Any time you exercise, you're using the large muscle groups in your upper and lower body to run, swim, cycle or stair climb. These muscles require increased oxygen to produce cellular energy, and your heart is the organ that delivers that oxygen via your blood. The more challenging the exercise, the more oxygen your muscles need and the faster your heart has to work.
The main reasons for the heart rate response to exercise are physiological, meaning 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.
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 also contribute to the heart rate response during exercise.
Certain environmental conditions can affect your heart rate response to exercise. For instance, exercising when it's very hot or cold typically increases your heart rate response to exercise. When it's hot, 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 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 because there is less oxygen in the air; thus, your heart must work harder to deliver enough oxygen to your muscles.
Read more: Fat Burning Vs. Cardio Heart Rate
Mode of Exercise
The type of exercise you engage in will affect your heart rate response. Exercise that uses your upper body, such as cross-country skiing, usually causes an increased heart rate response compared to lower-body exercise.
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 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 is not as conditioned will have a stronger heart rate response 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.
Read more: How to Calculate Maximum Heart Rate