1. A low profile, passive exercise device, configured to elevate aerobic metabolic activity compared to a baseline aerobic metabolic activity in the absence of the device, through a range of normal movement between a first region of the body and a second region of the body, comprising: a first attachment structure, for attachment with respect to a first region of the body; a second attachment structure, for attachment with respect to a second region of the body which is movable throughout an angular range with respect to the first region; a flex zone between the first and second attachment structures; wherein the flex zone imparts bidirectional resistance to movement between the first and second regions of the body, throughout a range of motion.