Zone 2 Training: An In Depth Analysis

What is Zone 2 Cardio?

Zone 2 cardio is simple: Think of this type of training as your easy cardio—like a long, slow run, or a recreational bike ride. The intensity of your Zone 2 workouts should feel low to moderate and normally falls between 60-70% of your max heart rate.

The goal of Zone 2 training is to build aerobic capacity. At a lower intensity, you can continue breathing in a steady supply of oxygen and delivering the oxygen to working muscles to create energy.  This process resists fatigue so you can continue exercising.

Zone 2 training is also called low-intensity steady-state cardio because unlike high-intensity interval training (HIIT), you don’t need to include rest periods to finish your workout. You also avoid that wiped-out feeling that sometimes comes with HIIT workouts. For that reason, Zone 2 training keeps your training volume up without overexerting yourself—helping you stay motivated to continue your exercise program and return fresh and ready for your HIIT sessions.

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The Science Behind Zone 2 Training, Metabolism, and Heart Rate

As the intensity of your workout moves into different target heart rate zones, your body uses different fuel sources (like carbohydrates or fat) to create energy, otherwise known as metabolism. Zone 2 science is based on maintaining your intensity within a certain metabolic system. These are three primary energy systems that help you create energy in the form of adenosine triphosphate (ATP):

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1. Aerobic Oxidation: Your body creates ATP at rest and in zones 1 and 2 by using oxygen in combination with stored fat and some carbohydrates. This process takes place in the mitochondria of muscle cells. With a nearly endless supply of fatty acids stored in your body and the overall volume of ATP fat oxidation creates, it’s the most efficient way to power long zone 2 workouts.
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2. Anaerobic Glycolysis: As you get into Zone 3, 4, and 5, your muscles will start to shift into using glucose (carbohydrate) for energy. We store a finite amount of glucose in our body as glycogen, but once these stores are used up in sustained high-intensity work, you run out of steam in your workout—like hitting the wall in a marathon.
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3. Anaerobic phosphocreatine system: Also outside of your mitochondria, this is your fastest energy system. Using creatine phosphate stored within the body for quick but limited ATP production helps power explosive movements without oxygen and may kick in closer to zone 5.

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Zone 2 training increases mitochondrial density and quality, boosting your fat oxidation capabilities and sparing the limited supply of glycogen (1, 2). Low-intensity cardio also increases capillary density, meaning the network of small blood vessels grows (3). As a result, you become more efficient at carrying blood and oxygen to the muscle, and your muscle cells are more efficient at pulling the oxygen out of the blood with more powerful mitochondria.

An important byproduct of glycolysis is lactic acid. While lactic acid has been villainized for years, it’s not the reason for your burning fatigue during a higher-intensity interval. The accumulation of lactate indicates a level of intensity above your aerobic threshold (which is why we test lactate) and is usually produced simultaneously with hydrogen ions and other byproducts, which causes momentary fatigue (4).

Lactic acid, however, is converted to lactate and can be transported to the mitochondria of the same muscle cells, other muscle cells, and the liver. Here, it will be converted into glucose to create new ATP. With a higher aerobic base after consistent zone 2 training, your lactate clearance capabilities improve, as do your abilities to neutralize the hydrogen ions that are causing fatigue (5). With zone 2 training, not only are you able to recover faster between higher-intensity intervals, but it also takes more effort and time for you to get to that lactate accumulation threshold.

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The Health Benefits of Zone 2 Cardio

