Percent body weight lifted, the number of sets, and time needed to complete 100 back squats were significantly associated with increased risk of ER. Affected athletes were more likely to report going to muscle failure, thinking they could complete the workout, and performing extra squats during the back squat assignment. Consuming protein shakes after the implicated workout was associated with a decreased risk. Clinicians, athletes, and athletic program staff must know risk factors for ER and early symptoms of ER.