Zone 2

Most of us have heard about zone 2 and the benefits it provides. By adopting a scientific perspective, how can we better understand it? In this article, we hope to address questions such as how accurate your target power needs to be to gain zone 2 adaptations, what will zone 2 specifically improve about your cycling ability, how do you best find your zone 2 intensity, and does zone 2 have a place in a low-volume training plan?

Zone 2 is the intensity at which the fat oxidation in your muscle cells is the highest. It is also called your fat threshold, LipOxMax, and FatMax. The chart below shows the average metabolic profile of more than 5300 tests from a variety of athletes at different fitness levels. We can see that fat oxidation, the blue line, has a moderately broad peak at a relatively low intensity - 47% of VO2max. [1]

The purpose of zone 2 training is to gain adaptations that occur due to your mitochondria primarily using fat as an energy source. Since the fat oxidation profile is bell-shaped, it suggests that zone 2 adaptations are not highly sensitive to slight variations in training intensity—whether slightly too intense or not intense enough.

The reason fat oxidation decreases despite energy demands increasing is because metabolites that are generated by carbohydrate oxidation inhibit it [2]. Along with high exercise intensity, a high carbohydrate intake also decreases your fat oxidation profile. So does decreasing carbohydrate intake lead to better adaptations? The very likely answer is no, as stated in the carbohydrate supplementation article [3].

Zone 2 is also the same exercise intensity as your first lactate turnpoint (LT1) and first ventilatory threshold (VT1) [4]. This is because your cells start to use more glycolytic fuel sources which produce more lactate and CO2. Zone 2 is the second zone in the 6 or 7 zone model, but in the three zone lactate model illustrated below, zone 2 happens at the border of Zone 1 and Zone 2. This chart is taken from a study of highly trained athletes, and notice how the FatMax occurs at a higher relative oxygen usage than the first figure [5].

Because peak zone 2 occurs at LT1 and VT1, the most accurate way to determine your zone 2 is by performing a ramp protocol. You can either use a blood lactate monitor to determine your lactate profile, or calorimetry equipment to determine your oxygen/CO2 usage profile. The authors recommend that each step in the ramp should last 6 minutes as it takes more than 3 minutes to eliminate CO2 production from buffering [1].

If you can’t determine your lactate or metabolic profile, a combination of the talk test and your rate of perceived exertion may be the most accurate way to determine your zone 2. The talk test identifies your zone 2 as the intensity at which you are able to have a full conversation with someone, but if they couldn’t see you they would still know you are exercising. Zone 2 is also the intensity that many people self-select if they are told to exercise for 45 minutes or more, and it is thought to be the pace that our Palaeolithic hunter–gatherer ancestors ran around all day at [1].

But what are the specific benefits to your mitochondria utilising fat as a fuel source? What are the specific benefits of zone 2? In the words of Dr. Inigo San Millan, who coached Tadej Pogacar to 2 Tour de France victories, zone 2 is the “exercise intensity that achieves or stimulates that mitochondrial function and fat oxidation and lactate clearance capacity the most” [6]. So, training zone 2 allows you to produce less lactate and clear it out faster at a given power output. That’s a huge deal, because for maximal efforts lasting more than ~20-30 seconds, acid starts to build up in your muscles. This decreases the force and velocity that your muscle cells are able to contract at, and unless you have bonked or you have high levels of muscular fatigue, it’s the limiting factor of any cycling performance. Another benefit of zone 2 training is that your intermediate and fast twitch muscle fibres get more oxygen, which further underscores the unintuitive, yet large impact of zone 2 training on shorter efforts [8].

The evidence we have that supports zone 2 is that we’ve overwhelmingly observed these adaptations in pro endurance athletes, who spend at least 80% of their time training in zone 2 [5]. Some reviews have attempted to examine the role of exercise volume and intensity on regulators of mitochondrial adaptations, however there is a lack of high-quality studies done on how or why zone 2 concretely affects the mitochondria [7].

A simple reason explaining why zone 2 appears to be so effective in improving mitochondrial function is that you can do a high volume without accumulating fatigue. One highly-cited study found that zone 2 may be the highest intensity that doesn’t flip a binary switch that causes lasting autonomic stress in your body. The study found that participants’ heart rate variabilities and immune system hormones recovered within 5-10 minutes after a zone 2 workout, but took much longer for sweet spot and high-intensity intervals [9].

Even though zone 2 is so effective because you can do a high volume, it still has a place in low volume plans. Based on many observational studies, we know that much less high intensity exercise is needed than zone 2 for the respective adaptations to plateau. The theory is that you’re going to saturate the pathways that adapt to high intensity training with approximately 2 sessions per week for 1-2 months, so the rest of your time should be spent doing zone 2. As paraphrased from one review, two primary signalling pathways for beneficial mitochondrial adaptations exist. One is based on calcium signalling, which is more likely used in zone 2 training, and the other is based on AMPK which is more likely used with high-intensity training. Only a small training volume of the AMPK based signalling is needed to reach a plateau in its benefits, however the findings of most studies imply that the adaptive potential of the calcium signalling pathway is much larger than that of the AMPK signalling pathway. [9]

With the science covered, what practical conclusions can we draw? Zone 2 adaptations are closely tied to the metabolic environment of muscle cells, with effectiveness likely higher when levels are near baseline. Therefore, Zone 2 training may be more beneficial before rather than after intervals, particularly if intervals disrupt metabolic balance. Additionally, incorporating short sprints on Zone 2 days could enhance neuromuscular adaptations, provided they don't cause a lasting metabolic imbalance.

