What is Zone 2 Running?

Zone 2 running is all the buzz on Instagram. Like many aspects of training theory, it’s nuanced. This article will delve into the science behind zone 2 training in endurance sports, including running. Additionally, you will learn how to accurately determine if you are running in zone 2.

Zone 2 training can be a bit confusing, because its exact definition varies on its context. In popular running literature, zone training is typically presented on a five zone scale. If you see zone 2 discussed on Instagram, it is likely in this five zone scale. Z1 is very low intensity; Z2 is low intensity, 

In academic literature, training intensity is presented on a three zone scale. Zone 1 is low-intensity, anything below the first ventilatory threshold (<2mmol/L lactate produced). This is easy, conversational pace running. Zone 2 is moderate intensity, ranging from just above first ventilatory threshold up to second ventilatory threshold (or what runners typically call lactate threshold) (2mmol-4mmol lactate produced). Anything from threshold runs to marathon pace runs will fall into this category. Zone 3 is high intensity, which is above the second ventilatory threshold (>4mmol/L lactate). Training in this high intensity zone involves high intensity intervals, with more anaerobic contribution than the other two zones. 

For the purposes of this article, Zone 2 training will be discussed in the context of the popular five zone scale. 

What is Zone 2 Training?

Zone 2 training refers to easy intensity training on a five zone scale. It is a way of measuring easy runs and training intensity, typically done using heart rate. Zone 1 is recovery, Zone 2 is easy, Zone 3 is moderate, Zone 4 is hard, and Zone 5 is very hard. 

Zone 2 is considered the aerobic training zone. Over the past 15 years, a large body of research has found that 70-80% of your training should be spent in zone two (on the five-zone scale). Most days should be easy runs and hard sessions are cushioned by warm-ups and cool downs in zone two. 

The Science of Zone 2 Running

Why is zone two so beneficial? Zone two optimizes the aerobic adaptations without accumulating significant training fatigue. Aerobic adaptations include increased mitochondria density, greater capillarization of the muscles, and enhanced oxidative enzyme activity. Essentially, your body becomes better at transporting and using oxygen for energy. You can spend more time training in zone two than in zones three, four, and five. You train your body to spare glycogen and increase its capacity for fat oxidation. 

For a short and very simplified lesson on bioenergetics:

• Zone two running is below the aerobic threshold.
• Below your aerobic threshold, your body relies on aerobic metabolism from fatty acids and carbohydrates to produce energy. Your body produces pyruvate from glucose or glycogen via glycolysis  (breakdown of carbohydrates). Then the pyruvate enters the mitochondria of the cell for oxidation in the TCA cycle to produce ATP. Fatty acid oxidation also contributes through a process called lipolysis. Your body breaks down the triacylglycerol into fatty acids, which then are oxidized into ATP in the TCA cycle. Fatty acid oxidation is a much slower process and cannot keep up with the demands of higher intensity exercise. 
• There is virtually no anaerobic metabolism occurring in zone 2. This means that lactate production is minimal, <2mmol/L. 
• At 65% of VO2max, triacylglycerol and plasma fatty acids contribute a slightly higher ratio of ATP production than do muscle glycogen and plasma glucose. As you increase intensity past the aerobic threshold, this ratio shifts; fatty acids and triacylglycerol contribute less and glycogen and glucose contribute more. 
• So, in zone two, you are using a higher ratio of fatty acid oxidation compared to aerobic glycolysis and you are producing very little lactate. Zone two is entirely aerobic. 

When Do I Use Zone Two in Running?

The time spent in zone 2 is dependent upon:

• Training experience
• Training goals
• Periodization of training 
• Individual recovery rates

Zone two training is beneficial for trained athletes seeking to improve performance. The more volume you do, the higher percentage of your training you need to spend in zone two.  In zone two, you mitigate the risk of overtraining via glycogen sparing mechanisms. Additionally, zone two training aids in recovery. According to a 2019 study in Frontiers in Active Living, easy intensity training minimizes autonomic nervous system stress.

