That heart-pumping, give-it-your-all feeling is one of the reasons many of us run in the first place. And a ramped-up heart rate during any type of exercise is not only normal, it’s necessary. But when is a high heart rate, especially on an easy run or jog, cause for concern?

As you increase your effort level from a walk to a jog and beyond, your muscles require more oxygen to produce energy. To get it there, your heart needs to increase your cardiac output—the number of liters per minute of oxygen-rich blood it pushes through your arteries, says Dr. Elaine Wan, Esther Aboodi associate professor of medicine in cardiology and cardiac electrophysiology at Columbia University Medical Center, College of Physicians and Surgeons and attending physician at New York-Presbyterian Hospital.

That figure is the product of your heart rate and one other factor: your stroke volume, or the amount of blood pushed out with each pulse. Regular training can boost your stroke volume over time, but in the moment, the only way for your heart to meet increased demands is to pick up the pace.