15 y. Volumes of 10 lower leg muscles were measured from magnetic resonance images of 208 typically developing children and 78 ambulant children with cerebral palsy. Deep learning was used to automatically segment the images. Reference curves for typical childhood muscle volumes were constructed with quantile regression. The median total leg muscle volume of a 15-y-old child is nearly five times that of a 5-y-old child. Between the ages of 5 and 15, boys typically have larger muscles than girls, both in absolute terms (medians are greater by 5 to 20%) and per unit of body weight (1 to 13%). Muscle volumes vary widely between children of a particular age: the range of volumes for the central 80% of the distribution (i.e., between the 10th and 90th centiles) is more than 40% of the median volume. Reference curves for individual muscle volumes have a similar shape to reference curves for total lower leg muscle volume. Confidence bands about the centile curves were wide, especially at the youngest and oldest ages. Nonetheless, the reference curves can be used with confidence to identify small-for-age muscles (centile < 10). We show that 56% of children with cerebral palsy in our cohort had total lower leg muscle volumes that were small-for-age and that 80% had at least one lower leg muscle that was small-for-age.