Ageing is linked to a decrease in mobility, which affects the quality of life of many elderly individuals. This is a growing challenge in industrialised societies since the proportion of elderly individuals is becoming larger. One potential solution that would keep these individuals active and independent is the use of mobility assist devices. These devices are designed to reduce the energy demand of the user with the use of electric motors providing torques at joints of the lower limb. Although promising, these devices have a problem: they become uncomfortable after prolonged usage. This is especially true for devices designed to produce substantial assistance. The research goal consisted of quantifying the performance of the physical interfaces, or points of attachments, of an experimental device with multiple interface adjustments. The device was fabricated with design criteria similar to active assist devices to simulate the mechanical behaviour of these particular devices. This analysis provided design recommendations that could ultimately enhance the performance of assist devices available on the market and thus the quality of life of many individuals. This research used force mapping and motion capture to quantify the kinetic and the kinematic compatibility of the device. Experimental results have shown that the position, shape and other parameters of the interfaces had an effect on the relative movement of the brace, or the brace performance. The device interface migration was greater when the interfaces were positioned furthest away form the joint. An increasing level of assistance showed more relative movement between the brace and the user. Interface geometry had a noticeable effect on force distribution over the interface. The results and methodology of this research offers an in depth understanding of the mechanical behavior of the physical interfaces of the developed assist device. Nevertheless, further research and development in the field of human machine interactions are needed in order to develop a physical human-machine interface that will ensure the success of powered assist devices in the future.