In base training, you will spend the majority of your time in zone 2 – perhaps up to 90-95% for a few weeks. (However, don’t skip important neuromuscular stimuli like strides and hill sprints!). When training for a race, you may spend 70-80% of your time in zone 2, with 20-30% of your time in higher intensity zones. (The exact distribution will depend on your training approach – here’s more on training intensity distribution.) However, this is not universal: beginners and injury-prone runners may have different intensity distributions and spend more time in zone two. 

How Do I Calculate Zone Two Training Based on Heart Rate?

Often, training zones are defined by percentages of maximum heart rate. However, methodologies may delineate the zones differently. 

• The Norwegian Olympic Federation (and many research articles) define zone 2 as 72-82% of maximum heart rate. 
• Dr. Stephen Seiler’s 2006 study on training intensity zones found that low intensity running cut off at 81% heart rate max in trained runners (+/- 2). This is the same as the NSCA recommends for easy zone running. 
• In a study of training qualities that predict performance, Casado et al. (2021) defined easy runs as “62 to 82% of HRmax,” with minimal mental strain. 
• Jack Daniels Formula defines easy zone training as 65-79% of max heart rate. 
• The Joe Friel method using lactate threshold heart rate defines zone 2 as 85-89% of lactate threshold heart rate. (Some though use <85% of LTHR to calculate easy zone heart rate, if they are functioning in a three-zone training paradigm.)
• Polar defines it as 60-70% of maximum heart rate. 

So, let’s look at how these methods would define zone two. This will use an arbitrary number 175 bpm for the lactate threshold and 195 bpm for the maximum. (LT and max heart rates do not always linearly correlate.)

• Norwegian: 140-159 bpm
• Selier: 157 bpm or lower
• Casado: 121-159 bpm
• Daniels: 127-154 bpm
• Friel: 148-156 bpm
• Polar: 117-136 bpm

As you can see, these methods all produce very different delineations of zone two. Most methods overlap. (The calculations from the Seiler and Casado studies wrap zone 1 and two together.) However, the Polar method is an outlier. Polar does note on their website that these calculations may vary based on training status, with some individuals reaching up to 85% of maximum heart rate while staying in zone two. 

Should I Use Heart Rate Zones for Zone Two Running?

Most heart rate calculations miss individual variation. The more trained you are, the higher your lactate threshold will occur in relation to your VO2max (maximum oxygen uptake). The higher your lactate threshold, then the higher your aerobic threshold (the cut-off of zone two). Even maximum heart rate field tests are unable to account for this variation. Using just heart rate to calculate training zones loses that individual variation. When heart rate is used to determine intensity zones in scientific research, it is calculated based off of heart rate at lactate threshold (see Seiler & Tonneson, 2009). If you use Friel’s method, you are able to calculate your lactate threshold heart rate and determine zone two most accurately. 

If you are using heart rate, you will want to calculate heart rate zones based on your actual maximum heart rate. The standard deviation of age-based maximum heart rate formulas can be profound (one standard deviation of +/- 10-12 bpm, with even greater variances for outliers). Age is only approximately 75% of the influence on maximum heart rate; training status also contributes significantly. You can perform a field test to determine either maximum heart rate or lactate threshold heart rate. Be sure to use a chest strap, as optical wrist-based monitors are more prone to error. 

How Can I Determine Zone Two Without Heart Rate?

In exercise science, training zones are typically defined by either blood lactate measurements or ventilatory rates. Training does not take place in a lab; you aren’t pricking your finger for blood during an easy run. Ventilatory rates is a fancy way of saying breathing patterns. With practice, most runners can learn to identify their training intensity based on breathing patterns. 

Zone Training Based on Breathing Rate:
1: Barely labored breathing
2: Able to carry on a conversation while breathing more than at rest
3: Able to speak in phrases or a sentence at a time
4: Short phrases, such as “this pace feels good”
5: Talking is hard

In a 2022 study in the International Journal of Sports Physiology and Performance, physiologists define low-intensity training (zone 1 in the three tier structure of the paper, zones 1 and 2 in a five zone paradigm) as “where speech is still ‘comfortable’ and where RPE is ≤4 [out of 10 on the 1-10 Borg scale].” 

