During vigorous aerobic exercise, such as running, cycling or swimming, your heart's cardiac output -- the amount of blood it pumps per minute -- rises steeply to keep your muscles supplied with oxygen. Long-term endurance training strengthens your heart, increasing its maximal cardiac output, although resting cardiac output remains unchanged.
Cardiac output can be calculated by multiplying stroke volume, or the amount of blood your heart pumps with each beat, times the number of times it beats per minute, or your heart rate. For an average adult male, resting cardiac output is about 5 liters per minute -- approximately the total volume of blood in the body. During strenuous exercise, cardiac output can more than triple to keep working muscles supplied with oxygen.
Resting Cardiac Output
Even after long-term, regular aerobic training, your resting cardiac output remains unchanged, However, the respective contributions of stroke volume and heart rate do change. As your heart gets stronger with training, it pumps more blood with each beat, resulting in an increase in stroke volume. At the same time, your resting heart rate declines. Thus, although there is no net change in cardiac output, your heart can work more efficiently.
Exercise Cardiac Output
With regular endurance training, cardiac output during moderate-intensity workouts also remains unchanged. In some cases, it may even decrease because your body is able to use oxygen more efficiently. However, long-term training increases your heart's maximal cardiac output, allowing your heart to pump more blood to your muscles during very strenuous exercise. This is one of the reasons your maximal oxygen consumption, or VO2max, increases with endurance training. The rise in maximal cardiac output is due to increases in maximal stroke volume. Maximal heart rate does not change appreciably with training.
Long-term resistance training, such as weightlifting, may result in some strengthening of your heart muscle, due to the resistance the heart must overcome during workouts. However, it does not lead to the large changes in cardiovascular function that endurance training does. Increases in stroke volume from resistance training tend to be small. Likewise, the resting heart rate declines slightly, if at all. However, resistance training does result in a lower heart rate at any given workload during a resistance workout.