In briefMore athletes are using mandibular orthopedic repositioning appliances (MORAs), and many have claimed they cause surprising performance improvements. However, most research on this subject has suffered from serious design problems. In this double-blind experiment, control, experimental, and placebo groups of college students were tested for left and right maximal grip strength and endurance, maximal quadriceps and hamstring dynamic strength, and maximal quadriceps and hamstring static strength. Statistical analysis of the results showed that the devices did not increase the strength of the muscle groups measured or the muscular endurance of the hand.