Zone two intensity is conversational pace. If you can comfortably talk, feel minimal mental strain, and are at a RPE of 4 or lower, then you are running easy. If you notice you are not recovering well or struggle to handle appropriately programmed hard workouts, slow down your easy days. (And assess energy intake!)

What Happens If I Run in Zone Three Instead?

Aerobic training adaptations do not disappear in zone three. Zone three is generally considered to be under your maximum lactate steady state (lactate threshold. Anaerobic contribution is still small in zone three. More carbohydrate oxidation occurs, which can produce slightly more metabolites that cause fatigue. However, steady running can have powerful training effects when used appropriately. 

If you catch yourself speeding up on an easy run, simply rein it back in. Check your ability to talk conversationally and adjust your effort accordingly. However, it’s not a waste of a run or significant training detriment. Going out of zone two doesn’t turn off all training adaptations like a switch. Rather, you are accumulating more fatigue. Extra fatigue may make it hard to recover, especially if you are running at higher volume and balancing the intensity of speed workouts.

If you train every run at zone three (and have been running for more than a year), then you are less likely to adapt as optimally as you could over the long-term. If you spend most of your training time at zone three or higher, you never recover fully. Performance plateaus and injury risk skyrockets. 

Should I Aim to Run in Zone Two As a Beginner Runner?

Many new runners (running for a year or less) will struggle to keep their heart rate in zone two. Their stroke volume hasn’t adapted to the demands, so naturally their HR will be higher. They simply don’t have the cardiac adaptations to run with good biomechanics at a low heart rate. As runners get fitter, their stroke volume increases, which means they can run at a lower heart rate. 

That doesn’t mean they should push every run as hard as possible. However, moderate intensity may actually be beneficial for low-volume recreational runners or novice athletes. In a 2015 review in Frontiers in Physiology, the researchers concluded that spending nearly half the time in a moderate intensity may be appropriate for runners who do not train at high volumes or who are newer to the sport. A large amount of time in zone three provides an aerobic stimulus and lets them work on good biomechanics. Once you have a more developed cardiovascular system (6-12 months of running), then you can focus on slowing down easy days if you want to focus on performance. 

Are There Any Downsides to Zone 2 Training?

When appropriately dosed in training for a trained runner, zone 2 running is beneficial. However, zone 2 should not be exclusively trained in. (Which is what is done in the MAF method).  If you only spend time in zone 1 and zone 2 for long periods, you will plateau or even get slower. In these lower intensity zones, you neglect working the neuromuscular system. Fast-twitch muscle fibers are not recruited, power is not increased via biomechanical mechanisms like the stretch-shortening cycle, and lactate clearance/shuttle is never improved. You need to spend some time – 10-30% of your training – in high intensity zones. Moderate running, interval workouts, and strides/hill sprints all have their value. 

Training for metrics can lead to performance plateaus. It is important to understand the purpose of each run and train to that. For example, if you are training for a hilly marathon, it may be more beneficial to let your heart rate drift into zone 3 on uphills. While walking the uphills keeps heart rate in zone two, it is not specific to the demands of race day. Running the uphills at a zone 3 effort allows you to practice the biomechanics you will use for running uphill on race day. Walking uphill neglects the important training of your muscle fibers and biomechanical patterns.  

Key Takeaways on Zone Two Training

• Spend most of your time in zone 2, but not all.
• Don’t neglect your neuromuscular system.
• Moderate and high intensity running have their place in training.
• Heart rate calculations can be messy; if you want to use heart rate to determine zone two, it’s best use lactate threshold heart rate. You can determine that using a field test to calculate your lactate threshold heart rate. 
• If you can talk comfortably, you are running easy. 
• Train for performance, not for metrics.