Pumping your heart rate is more important than measuring it for fitness, know what science says

Aerobic exercise such as jogging, biking, swimming or hiking is a basic way to maintain cardiovascular and overall health. The intensity of aerobic exercise is important to determine how much time you should spend training to reap its benefits. As an exercise science researcher, I support the American College of Sports Medicine’s recommendation of a minimum of 150 minutes of moderate aerobic exercise per week, or 75 minutes of high-intensity exercise per week. But what does exercise intensity mean? There is a linear relationship between heart rate and exercise intensity, that is, as the intensity of exercise increases, so does the heart rate. Heart rate zone training, which uses heart rate as a measure of exercise intensity, has increased in popularity in recent years, partly due to the ubiquity of wearable heart rate measuring technology. The way exercise intensity is commonly described is problematic because “very intense” for one person may be “moderate” for another. Heart rate zone training attempts to provide an objective measure of intensity by dividing it into different zones. But heart rate can also be affected by temperature, medications and stress levels, which can affect readings during exercise.

heart rate and exercise intensity

The gold standard for determining the intensity of aerobic exercise is to measure the amount of oxygen consumed and carbon dioxide exhaled. However, this method is cumbersome because it requires people to wear a breathing mask to capture the exhaled gases. This is an easy way to estimate a person’s maximum heart rate. This can be done with an equation that subtracts the person’s age from 220. Although there is controversy regarding the best way to calculate maximum heart rate, researchers suggest that this method is still valid. The American College of Sports Medicine outlines five heart rate zones based on a person’s maximum estimated heart rate. Zone 1, or very mild intensity, equals less than 57 percent of maximum heart rate. Zone 2, or light intensity, is 57 percent to 63 percent; Zone 3, or moderate intensity, is 64 percent to 76 percent; Zone 4, or severe intensity, is 77 percent to 95 percent, and Zone 5, or near-maximum intensity, is 96 percent to 100 percent. However, other organizations have their own measures of exercise intensity with different ranges and descriptions. For example, Orange Theory recommends their Zone 2 training as 61 percent to 70 percent of maximum heart rate. To make matters even more complicated, companies that make heart rate monitors also have higher limits for each zone. For example, Polar’s Zone 2 is up to 70 percent of maximum heart rate, while the American College of Sports Medicine recommends Zone 2 up to 63 percent.

Adapting heart rate zones

Zone training is based on the idea that how the body responds to exercise is determined, at least in part, by the intensity of the exercise. These adaptations include increased oxygen consumption, significant cellular adaptations, and improved exercise performance. Zone 2 has attracted a lot of attention from the fitness community because of its potential benefits. Performance trainers describe Zone 2 as “light cardio”, where the intensity is low and the body relies primarily on fat to meet energy demands. Fats provide more energy than carbohydrates, but deliver it to cells more slowly. Because fat is more abundant than carbohydrates in the body, the body responds to cellular stress caused by exercise in muscle cells by increasing the number of mitochondria, or the energy-producing component of cells. By increasing the number of mitochondria the body can become better at burning fat.

At the other end of the spectrum of exercise intensity is high-intensity interval training, or HIIT. These workouts involve exercising at a high intensity for short periods, such as an all-out sprint or cycling for 30 seconds to a minute, followed by a lower intensity activity. This is repeated six to 10 times. During this type of high-intensity activity, the body primarily uses carbohydrates as a fuel source. During high-intensity exercise, the body preferentially uses carbohydrates because energy demands are high and carbohydrates provide energy twice as fast as fat.

Some people who turn to exercise to lose fat may avoid high-intensity training for Zone 2, as it is considered the ”fat burning zone.” Researchers have found that high intensity interval training produces similar increases in markers for mitochondria production compared to long, moderate aerobic training. Studies have also shown that high-intensity exercisers build muscle and improve insulin resistance and heart health at the same rate as moderate-intensity exercisers, and they achieved these benefits faster. The main problem was discomfort during high intensity exercise.

Moderate or high intensity exercise?

With varying guidelines regarding heart rate zones and conflicting evidence on the potential benefits of training in each zone, exercisers may be left wondering what to do. The most important factor in obtaining the health benefits of exercise is to follow an exercise routine, regardless of intensity. Because the body adapts to moderate and high-intensity exercise in similar ways, people can choose which intensity they like best or dislike least. Note that the American College of Sports Medicine’s recommendation for exercise falls under moderate intensity. This equates to zone 3, or 64 percent to 76 percent of maximum heart rate, a range you can only meet in the upper levels of most zone 2 workouts. If you’re not seeing the desired results with your Zone 2 workouts, try increasing your intensity to reach a moderate level. A commonly cited reason for not exercising is lack of time. For those who are short on time, high intensity training is a good alternative to steady-state cardiovascular exercise. People who find exercising at such high intensity uncomfortable can get the same benefits by doing moderate-intensity exercise for a longer period.