Architecture of lower leg muscles in children: Reference curves and potential mechanisms of growth

Muscle architecture [i.e. physiological cross-sectional area (PCSA), fascicle length and pennation angle] changes significantly during childhood. Previous studies have described the architecture of selected muscles in small samples of children over narrow age ranges, but a comprehensive analysis of the distribution of muscle architectural parameters during childhood development is currently lacking. The primary aim of this study was to estimate age- and sex-conditional distributions (reference curves) of architectural parameters of seven lower leg muscles (soleus, medial gastrocnemius, lateral gastrocnemius, tibialis anterior, tibialis posterior, flexor digitorum longus and flexor hallucis longus) in typically developing children aged 5-15 years. We used anatomical and diffusion-weighted magnetic resonance imaging to quantify the three-dimensional architecture and aponeurosis dimensions of seven lower leg muscles in 192 typically developing children aged 5-15 years. Quantile regression with b-splines was used to estimate muscle- and sex-specific reference curves for PCSA, fascicle lengths and pennation angles. In the median 15-year-old, PCSAs were 3.0-4.7 times (range is across muscles and sexes), and fascicle lengths were 1.1-1.7 times that of the median 5-year-old, respectively. Thus, lower leg muscle volumes (product of PCSA and fascicle length) increase primarily through transverse growth, rather than longitudinal growth, especially in children above 5 years of age. There was considerable overlap in PCSA, fascicle length and pennation angle distributions between boys and girls at all ages. Further analysis showed that longitudinal growth of muscle-tendon units is achieved primarily by lengthening of intramuscular aponeuroses and that aponeurosis surface areas scale in proportion with PCSA. The reference curves presented here provide normative data for muscle architecture in children and provide insights into the mechanisms of childhood muscle growth.