"I Heart Running" is a case study in which students diagnose the cause of exercise-induced tachycardia in an otherwise healthy, 27-year-old female. The patient, Sam, is a long-distance runner and realizes that her exercising heart rate reaches over 200 beats per minute. As the story of Sam's encounters with her physician develops, students are given results from multiple laboratory tests and detailed patient background information.  Students are guided through the material with a series of questions with the ultimate goal of determining the cause of the exercise-induced tachycardia that Sam experiences. The case study is based on actual laboratory results and was designed for students in an upper-level undergraduate Human Physiology course. In this activity students will learn about heart rate, cardiovascular physiology, and oxygen carrying capacity, and factors which influence these concepts, as they try to diagnose Sam.