Virtual Reality (VR) technology can be used to influence performance on endurance exercises. In this study, we focused on manipulating perception of own-body strength by exercising in VR as a muscular avatar. In this repeated-measure study, twenty-nine participants performed biceps curl exercise in a maximum repetitions protocol, up to exhaustion. The exercise was done either in VR as a muscular avatar, or without VR, in front of the mirror. Dependent variables were the number of exercise repetitions and self-reported exertion. We also controlled blood glucose level, perceived weight of the barbell and level of embodiment. Participants performed significantly more biceps curl repetitions in the VR condition (Z = -2.05, p < .05) with an effect size of d = 0.36. The self-reported effort did not differ significantly between conditions. The results of this study may have an applied significance since number of exercise repetitions is an ecologically valid measure, directly relevant to real training protocols.