The benefits of zone 2 cardio go well beyond the well-known heart health perks. The following potential physical and mental health benefits of zone 2 training will have you tying your sneakers for a workout in seconds flat.
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1. Reduces risk of cardiovascular, brain, and metabolic disease and some cancers: The Centers for Disease Control and Prevention (CDC) indicate that zone 2 aerobic training within a holistic physical activity program has a protective effect to some of the diseases that make up the leading causes of premature death in the United States (6).
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2. Boosts immunity: Fighting off the common cold or even COVID-19 is easier when you’re physically fit. Zone 2 training keeps you active, which improves health outcomes for the flu, pneumonia, upper respiratory infections, and common viruses (7, 8).
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3. Increases VO_2max (aerobic capacity): Zone 2 training increases the amount of oxygen your muscles use, otherwise known as VO_2max. Often a marker of your current fitness level, a higher VO_2max means you have more energy and stamina during your workout and all day long.
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4. Increases insulin sensitivity: Improved mitochondrial function helps regulate your metabolism. Your body becomes more efficient at oxidizing fat for energy and selectively using glucose when needed—making your body more sensitive to the release of insulin and reducing your risk of developing Type 2 diabetes (9).
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5. Improves sleep quality and quantity: While most types and intensities of exercise can positively affect sleep patterns, the unique recovery benefits of zone 2 training can enhance your shuteye on both days you perform high- and low-intensity workouts (10).
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6. Reduces stress and sympathetic nervous system response: Exercise is a positive stressor that helps you relax better at rest. Consistent zone 2 training reduces resting heart rate and perceived stress and increases heart rate variability (11).
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7. Boosts mood and reduces symptoms of anxiety and depression: Physical activity is well-known for its mood-enhancing qualities and zone 2 training is no exception. Oftentimes, zone 2 cardio avoids any negative emotions associated with high-intensity workouts, and you can perform it for longer durations (12).
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8. Improves longevity and quality of life: Improved physical mental and physical health, fitness, stress, and sleep over decades are bound to add fruitful years to your life.

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The Performance Benefits of Zone 2 Training

A robust cardiovascular or aerobic base can help you go faster for longer. Consider this: the more resistant you are to fatigue, the better—that’s the power of zone 2 cardio. Endurance athletes like runners or cyclists have been performing zone 2 training for decades, but any athlete can benefit from building their cardiovascular endurance.

Zone 2 training improves lactate clearance rates when performing higher-intensity exercise, which helps you recover faster between intervals. This can also be advantageous for resistance training. While the stimulus and adaptations to the muscle are different in resistance training, the energy systems are the same across any type of exercise.

Training low to race fast is a common practice, but high-intensity exercise still plays a vital role in performance. Popularized by Stephen Seiler as a general guideline (13), you can aim for 80% of your training to be in zone 2 and 20% dedicated to higher-intensity work (closer to zone 4). You can also think of this training structure as a pyramid with zone 2 as your foundation and the other zones building on top of it based on your specific goals.

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Decoding Training Zones: How to Calculate Zone 2 Intensity

Intensity is different for everyone, depending on the fitness level, experience, and age. Here are a few ways to determine intensity so you know you’re reaching and maintaining your zone 2 target heart rate:

1. Max Heart Rate: The simplest way to use heart rate to measure your zones is with max heart rate. Subtract your age from 220, then multiply that number by 0.60 and 0.75—giving you a range of 60-75% of your max heart rate to work towards for your Zone 2 workouts (14).
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2. Heart Rate Reserve: You can use a personalized heart rate strategy based on your current fitness level by calculating your heart rate reserve. After subtracting your age from 220, go a step further and subtract your resting heart rate from this number. Multiply by 0.6 and 0.7, and then add your resting heart rate back into each value and you have a more finetuned heart rate range (60-70%) to aim for in Zone 2 training.
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3. RPE: How you feel during exercise is an accurate way to assess intensity (15). On a scale of 1-10, your zone 2 training should fall around 3-5. On the more technical Borg 6-20 scale, zone 2 would fall around 10-12 (16).
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4. Talk Test: You should be able to hold a conversation when you’re in Zone 2, with a little effort. Think: a leisurely bike ride or a long walk when you’re slightly breathy. Once you can’t complete full sentences, you’re most likely creeping into Zone 3.

In certain instances, a combination of metrics may be recommended, like if you’re on a beta blocker where heart rate is no longer useful to measure intensity. Psychological stress can also affect both heart rate and/or RPE depending on the individual, so try out different methods and see what works best for you. Keep in mind that as you gain fitness, the same workouts will feel easier over time. Reassess if your resting heart rate has lowered or if your everyday walk needs a boost with some extra hills. Wearables are also great tools to monitor your intensity during your workout to stay in Zone 2.

