Zone 2 Training: Is it the missing link in your cardio regimen?
Or is it just an excuse to slow down?
My first article on Substack outlined the basics of HIIT training and offered practical ways to incorporate it into your training regimen. Since HIIT training is designed to be used conservatively, it begs an important question: How should you train the rest of the time? Research indicates that low intensity, steady state, Zone 2 training is the answer.
Consider this a simplified guide to Zone 2 training.
What is it?
Zone 2 (Z2) training is aerobic, low intensity, steady state training. It doesn’t incorporate the use of intervals, and it utilizes around 60-70% of your maximum heart rate (HR) or other intensity metric of your choosing, like VO2max. If you were utilizing a talk test, you could easily carry on a conversation with a friend while completing a Z2 training session.
Since this article also references VO2max, let’s define it. VO2max is the maximal amount of oxygen your body can utilize during exercise. This amount of oxygen is important, because it directly affects the amount of energy your body can produce. You’ll see it used in research because it is the gold standard for cardiovascular exercise prescription. However, without access to a lab for testing, most people won’t know what their VO2max is. Thus, heart rate ranges are the more practical option.
Z2 has been labeled as the “fat-burning” zone on most cardio zone training charts that you see in the gym. That’s because Z2 training relies on fat oxidation for fuel. Fat oxidation is often misinterpreted for fat loss, but that’s not necessarily the case. For far too long, people have believed that if they just go low (intensity) and slow that they’ll lose fat.
Fat is used as the primary source of fuel in submaximal exercise (<65%VO2max). However, as exercise intensity increases, carbohydrates begin to replace fats for fuel (Brooks & Mercier, 1994). Fat utilization varies with gender, training status, and nutrition (Achten & Jeukendrup, 2004). Endurance training increases levels of maximal fat oxidation, indicating that trained individuals continue to use fat for energy at higher exercise intensities (Bircher & Knechtle, 2004).
However, the body doesn’t solely use fat stores from adipose tissue, which is why it doesn’t directly alter body composition. Fatty acids are pulled from dietary fat, cholesterol, intramuscular stores, and subcutaneous adipose tissue (Purdom et al., 2018).
Why do people do it?
1) Despite its easy-going nature, it offers participants a chance to improve their overall aerobic base and increase their training volume, while reducing the risk of overtraining. The goal of aerobic training is to economically improve our body’s utilization of oxygen, meaning that ideally, we want to be able to run faster while using less oxygen than our untrained friends.
2) Endurance training positively affects the function of the mitochondria, which can result in improvements in speed, VO2 max, and reductions in muscular fatigue (Zoladz et al., 2022).
ATP, which the body uses as energy, is produced in the mitochondria of the cell. (I can still hear my high school biology teacher’s voice saying, “The mitochondria are the powerhouse of the cell.”) Mitochondrial function is improved with aerobic and anaerobic exercise, but those improvements aren’t endless. Too much exercise and the cell experiences stress, resulting in oxidative damage. Oxidative stress is a primary contributor to overtraining syndrome (Vargas-Mendoza et al., 2021).
I know there’s a subset of people that fully believe if you train slow, you will be slow. However, I liken the slower training to the same phenomenon that happens when you take a break in strength training. With proper recovery, your body is able to heal and repair because sleep is an incredibly powerful recovery tool.
If you spend the majority of your training at or around race pace, when it comes time for the main event, theoretically, you’ll have left part of your race behind with your training. Zone 2 is also a measure of energy conservation and the opportunity to connect with your body.
3) Z2 allows participants a greater volume of training (frequency and duration) with a reduction in necessary recovery time. While HIIT training requires rest in between sessions due to the physiologic response to the acute stress it places on the body, Z2 training can be completed daily because of its light intensity. As an instructor, my favorite part is that anyone can do it. It requires minimal skill, minimal equipment, and is more likely to increase long term exercise adherence because of the cost/benefit ratio.
How should it be programmed?
The most common recommendation for the breakdown of Z2 to HIIT training is 80/20, meaning that 80% of training should be aerobic in nature and 20% of training should be anaerobic. This is also supported by the fact that the recommendation for HIIT session frequency is one to two times per week (NSCA, 2021).
