With a growing elderly population, shoulder injuries are becoming more common, and part of the recovery plan is to go to physiotherapy. However, to alleviate the demand for physiotherapists, robots could help with shoulder rehabilitation. To do this safely and enjoyably, the robot will need to prevent re-injury caused by fatigue while keeping the patient interested and motivated to continue with their therapy. In this study, a method for managing fatigue of the two most commonly injured shoulder muscles, the supraspinatus and infraspinatus, in a game is proposed and tested. To validate the developed method, a human factors experiment was conducted. The fatigue-adaptive game was compared to a baseline in which participants controlled fatigue themselves. The participants played three cases for each version of the game. To minimize the risk of over-fatiguing during physiotherapy and not crossing the line of being too fatigued. Therefore, we measured the overshoot of fatigue in both versions of the game. The mean of the overshoot is compared with a Welch's t-test with Bonferroni correction for each fatigue case. The results show a significant difference for some of the fatigue cases, where the controller is either significantly better or there is no significant difference in the overshoot. The fatigue-adaptive game shows consistency across the cases, whereas the baseline does not. Therefore, the fatigue-adaptive game can compete with a person in managing fatigue, while being easier to learn and automatically identifying and removing risky shoulder positions where fatigue changes rapidly. The fatigue-adaptive game can also be played with an industrial robot arm and still demonstrates the capability to manage the fatigue of the two most commonly injured muscles.