Three-dimensional skeletal muscle architecture in the lower legs of living human infants

Little is known about the skeletal muscle architecture of living humans at birth. In this study, we used magnetic resonance imaging (MRI) to measure the volumes of ten muscle groups in the lower legs of eight human infants aged less than three months. We then combined MRI and diffusion tensor imaging (DTI) to provide detailed, high-resolution reconstructions and measurements of moment arms, fascicle lengths, physiological crosssectional areas (PCSAs), pennation angles and diffusion parameters of the medial (MG) and lateral gastrocnemius (LG) muscles. On average, the total lower leg muscle volume was 29.2 cm3. The largest muscle was the soleus muscle with a mean volume of 6.5 cm3. Compared to the LG muscles, the MG muscles had, on average, greater volumes (by -35%) and greater PCSAs (by -63%) but similar ankle-to-knee moment arm ratios (-0.1 difference), fascicle lengths (-5.7 mm difference) and pennation angles (-2.7 degrees difference). The MG data were compared with data previously collected from adults. The MG muscles of adults had, on average, a 63-fold greater volume, a 36-fold greater PCSA, and 1.7-fold greater fascicle length. This study demonstrates the feasibility of using MRI and DTI to reconstruct the three-dimensional architecture of skeletal muscles in living human infants. It is shown that, between infancy and adulthood, MG muscle fascicles grow primarily in crosssection rather than in length.

Automated summary

This study used MRI and DTI to measure and reconstruct the skeletal muscle architecture in the lower legs of human infants. The results showed that the medial gastrocnemius (MG) muscles had greater volumes and physiological cross-sectional areas (PCSAs) compared to the lateral gastrocnemius (LG) muscles. However, the ankle-to-knee moment arm ratios, fascicle lengths, and pennation angles were similar between MG and LG muscles. The study also found that the cross-sectional area of the MG muscles increased primarily in infancy and adulthood.