Both are important for improving performance, cardiovascular health, and overall stamina. However, these recommendations shouldn’t be so prescriptive that training becomes Z2 or bust. If a banger pops up on your playlist, and you hit Z3 or even threshold because you speed up to the beat, your training isn’t “ruined”.
Unless you’re doing something unsafe, or pushing yourself toward overtraining syndrome because you’re failing to recover, all movement is good movement.
Wearing a device makes Z2 easy to monitor, but if you train in Z2 enough, you’ll be able to go by feel. If you live in a climate that is rather temperamental (looking at you, Southern US), environmental conditions are likely to skew your HR response.
When it’s impossibly hot and humid outside, it may feel impossible to keep your HR low enough. It can be equally as impossible to go slow enough to get your HR in a Z2 range . If you planned on a Zone 2 run, but end up at Zone 3, consider another important metric we exercise physiologists love: RPE or Rating of Perceived Exertion.
RPE is a measure of how you feel, typically on a 1-10 scale. Yes, this is the same type of smiley face chart you’ve likely seen at your doctor’s office. Coupled with metrics, like HR or VO2max, it’s a great tool to give you a bigger picture of your overall training.
Using the example above, while my Garmin may indicate I’m well within Zone 3 for my afternoon run, my RPE may only be a 3/10. This is a sure sign that something else is contributing to my elevated heart rate, because it’s not my true exercise intensity. If the talk test checks out and you can sing the full chorus of your favorite T. Swift song, then your intensity matches Zone 2. Your heat tolerance is what’s lacking.
If you’re new to exercise or trying to get back into a routine, walking as frequently as you can is a great place to start. Those who are training or tracking performance metrics, I’d encourage you to consider the following questions:
How frequently is your training at or above 80% of your maximum heart rate?
What intensity is your submaximal training performed at? Is it continually Z3 or close to threshold?
Do you take rest days? Are they adequately programmed in your training (i.e. after an anaerobic or high intensity training day)?
For some, adopting Zone 2 training might feel like a massive shift in your exercise routine. However, when used correctly, it will allow you to increase your training volume while keeping burnout at bay.
Are you new to Zone 2 training or are you a seasoned pro? I’d love to hear about your training experience in the comments.
References
Achten, J., & Jeukendrup, A. E. (2004). Optimizing fat oxidation through exercise and diet. Nutrition, 20(7-8), 716–727. https://doi.org/10.1016/j.nut.2004.04.005
Bircher, S. & Knechtle, B. (2004). Relationship between fat oxidation and lactate threshold in athletes and obese women and men. Journal of Sports Science and Medicine, 3(3), 174–81. PMID: 24482595
Brooks, G. A., & Mercier, J. (1994). Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. Journal of Applied Physiology, 76(6), 2253–2261. https://doi.org/10.1152/jappl.1994.76.6.2253
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, 3. https://doi.org/10.1186/s12970-018-0207-1
Vargas-Mendoza N, Angeles-Valencia M, Morales-González Á, Madrigal-Santillán EO, Morales-Martínez M, Madrigal-Bujaidar E, Álvarez-González I, Gutiérrez-Salinas J, Esquivel-Chirino C, Chamorro-Cevallos G, Cristobal-Luna, J., & Morales-Gonzalez, J. (2021). Oxidative stress, mitochondrial function and adaptation to exercise: New perspectives in nutrition. Life, 11(11), 1269. https://doi.org/10.3390/life11111269
Zoladz, J. A., Majerczak, J., Galganski, L., Grandys, M., Zapart-Bukowska, J., Kuczek, P., Kołodziejski, L., Walkowicz, L., Szymoniak-Chochół, D., Kilarski, W., & Jarmuszkiewicz, W. (2022). Endurance training increases the running performance of untrained men without changing the mitochondrial volume density in the gastrocnemius muscle. International Journal of Molecular Sciences, 23(18), 10843. https://doi.org/10.3390/ijms231810843