To understand the muscle architecture of healthy individuals in vivo, and in diseases such as cerebral palsy, structural and architectural parameters of skeletal muscles are often investigated using medical imaging. Diffusion tensor imaging, in particular, allows for in vivo determination of muscle fibre arrangement within a muscle. Common measurements taken are fascicle length and 2D pennation angle. However, muscle fibres are arranged 3-dimensionally, and cannot be fully described with a single or even multiple 2D pennation angles. Consequently, quantitative comparisons between individual muscle fibre arrangements are extremely limited, leaving a knowledge gap in understanding how fibre architecture varies between people and how it is impacted by pathologies such as cerebral palsy. In this abstract, we propose a new method to quantitatively and statistically assess fibre tracts arrangement in lower-limb muscles, focusing on the gastrocnemius muscle. Our method relies on the mathematical underpinnings of statistical shape modelling, but we add fibre directionality in the form of 3D vector fields in the muscle data. We obtained DTI data of healthy children from previous studies [1][2]. We constructed fibres using MRtrix and FSL software. We embedded vector fields obtained from DTI into volumetric meshes and ran Principal Component Analysis (PCA). Shape and vector field modes (i.e., principal components) were acquired. These weighted modes describe the dominant features of the muscle combined shape and vector field variations. Dominant features may reflect biomechanical variations in the functionality of gastrocnemius muscles. In short, a shape and vector field model was constructed to capture the dominant vector field features of fibre tract variations in gastrocnemius muscles. In future studies, differentiation of the shape and vector field dominant features between children with CP and healthy controls will be carried out; and the biomechanics behind the dominant features in a group of children with CP will be investigated.