Any time you move you must utilize the muscles of your limbs and trunk, called the skeletal muscles. These muscles require increased oxygen during use, so your heart needs to pump faster in order to deliver more oxygen through your arteries to your working muscles. Exercise involves challenging many muscles and results in a significant increase of the pumping rate of your heart, which is known as the heart rate response to exercise.
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The main reasons for the heart rate response to exercise are physiological, meaning that they are normal functions of your organ systems. Increased activity in the nerves of your working muscles triggers an increase in activity of the sympathetic nervous system. The sympathetic nervous system induces what is known as the stress, or fight or flight, response, which includes an increased heart rate as well as increased blood pressure, body temperature and slowing of digestion among other changes. The harder you exercise, the higher your sympathetic nerve activity and thus the higher your heart rate until you reach your maximum achievable heart rate. In addition to an increase in sympathetic nerve activity, exercise causes your body to release stress hormones. The main stress hormones are epinephrine and norepinephrine, which both further contribute to the heart rate response during exercise.
Certain environmental conditions can affect your heart rate response to exercise. For instance, extreme heat or cold typically increases your your heart rate response to exercise. The reason for the heat effect is that your heart has to pump blood to your skin surface for cooling in addition to providing oxygen to your muscles. Extreme cold induces an increased heart rate response to exercise because your circulatory system must work harder to keep you warm and your blood vessels become constricted. Constricted blood vessels mean that blood flows less freely and the heart needs to pump faster as a result, in order to get enough oxygen-filled blood through these narrow arteries. High altitude causes an increase in your heart rate response to exercise because there is less oxygen in the air, and thus your heart must work harder to deliver enough oxygen to your muscles.
Mode of Exercise
The mode, or type, of exercise you engage in will affect your heart rate response to exercise. Exercise that uses your upper body, such as cross-country skiing, usually causes an increased heart rate response. This greater response is because your arms contain smaller blood vessels and when they are constricted during exercise, your heart has to work harder to deliver enough oxygen to the muscles they supply. Your body position also affects your heart rate response. Exercise done in a horizontal position, such as swimming, results in a lower heart rate response to exercise because your heart does not have to work against gravity to deliver blood to your upper body and brain.
Exercise training over long periods of time can affect your heart rate response to exercise. Someone who has low fitness will have a stronger heart rate response to exercise than someone who is physically fit. The human body has evolved to try to remain in homeostasis--or a state of equilibrium--because it saves energy. If you exercise consistently, your body adapts in ways that keep it more homeostatic even during times of physical stress. Some of these methods include more efficient use of oxygen in the muscles, more powerful heart pumping and more relaxed, elastic blood vessels. Thus, if you are trained, your heart rate will still increase during exercise, but you will have to exercise at higher intensities to get the same heart rate increase as you did when you were untrained.