Heart rate response to exercise - Tech Brief

Your heart is about the size of your fist and shaped like an upside-down pear. From the moment it begins beating until the moment it stops, the heart works tirelessly, in an average lifetime, the heart beats more than two and a half billion times.

The heart has an electrical conduction system made of two nodes (special conduction cells) and a series of conduction bundles (pathways). The heart begins beating with an electrical impulse from the sinoatrial (SA) node, located high in the right atrium. The SA node is the pacemaker of the heart, responsible for setting rate and rhythm. The impulse spreads through the walls of the atria, causing them to contract. Then, the impulse moves through the atrioventricular (AV) node (a relay station) into the conduction bundles located in the ventricles. As the impulse travels down the bundles, the ventricles contract and the cycle repeats itself. This cycle of atrial and ventricular contractions pumps blood out of the heart.

During exercise, the quantity of blood pumped by the heart increases to match the increased skeletal muscle demand. Heart rate (HR) monitoring is a method commonly used to determine and assess exercise intensity levels. Exercise intensity is a key component of the training response. Therefore, it is important to understand the factors that can influence HR during exercise, so modifications can be made when establishing training heart rate (THR).

Neural Control

Resting and exercise HR are controlled by the sympathetic and parasympathetic nervous system. The sympathetic division of the autonomic nervous system prepares the body for physical activity by increasing HR, blood pressure and respiration. The sympathetic division also stimulates the release of glucose from the liver for energy. Once exercise begins, the sympathetic nervous system is activated and the HR rises quickly. Heart rate also rises by thinking about exercise, which is referred to as anticipatory HR response.

The parasympathetic division helps slow down HR and respiration. At rest, the heart is controlled by the parasympathetic division, which is why the average resting HR is 60 bpm or less. One of the explanations of why endurance athletes have such low resting HR following training is due to increased parasympathetic response.

Catecholamines

During exercise, the release of epinephrine and norepinephrine stimulate receptors in the heart which causes HR to increase.

Medications

Over-the-counter and prescribed medications can affect HR. Therefore, trainers should regularly ask clients if they are taking or have had a change in medications. ACSM's Guidelines for Exercise Testing and Prescription-6th Edition contains an appendix that lists common medications and their effects on exercise.

Environment

High and low ambient temperatures affect HR. When exercising in hot temperatures, HR is often higher; whereas when the temperature is cold, HR is lower.

Body Temperature

A rise in body temperature (which normally occurs during exercise) can increase HR, while a lower body temperature reduces it.

Body Position

When swimming, the HR response is often lower compared to vertical exercise. This is due to the horizontal position of the body and the cooling effect of the water.

Illness

When ill, heart rate is often elevated.

Upper Body vs. Lower Body Exercise

Exercise involving only the arms elicits a higher HR response than lower or whole body exercise.

Age

Age-predicted maximal HR reduces with age.

Gender Difference

At the same relative workload, women have a higher HR response than men. This response compensates for the lower stroke volumes women have compared to men. The average amount of blood pumped out of the heart per minute is referred to as cardiac output. Cardiac output is the product of HR x stroke volume (i.e., the average amount of blood pumped out of the heart per beat). To maintain a given level of cardiac output during exercise, stroke volume and HR must increase. If the degree of stroke volume is limited, HR will be proportionally higher to support the increase in cardiac output.

Altitude

Immediate and prolonged exposure to higher altitude results in a higher resting and exercise HR.

Effect of Exercise on Heart Rate

Resting Heart Rate

Regular exercise can cause as much as a 20 to 30 bpm reduction on resting HR. This is due to greater parasympathetic control and improved myocardial efficiency.

Submaximal Exercise

A benefit of endurance training is improved myocardial efficiency--the heart works harder with less effort. Stroke volume increases following training, thus generally reducing submaximal exercise HR by 10 to 15 bpm.

Maximal Exercise

Maximal HR remains unaffected or slightly reduced with age. With improved cardiovascular efficiency, greater blood flow is achieved via a lower HR response. As the heart grows stronger, the amount of blood pumped with each beat (i.e., stroke volume) increases. Since the amount of blood pumped is greater, the heart relaxes more with each beat. Thus, resting and submaximal HR is lower.