An optimal training week incorporating Zone 2, depending on the training phase and volume, usually features Zone 2 training five days a week, one VO2 max session, and one threshold session in the 1-2 months before competition. Further details and justification will be provided in an upcoming article about training plans.

Notes

[1] Based on this review published in the scientific journal Nutrients. The first figure comes from this review.

[2] From this review published in the Acta Physiologica scientific journal.

[3] As justified in the carbohydrate supplementation article, “A potential criticism of a high carb diet is that it reduces fat oxidation in your cells. This has given rise to the ‘train-low’ or ‘train fasted’ methodology, which aims to increase the beneficial adaptations during low-intensity rides by promoting fat oxidation. While it’s true that the more carbohydrates you consume, the less your cells oxidise fat, you should probably not consider any form of low carb diet. As put by the authors of a recent meta-analysis that tested the efficacy of this “train-low” methodology, “In general, LCHF (Low Fat High Carb) diets have been associated with an impaired ability to perform high intensity exercise, a reduced CHO (Carbohydrate) oxidative capacity, a lower energy yield per litre of O2, and reduced mitochondrial respiration, and together this can explain the absent effects of this diet on performance in elite endurance athletes”. Furthermore, the meta-analysis investigated 9 studies and found that completing low-intensity rides fasted had no effect on the training adaptations of trained endurance athletes.”

Based on the meta analysis in the Journal of the International Society of Sports Nutrition.

[4] Below LT1 your blood lactate concentrations will stay very similar. LT1 is when your blood lactate concentrations rise noticeably with a small increase in power, usually around 2mmol/L. VT1 is when your rate of breathing first starts to increase at a rate higher than your oxygen usage. This happens when your body needs to get rid of more CO2 due to glycolytic oxidation.

[5] Based on this review in Sports Science. The 2nd figure comes from this review. The curve is not based on any particular dataset. Seiler and Stoggl also state that ~80% of elite endurance athletes' time is spent in zone 2.

[6] In this youtube video, Dr. Inigo San Millan gives his definition for zone 2 training. He has previously coached Tadej Pogacar to 2 Tour de France victories.

[7] The scientific term for the beneficial adaptations Zone 2 provides to mitochondria is "mitochondrial biogenesis," which involves enhancements in the mitochondrial network that improve its function. The best scientific publication addressing the specific adaptations occurring at Zone 2, which are absent at higher intensities, is “Principles of Exercise Prescription, and How They Influence Exercise-Induced Changes of Transcription Factors and Other Regulators of Mitochondrial Biogenesis” This review concludes that evidence is scarce regarding how volume and Zone 2 affect pathways leading to mitochondrial biogenesis. The authors propose that either the high volume enabled by low-intensity training increases the level of regulators produced by high-intensity exercise, thus enhancing mitochondrial biogenesis, or that submaximal exercise generates different regulators with a similar effect. However, these hypotheses remain unproven due to limited studies.

[8] This review finds that another adaptation in zone 2 is increased capillarization in type 1 muscle, which leads to higher oxygen availability in type 2 fibres (intermediate and fast twitch muscle).

We paraphrased the following excerpt from the review:

“Two primary signaling pathways for mitochondrial proliferation (both convergent on PGC1-α expression) exist. One is based on calcium signaling, which is more likely used with high-volume training [57,58], and the other is based on signaling derived from adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway, which is more likely used with high-intensity training, as [ATP] and AMP levels are reduced and increased, respectively [59,60]. As recruiting certain motor units elicited during competitive intensity exercise is needed in order to generate adaptative responses leading to increase mitochondrial density and aerobic metabolism, it can be achieved through the completion of at least a modicum of high-intensity training. The fact that most studies conclude that most of the training volume in distance runners should be covered at easy intensity to optimize performance development implies that adaptive potential of calcium signaling pathway is much larger than that of the AMPK signaling pathway. Accordingly, only relatively small training volume of the latter is needed to reach saturation in the adaptive response using this pathway.”

[9] Based on this highly-cited study by Stephen Seiler. The authors of this review find that the conclusions by Stephen Seiler’s study fit within the broader context of the scientifically proven polarised/pyramidal training intensity distributions, which are characterised by high volumes of zone 2.

Because the authors found no difference in the heart rate variability recovery times for sweet spot and high-intensity intervals, they concluded that the stress response to an endurance workout might more closely approximate a binary switch than a continuous spectrum.