Running-specific prostheses enable amputee athletes to perform at the highest level. However, current prosthesis design still impairs sprinting performance in multiple ways. An example is the constant mechanical stiffness of the prosthetic devices. The dynamic behaviour of the blade is determined by this property, but it is static and cannot change according to the gait phase-specific sprinting dynamics. During acceleration a different stiffness might be required as opposed to steady state running. Therefore, it is hypothesised that amputee athletes can improve their performance with prostheses that have a gait phase-specific stiffness. In order to investigate the effect of stiffness on amputee sprinting performance a novel and unprecedented modelling approached is used. Modelling is economical and straightforward in comparison to other research methods such as laboratory experiments. In this thesis, an extension of the established Spring-Loaded Inverted Pendulum model is proposed that qualitatively describes amputee sprinting motion. The Actuated Spring-Loaded Inverted Pendulum (ASLIP) is capable of predicting stable forwardly integrated sprinting motion with the inclusion of the start and acceleration phase. Optimisation of the model predicts that phase-specific spring stiffness leads to a significant time reduction on the 100m sprint for given physiological parameters. In general it can be said that a stiffer spring results in better performance. More specifically, the model benefits from a stiff spring during acceleration and a more compliant one in steady state. Although it has its limitations, the ASLIP model additionally provides a valuable insight into the mechanics of amputee sprinting. For example, it seems that optimal phase-specific stiffness is strongly dependent on biomechanical parameters such as touchdown angle, force angle and CoM velocity. Future work in this direction can provide a better understanding of the underlying mechanisms that determine amputee sprinting performance. The outcomes of this study suggest that amputee sprinters might be able to achieve a reduction in finishing time with prosthetic devices that have a phase-specific stiffness. The modelling approach used in this thesis is promising and lends itself well to investigate this opportunity in more detail.