Clinical devices exist that aid in injury rehabilitation by “unweighting” a runner on a treadmill. However, these devices are stationary. For my thesis, I designed, built and tested a Mobile Reduced Gravity Cart (MRGC) to be used outdoors. The MRGC exploits the principle of Mechanical Advantage (MA). The fulcrum of the device was a stripped-down bicycle trailer modified to attach to a runner via aluminum bars and special Velcro shorts. A carbon fiber “uni-tail” supported counterweights at the rear of the device, providing a lifting force to unweight the runner. Throughout the design process, MA was increased from 1 to 5. Three pilot tests were conducted to determine if the MRGC enhanced distance and/or sprint running performance. Heart rate (HR) was used as a metric for exercise intensity. Pilot tests showed no substantial decrease in HR when using the MRGC compared to normal running. Numerous design iterations improved the efficacy of the device, however, none were fully successful. I conducted four diagnostic tests to determine why the MRGC was not working as expected. After removing the uni-tail and counterweight, I found that adding mass to the cart itself increased HR by ~0.37 beat per minute per kilogram. Similar small increases in HR were found when adding mass to the MRGC while the runner was unweighted. After modifying the MRGC-Runner interface from compliant shorts to a more rigid option, no change in HR resulted. Using a force plate, average vertical force during running in place was no different between compliant and rigid interface methods. Stride kinematics were investigated outdoors with and without the MRGC. Contact time decreased 3.3% and stride frequency (SF) decreased 1.5% in conditions of 0.73G (where G means “times normal gravity). Those changes were much smaller than anticipated. It appears that the interaction between reduced gravitational forces (GF) and increased inertial forces (IF) may be responsible for the lack of change in both exercise intensity and stride kinematics. Future studies should examine this interaction in more dep