The mechanical environment of tendons, specifically the Achilles tendon and its strain under load, drives its geometry and material properties adaptation. Strain is reciprocally governed by its geometry and material properties (i.e., structure) in response to the applied mechanical load. Therefore, estimating the tendon's structure and mechanical environment may support the development of novel training paradigms able to target specific Achilles tendon adaptation (e.g., improved material properties, hypertrophy), with applications ranging from rehabilitation through to improved sports performance. How is this to be done? Achilles tendon geometry can be quantified using medical imaging, such as 2D and 3D ultrasound, and when combined with biomechanical measurement systems, such as dynamometry, Achilles tendon mechanical properties and strain can be estimated. However, ultrasound methods are limited when attempting to measure free Achilles tendon strain during dynamic motor tasks, e.g., rehabilitation tasks, locomotor, jumping, and landing. Specifically, 2D ultrasound suffers from measurement errors arising from motion artefacts and changes in muscle-tendon junction shape during locomotion, while freehand 3D ultrasound is limited to quasi-static tasks. Alternatively, Achilles tendon loading and strain during dynamic tasks can be investigated using neuromusculoskeletal modelling, which combines biophysical and phenomenological models of the human neuromusculoskeletal system. However, understanding how to best measure in vivo Achilles tendon geometry and material properties is pivotal to personalise neuromusculoskeletal models and enable physiologically plausible estimations of Achilles tendon strains. Subsequently, the general aims of this thesis were to develop medical imaging methods to quantify Achilles tendon 3D geometry, material, and mechanical properties to create personalised Achilles tendon models, which could next be incorporated into neuromusculoskeletal models to assess Achilles tendon loading and strain in a range of dynamic tasks. To achieve this goal, the thesis was developed via three main studies. [...]