This paper and patent provided a detailed overview of an cool system for generating electrical power passively from everyday human movement. The unique thing about this system is that it uses generative braking – only providing resistance when the leg is trying to slow itself down and thus assisting the gait. This is similar to how some hybrid cars generate electricity during braking.
My primary concern with this device is the physiological impact of wearing it long-term. The body would come dependent on assistance in this part of the gait, thus leaving the corresponding muscles underutilized and left to atrophy. However, I don’t think anyone intends this for long-term use.
[J. M. Donelan, Q. Li, V. Naing, J. A. Hoffer, D. J. Weber, and A. D. Kuo, “Biomechanical Energy Harvesting: Generating Electricity During Walking with Minimal User Effort,” Science, vol. 319, no. 5864, pp. 807 -810, Feb. 2008]