This study investigated the effect of cognitive fatigue on physical performance in a paced running task. Experienced runners (
n= 20) performed two 3,000-m runs on an indoor track, once after cognitive fatigue, and once under nonfatigued conditions. Completion times were significantly slower in the cognitive fatigue condition ( M= 12:11,88 min, SD= 0:54,26), compared with the control condition ( M= 11:58,56 min, SD= 0:48,39), F(1, 19) = 8.58, p= .009, eta2p = .31. There were no differences in heart rate, t(17) = 0.13, p> .05, blood lactate levels, t(19) = 1.19, p> .05, or ratings of perceived exertion F(1, 19) = .001, p3 .05. While previous research has examined the impact of cognitive tasks on physical tasks, this is the first study to examine a self-paced physical task, showing that cognitive activity indeed contributes significantly to overall performance. Specifically, cognitive fatigue increased the perception of exertion, leading to lesser performance on the running task.