People with chronic arm impairment should exercise intensely at home after completing their clinical rehabilitation program, but frequently lack motivation. To address this issue, we present a home rehabilitation system that motivates patients by allowing them to perform arm exercises together with friends or relatives in competitive and cooperative games. Inertial sensors are used to track the patient's arm and control the game. The system was tested with seven adults with arm impairment as well as their friends or spouses. They tested four exercise games (single-player, competitive and two different cooperative games) for 3 minutes each. Of the 7 participants, 4 preferred the competitive game, 2 preferred a cooperative game, and 1 preferred to exercise alone. Competition also increased exercise intensity (measured using inertial sensors) compared to exercising alone. Though preliminary, these results indicate that competitive exercise games could improve arm rehabilitation at home for survivors of neurological and orthopedic injuries.