We used diffusion tensor imaging (DTI) to study macroscopic and microscopic features of skeletal muscles during childhood development (5-17 years). From muscle volume and fibre length measurements, we determined the summed cross-sectional area of all fibres. From measurements of diffusion properties and simulations of restricted diffusion in skeletal muscle, we estimated mean cross-sectional areas of individual fibres. Our findings suggest that human muscles grow both by adding fibres and by increasing fibre cross-sectional areas. DTI-based measurements of skeletal muscle micro- and macrostructure could have important applications in understanding both normal and disordered muscle growth.