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Tips on Integrating Zone 2 Training into Your Fitness Regime

If the thought of toiling away on the treadmill for an hour puts you to sleep, the following practical tips can help you effortlessly integrate zone 2 training into your fitness routine.
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1. Choose a modality of exercise you enjoy: The more you like a workout, the better the chances you’ll continue engaging in that workout. For example, if you love walking your dog, use that as an opportunity for your zone 2 by picking up the intensity to a brisk pace. If you’re just starting out and struggling to keep your heart rate in zone 2 running, try a walk/run cadence or opt for a bike workout until your body adapts.
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2. Use the resources at your disposal: AIM7 can help guide your zone programming, providing you with a personalized and precise zone 2 heart rate range, which you can use in real-time to measure intensity during a workout. AIM7’s science-driven volume recommendations take into account your exercise history, fitness level, and overall readiness so you know when to opt for zone 2 cardio and how long you should spend in your exercise session that day.
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3. Schedule your workouts ahead of time: You’ll get the biggest bang for your buck from zone 2 training in high volumes. The Physical Activity Guidelines for Americans and the World Health Organization recommend at least 150 minutes of your exercise should come from moderate aerobic activity (17). And with zone 2 training specifically, it’s recommended you work up to sessions that are at least 45 minutes long (18). If you’re a weekend warrior, opt for your longer weekend workouts to be zone 2 and your shorter bouts during the week to be higher intensity.
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4. Build up your volume over time: When you’re just starting a new fitness program or modality, your body needs time to adapt to the new stimulus. Aim for shorter bouts first, like 20 minutes at a time, then 20 minutes and a short break, then another 10 minutes, and continue to build on those intervals and reduce rest periods until you’re accustomed to the new volume.
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5. Perfection is overrated: It takes time to reach and maintain a steady-state heart rate. Don’t sweat it if your heart rate jumps up to zone 3 or dips down to zone 1 momentarily—it’s difficult to control your heart rate with absolute precision. Stay consistent in your training, and you’ll see that it’s easier to feel your zones and adjust your effort accordingly.
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6. Go easy when it’s hot or humid: If you’re not accustomed to elevated heat or humidity, your body works in overdrive to maintain your internal temperature. So, your exercise not only feels harder, but it’ll drive your heart rate up, easily moving you out of zone 2.

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Start Zone 2 Training

Whether you’re training for an ultramarathon, want a little more pep in your step, or chasing longevity, Zone 2 training is an essential component of any exercise program regardless of fitness level and goals. The recommended dose: at least 150 minutes per week at a moderate intensity (60-75% of your max heart rate), preferably in longer bouts. Zone 2 training not only builds a robust athletic foundation for any sport, but the protective effects of consistent training can help you live a long, fuller life free from disease.

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References

1. Exercise training increases skeletal muscle mitochondrial volume density by enlargement of existing mitochondria and not de novo biogenesis
2. Understanding the factors that effect maximal fat oxidation
3. Exercise‐induced capillary growth in human skeletal muscle and the dynamics of VEGF
4. Acidosis and Phosphate Directly Reduce Myosin’s Force-Generating Capacity Through Distinct Molecular Mechanisms
5. Lactate as a fulcrum of metabolism
6. Health Benefits of Physical Activity for Children, Adults, and Adults 65 and Older
7. Brief Summary of Findings on the Association Between Physical Inactivity and Severe COVID-19 Outcomes
8. Leisure-time physical activity and mortality from influenza and pneumonia: a cohort study of 577 909 US adults
9. The essential role of exercise in the management of type 2 diabetes
10. The bidirectional relationship between exercise and sleep
11. Effects of Exercise Training on the Autonomic Nervous System with a Focus on Anti-Inflammatory and Antioxidants Effects
12. Exercise for the treatment of depression and anxiety
13. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes?
14. ACSM's guidelines for exercise testing and prescription
15. Validity, Reliability, and Application of the Session-RPE Method for Quantifying Training Loads during High Intensity Functional Training
16. A new approach to monitoring exercise training
17. Physical Activity Guidelines for Americans
18. Dose-response relationship of the cardiovascular adaptation to endurance training in healthy adults: how much training for what benefit?
19. Do Olympic athletes train as in the Paleolithic era?

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