Musculoskeletal tissues have an incredible structure-function relationship. The size and shape of skeletal muscles and tendons are closely related to their functional capacity. Understanding the function and mechanics of these tissues begins with a strong foundational understanding of their size and shape. The current knowledge of human muscle architecture is based on cadaver dissection which is limited by the old age, poor health, and limited subject pool of cadaver donors. Modern medical imaging avoids these limitations and allows for muscle architecture data to be obtained in vivo from healthy subjects ranging in size. Medical imaging can be extended to the muscles of pathological populations such as cerebral palsy and elite athletic populations such as sprint runners. Coupled with knowledge of the muscle sizes in healthy subjects in vivo, these assessments can be used to quantify muscle impairments for pathological populations or quantify muscle hypertrophy patterns due to exercise for athletic populations. Additionally, modern medical imaging approaches allow for the determination of tendon architecture in vivo which can be used to create computational finite element models to simulate and probe the mechanics of tendon function. In the case of the Achilles tendon, these sophisticated approaches may be used to elucidate previously unexplained observations of complex, non-uniform tendon deformations. In this dissertation, I utilize non-Cartesian magnetic resonance imaging (MRI) to generate a comprehensive data set of muscle lengths and volumes in the lower limbs of 24 healthy subjects. Using these data, I explore size scaling relationships in the human lower limb, develop a metric for assessing muscle sizes in populations of interest, and extend this to two populations:​ subjects with cerebral palsy and competitive collegiate sprinters. I go on to use non-Cartesian MRI and finite element modeling to explore the effects of Achilles tendon morphology on non-uniform tendon displacements in the Achilles.