Zone 2 Heart Rate Training: The Science Behind the Method

Zone 2 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 training may be more sustainable for some people.

How do your muscles use energy?

Zone 2 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 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.

What are heart rate zones?

Exercise is often broken down into five zones, including:

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:

Zone 2 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

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

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 in Zone 2?

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 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.

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.

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Disclaimer

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.

References

1. Nes, B. M., Janszky, I., Wisløff, U., Støylen, A., & Karlsen, T. (2013). Age-predicted maximal heart rate in healthy subjects: The HUNT fitness study. Scandinavian journal of medicine & science in sports, 23(6), 697–704. https://doi.org/10.1111/j.1600-0838.2012.01445.x
2. Heinzmann-Filho, J. P., Zanatta, L. B., Vendrusculo, F. M., Silva, J. S. D., Gheller, M. F., Campos, N. E., Oliveira, M. D. S., Feoli, A. M. P., Gustavo, A. D. S., & Donadio, M. V. F. (2018). Maximum heart rate measured versus estimated by different equations during the cardiopulmonary exercise test in obese adolescents. Revista paulista de pediatria : orgao oficial da Sociedade de Pediatria de Sao Paulo, 36(3), 309–314. https://doi.org/10.1590/1984-0462/;2018;36;3;00015
3. Saw AE, Main LC, Gastin PB. Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review. Br J Sports Med. 2016 Mar;50(5):281-91. doi: 10.1136/bjsports-2015-094758. Epub 2015 Sep 9. PMID: 26423706; PMCID: PMC4789708.
4. Keytel LR, Goedecke JH, Noakes TD, Hiiloskorpi H, Laukkanen R, van der Merwe L, Lambert EV. Prediction of energy expenditure from heart rate monitoring during submaximal exercise. J Sports Sci. 2005 Mar;23(3):289-97.
5. Purdom, T., Kravitz, L., Dokladny, K., & Mermier, C. (2018). Understanding the factors that effect maximal fat oxidation. Journal of the International Society of Sports Nutrition, 15(1). https://doi.org/10.1186/s12970-018-0207-1
6. Carey, Daniel G. Quantifying Differences in the “Fat Burning” Zone and the Aerobic Zone: Implications For Training. Journal of Strength and Conditioning Research 23(7):p 2090-2095, October 2009. | DOI: 10.1519/JSC.0b013e3181bac5c5
7. Seiler, K.S. and Kjerland, G.Ø. (2006), Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution?. Scandinavian Journal of Medicine & Science in Sports, 16: 49-56. https://doi.org/10.1111/j.1600-0838.2004.00418.x
8. Gottschall JS, Davis JJ, Hastings B, Porter HJ. Exercise Time and Intensity: How Much Is Too Much? Int J Sports Physiol Perform. 2020 Feb 28;15(6):808-815. doi: 10.1123/ijspp.2019-0208. PMID: 32365286.
9. Huertas JR, Casuso RA, Agustín PH, Cogliati S. Stay Fit, Stay Young: Mitochondria in Movement: The Role of Exercise in the New Mitochondrial Paradigm. Oxid Med Cell Longev. 2019 Jun 19;2019:7058350. doi: 10.1155/2019/7058350. Erratum in: Oxid Med Cell Longev. 2021 Jan 18;2021:9274841. PMID: 31320983; PMCID: PMC6607712.

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