Exercise videogames are increasingly popular as individuals seek to increase daily exercise. This study assessed physiological reactions to perceived task demands from a virtual exercise game and their influence on exercise during the following week. Participants completed a biking task in a virtual environment with an avatar that they were told would slim if they cycled fast enough (low task demand manipulation). Only half of participants' avatars actually slimmed (high task demand manipulation). Individuals with high exercise self-efficacy (ESE) showed similar results in both task demand conditions with high levels of work output and cardiovascular reactivity patterns indicative of threat. However, individuals with low ESE with an avatar that remained constant (high task demand) worked less hard and completed less exercise than those with low ESE who observed their avatar slimming. Results confirm that high task demands for those with low ESE levels can be particularly damaging by reducing future exercise levels.