The longer it takes for the heart rate to recover to its resting state, the more intense the activity. Heart rates return to normal after 10-20 minutes of low-moderate intensity aerobic fitness exercise (as seen in the graph). High-intensity activities can cause the heart rate to rise quickly and remain at a high level for some time after the activity has ended. In other words, the more efficient your body is at using oxygen, the higher your heart rate will be during exercise. The more efficient your body is at moving blood throughout your tissues, the lower your heart rate will be once you have stopped exercising. People who are very aerobically fit may be able to maintain their heart rates above 180 beats per minute for several hours after a hard workout. This is called "high endurance" because it takes them longer to recover than someone who is less aerobically fit. As you get older, your cardiovascular system changes. It becomes less capable of responding to stressors such as exercise. This is why older adults often experience symptoms such as shortness of breath, fatigue, and dizziness after just a few minutes of walking or standing still. Their heart rates cannot stay elevated for as long as those of younger people. Also, their lungs may not be able to supply enough oxygen to muscles cells during exercise.