NASA's vision for future exploration-class missions has made countermeasures for muscle atrophy, bone loss and cardiovascular deconditioning areas of major research design and development within the U.S. space program. Due to restricted volume and mass capabilities within the newly-developing Crew Exploration Vehicle (CEV) and Lunar Surface Access Module (LSAM), there is a need for a multi-functional, compact exercise machine that can incorporate both resistive and aerobic exercise capabilities during lunar sortie missions. The proposed innovation is an exercise device, the multi-functional Constant Force Resistive Exercise Unit (CFREU), that can provide a whole-body workout for aerobic exercise and resistive exercise. The device provides constant force eccentrically and concentrically during multiple exercise configurations, allows resistance selection in 2.5kg increments, requires no power to operate, requires no on-orbit maintenance, and can be stowed in an area of 1 cubic foot. During the Phase I performance period, we propose to develop a prototype of this device and a feasibility assessment of the design for spaceflight and commercial use.