Cardiac Drift: What is a Safe Heart Rate Level with Exercise?

Exercise has numerous health benefits. What is the importance of heart rate during exercise, and why does heart rate increase even if you don't increase the intensity of your workout? This article will explore what cardiac drift is and where your optimal heart rate should be while exercising.

What is cardiac drift?

Cardiac drift is when there is a progressive increase in heart rate and decreased stroke volume (the volume of blood your heart pumps out during each contraction) during prolonged moderate to intense exercise.

Essentially, your heart rate starts increasing even though the intensity of the workout remains constant. In trained athletes and people engaging in low-intensity workouts, there is a lower cardiac drift because the heart rate rises with intensity over a fixed time to meet the energy demands of muscles and tissues during exercise.

Causes of cardiac drift

Causes of cardiac drift are multifactorial. One hypothesis states that the blood volume displacement associated with hyperthermia may contribute to reduced stroke volume. This increases heart rate or reduces ventricular filling time, decreasing stroke volume.

Several factors can modify cardiac drift:

• Ambient and core body temperature
• Hydration status
• Exercise duration

Ambient and core body temperature

A link between cardiac drift and body temperature showed that in warm and hot environments, there is a decrease in VO2 max, the maximum amount of oxygen that can be used during intense or maximal exercise. Studies show a larger cardiac drift associated with warmer temperatures when there is a larger VO2 max. Subsequently, a smaller cardiac drift was associated with cooler temperatures and a smaller VO2 max.

Hydration status

Studies show a correlation of fluid status in relation to cardiac drift. Remaining hydrated during exercise can reduce cardiac drift during sustained exercise. Staying hydrated can also help maintain VO2 max levels. Dehydration resulted in increased cardiac drift. However, there was also an increase in heart rate due to dehydration, leading to a rise in core body temperature. The physiologic effects of dehydration are believed to cause an increase in cardiac drift.

Exercise duration

Studies have shown that cardiac drift negatively affects performance. When athletes try to hold their heart rate max, it slowly reduces cardiac output and results in a performance decline. Some studies have shown that steady-state training can reduce performance, whereas interval training stimulates both the aerobic and anaerobic systems. This gives your body more stamina and improves performance.

What is a normal heart rate at rest and with exercise?

Measuring your heart rate can gauge your exercise intensity and determine how hard your heart works during physical exercise. A normal resting heart rate is 60 to 100 beats per minute; however, most healthy adults range from 55–85 beats per minute.

The American Heart Association recommends a target heart ratio of 50–70% of your maximum heart rate for moderate exercise intensity and 70–85% for vigorous exercise. To determine your maximum heart rate, you subtract your age from 220. For example, if you are 37 years old, subtract 37 from 220 to get a maximum heart rate of 183 beats per minute. This is the target heart rate for exercise.

There are online target heart rate calculators you can use to determine your target heart rate zone while exercising based on your level of intensity.

You can also figure it out by completing this equation:

• Subtract your age from 220 (if you are 37, subtract that from 220 to get 183). This is your maximum heart rate.
• Check your resting heart rate first thing in the morning. Let’s say your resting morning heart rate is 70 beats per minute.
• Calculate your heart rate reserve by subtracting your resting heart rate from your maximum heart rate (183-70=113)
• Multiply your heart rate reserve by 0.7 (70%) and then add your resting heart rate to this number (0.7×113=79.1+70=149.1)
• Next, multiply your heart rate reserve by 0.85 (85%) and add your resting heart rate to this number(0.85×113=96.05 + 70=166.05)
• Your target heart rate zone for vigorous exercise is the number you calculate for 70–85%. In this example, the target heart rate zone is 149.1 to 166.05 beats per minute.

You can use this information to check if you are in your target heart rate zone while working out. You can use a wearable device, such as a smartwatch, to monitor your heart rate while exercising. Alternatively, you can take short breaks during your workout and check your heart rate manually.

Training in your target heart rate zone rewards you the most for your workout. It should be noted that the target heart rate zone is only a guide, and some people may have a lower or higher zone than the calculation. Talk with your healthcare provider regarding this if you have any concerns.

Effects of heart rate and exercise

If your heart rate gets too high, it can affect the blood flow through your heart. Exercise increases your heart rate to meet the extra demands of oxygen, which is why your heart rate increases during intense exercise. If your heart rate didn't increase with exercise, your heart wouldn't get the oxygen needed, and you wouldn’t be able to sustain the exercise.

What if you exercise above your target heart rate? In most people with healthy hearts, going above your target heart rate shouldn't cause problems. However, if you consistently exercise above your target heart rate zone, it is a sign that you are pushing yourself harder than you should be and should slow down. This increases the risk of overtraining and can increase the risk of injury.

Exercising consistently above your target heart zone can increase your risk of worsening your condition if you have uncontrolled hypertension, high blood pressure, or an underlying cardiovascular disease. It is always best to consult with your healthcare provider to determine what levels of exercise are safe for you.