In endurance training and longevity circles, ‘Zone 2’ has become a buzzword, often tossed around… but not fully understood. While many associate ‘Zone 2’ with specific heart rate or power output targets, its true importance lies in the physiological response it evokes, the underlying fuel partitioning, and the adaptations we seek to improve upon. In this, I delve into why true low heart work is important, what we should seek to accomplish, and why we also need high heart rate domain work.
Understanding Training Zones
The 5-zone training model (there are many others) is a cornerstone in endurance sports, with each zone targeting different physiological responses. Zone 1 is the lightest and is useful for recovery and building an aerobic base in trained and untrained athletes. In zone 2, athletes optimize fat oxidation (Fat Max) and enhance mitochondrial number, efficiency, and flexibility – all key factors for long-term endurance training and optimal health.
In Zone 3, we transition our fuel partitioning, favoring glycolysis over fat oxidation. Lactate production increases. If the athlete is well trained, that lactate can be shuttled back to our type 1 oxidative fibers and used for fuel. In Zone 4, we are no longer oxidizing fat for energy. Why? Because fat oxidation produces ATP too slowly. In athletes with a well-developed lactate shuttle, lactate production can be decently balanced with its clearance, allowing them to stay in Zone 3 for a while. Furthermore, the better trained we are, the longer we can stay in Zone 3, and the more we can still utilize fat oxidation to produce at least some of our energy demands. In Zone 4, we are pushing hard. Our energy demands are now outpacing the amount of ATP that fat oxidation can provide, so fat oxidation goes to zero (this often occurs in zone 3 for many of us). Our lactate threshold occurs in Zone 4. This is where lactate will increase steadily because we have saturated our lactate shuttle. We are working hard and feeling it in Zone 4. Zone 5 is our maximal effort. We can only hold this for very short periods of time. Lactate production is increasing rapidly, and fatigue will be upon us shortly.
You should think of your AeT as a ceiling and not a goal.
Zone 1 for trained athletes? Really?
Yes, really! When trying to improve our endurance capacity and the numerous mitochondrial adaptations I mentioned, we need to be able to perform these long, low heart rate sessions at an ‘easy’ pace, cadence, etc. If highly trained athletes had to run/cycle in zone 2 for hours, it would be a very high load or stimulus. Therefore, they will do a fair amount of training in Zone 1.
Aerobic Threshold (AeT) and Lactate Threshold
Key to understanding zone training is grasping the concepts of AeT and Lactate Threshold. AeT marks the intensity at which lactate accumulates in the blood above baseline levels. It’s a critical point where the body shifts from primarily using fat oxidation to glycolysis. Fat oxidation does not produce lactate, but glycolysis does. That’s why you will often read about athletes checking their lactate levels. It helps define our zones from a physiological perspective. Your Aet is above your fat max, or maximal fat oxidation rate, and serves as the transition from Zone 2 to Zone 3.
Lactate Threshold, on the other hand, is the point at which the body accumulates lactate significantly and cannot clear it as quickly as it’s produced. Again, this is where our lactate shuttling capacity has been saturated. