Although running is a low cost and convenient means of obtaining a number of health benefits, these benefits are often not optimized due to injury. Many experts advocate core stability exercises to maintain control of the torso and reduce injury risk. However, no one has tested if changing one’s core stability actually causes changes in running form. In this study, the researchers developed an exercise circuit to tire the core muscles and reduce core stability without fatiguing the rest of the body. Twenty-five inexperienced runners ran in the lab, then did as many repetitions of the exercise circuit as they could, and then ran again. After core stability was reduced, the runners significantly increased forces on their knees in a way that may lead to patellofemoral (kneecap area) pain. No changes in running form that lead to other common running injuries were observed. Despite a small sample size, these results suggest that low core stability may be a risk factor for certain running injuries. More research is needed to determine which core exercises are most effective and how much core stability is enough for running safely.