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Zone 2 Heart Rate Training: The Science Behind the Method

Sep 20, 2023
Zone 2 Heart Rate Training: The Science Behind the Method

Zone 2 heart rate training is used to improve aerobic endurance, weight management, and overall fitness by staying in a specific heart rate zone. It involves exercising for a long time at a relatively low intensity. A rough guide to whether you are in zone 2 is your ability to hold a conversation. Any exercise you do improves your overall health. Zone 2 heart rate training may be more sustainable for some people.

How do your muscles use energy?

Zone 2 heart rate training aims to exercise at the highest level of exertion that allows your body to process lactate and keep it from accumulating. You want to exercise at the highest level possible, but you are still below your aerobic threshold.

Your muscles can use fat, glucose, creatine phosphate, and lactate as sources of energy. Ultimately, all fuel sources are used to produce adenosine triphosphate (ATP), the energy currency used in the body.

There are three ways ATP can be regenerated in muscle cells:

  • Creatine phosphate: Creatine phosphate can hold the energy in a high-energy phosphate bond and transfer the energy to ADP to make ATP. Creatine phosphate provides a small amount of energy to power a muscle cell for about 15 seconds.
  • Anaerobic metabolism: This process is faster than aerobic metabolism but incompletely breaks down glucose and creates lactate, resulting in an oxygen debt that must be paid back as lactate is converted to pyruvate. Muscles using anaerobic metabolism will fatigue within minutes.
  • Aerobic metabolism: This process is the complete breakdown of glucose, proteins, and fats in the mitochondria by oxidative phosphorylation to provide ATP, carbon dioxide, and water. It is a relatively slow process but provides the most ATP per glucose molecule.

Aerobic endurance is the amount of time muscles can contract using aerobic pathways. Low-level, long-term exercise will stay below the aerobic threshold. Under this threshold, muscle cells can access enough oxygen to fully metabolize glucose and fats.

If you stay below the aerobic threshold, your body can oxidize fats for energy. With low-intensity exercise, fats are your primary fuel. With high-intensity exercise (>70% Vo2 max), carbohydrates are your primary energy source. Between 30% and 40% VO2 max, your body transitions from using fats to using carbohydrates for energy. As exercise intensity increases, fats cannot be oxidized fast enough to keep up with energy demands; fast-twitch muscle fibers are recruited, but they do not metabolize fats well; and increasing blood levels of epinephrine in the bloodstream increase glycogen breakdown.

Zone 2 Heart Rate Training: The Science Behind the Method

What are heart rate zones?

Exercise is often broken down into five zones, including:

ZoneTraining EffortRate of Perceived Exertion (RPE)Fitness Goal
Zone 150-60% of max heart rate2-3/10Warm up, cool down, and recovery
Zone 260-70% of max heart rate4-5/10Fat-burning, aerobics, and basic fitness
Zone 370-80% of max heart rate6-7/10Aerobic endurance
Zone 480-90% of max heart rate8-9/10Anaerobic endurance
Zone 590-100% of max heart rate9-10/10Develop fast-twitch muscle fibers, speed training

Many people estimate their heart rate using the 220-age formula. The validity of this formula has been questioned. Other formulas researchers have suggested to get a better estimation over a wide age range include:1  

  • 208-0.7 age
  • 211-0.64 age
  • 200-0.48 age

Using these formulas, the max heart rate for 20, 40, and 60-year-olds would be:

Age220-age208-0.7 age211-0.64 age200-0.48 age

Zone 2 heart rate training based on heart rate is not the best way to measure whether you are staying at the top of Zone 2 because it does not accurately measure mitochondrial efficiency. VO2 max and lactate measurements are better but not practical for most people. Instead, use your rate of perceived exertion. If you are able to sustain a conversation, though it feels uncomfortable, and you can keep your mouth closed when you are not talking, you are likely in zone 2.3

Zone 2 Heart Rate Training: The Science Behind the Method

How many calories are burned when you exercise in each zone?

Exercising in zones 1-3, or 50 to 75% of your maximum heart rate, is when your body will utilize fats most efficiently for energy. However, it is essential to recognize that weight loss occurs when you are in a calorie deficit, and it is difficult to burn enough calories for weight loss through exercise alone. Diet and calorie consumption will determine weight loss, but exercise can help speed up the process.

Below is a sample calorie burned chart for a 135-pound, 20-year-old female with a max HR of 190.

Using the formula4 or an online calculator

Women: Calories/min = (-20.4022 + (0.4472 * Heart Rate) – (0.1263 * Weight) + (0.074 * Age)) / 4.184

Men: Men: C/min = (-55.0969 + 0.6309 x HR + 0.1988 x weight + 0.2017 x age) / 4.184

50% max HR Zone 160% max HR Zone 270% max HR Zone 380% max HR Zone 490% max HR Zone 5
Calories burned per minute2.524.556.598.6210.6
Calories burned per hour151.2273395.4517.2636

As you exercise in zone 2 for long periods, you burn more fat for energy, but as you exercise in the higher zones, you burn calories more quickly. At low exercise intensities, a higher percentage of energy expenditure is derived from fat. However, total energy expended and total fat oxidation are low. As energy intensity increases, a lower percentage of energy comes from fat, but the total energy expended and total fat oxidized are higher. It is also important to recognize that your body can convert one form of energy into another. For example, if you consume a lot of carbohydrates that your body cannot use, they will be converted to fat.

How well your body burns fat for energy depends on factors such as training status, exercise duration and intensity, diet, and sex. For example, consuming carbohydrates before exercise will favor carbohydrate oxidation; adopting a higher-fat diet will favor fat oxidation even at rest; and female athletes have higher fat oxidation rates than male athletes.5,6

How much of your exercise should be done using Zone 2 heart rate training?

Approximately 75–80% of your training time should be in zone 2, and 15-20% of training should be in VO2 max.7 Approximately 4% to 9% of your training time (of your 20% VO2 max training) should be above 90% of your maximal heart rate to maximize fitness and minimize overtraining. 8

It takes time and effort to reach these goals. Most people find they cannot exercise at an intensity that keeps them at 90% of their heart rate for more than a few seconds. Zone 2 heart rate training can help because it builds aerobic endurance. Plan zone 2 sessions for a minimum of 45–60 minutes. Cycling, swimming, walking, jogging, rowing, or using an elliptical machine can all provide a steady-state exercise environment for achieving your zone 2 goals.

People exercising  using zone 2 heart rate training

What are the benefits of zone 2 training?

Zone 2 training helps improve aerobic performance and energy efficiency. It also increases mitochondrial size, number, and function, leading to better metabolic health.9 It can also help you avoid fatigue, improve recovery, and manage your weight. The human body is complex, and physiology is affected by many things, such as genetics, diet, and environmental factors. Your training plan should differ slightly from others, depending on your goals.

Exercise and a healthy diet are the best ways to lose weight, but they do not work for everyone. If you meet the criteria for taking a weight-loss drug, talk to a treatment specialist at Invigor Medical to learn more about GLP-1 agonists. This medication class has demonstrated its safety and effectiveness in treating obesity in multiple clinical trials.

Get started today with a monthly subscription of Liraglutide.

Zone 2 Heart Rate Training: The Science Behind the Method
While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice.

Zone 2 Heart Rate Training: The Science Behind the Method

Leann Poston, M.D.

Dr. Leann Poston is a licensed physician in the state of Ohio who holds an M.B.A. and an M. Ed. She is a full-time medical communications writer and educator who writes and researches for Invigor Medical. Dr. Poston lives in the Midwest with her family. She enjoys traveling and hiking. She is an avid technology aficionado and loves trying new things.


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