When exercising to improve health and fitness level, it is important to train at a proper pace. Run or bike too slowly and the health benefits of exercise will not be maximized. Train at too fast of a pace and the exercise bout will be short, painful and increase the risk of injury.
Heart rate is an excellent indicator of work performed by the body.
During exercise, muscles require an increased supply of oxygen to produce energy. Since the heart is responsible for circulating oxygen-carrying blood, the cardiac output must increase during exercise. This is accomplished by increasing stroke volume and heart rate. Stroke volume can double during exercise, but most of the increase in cardiac output is due to elevated heart rate. For example, if a person's stroke volume can increase from 60 ml per heartbeat at rest to 120 ml during exercise, at a heart rate of 70 beats per minute the cardiac output can only increase to 8.4 liters per minute.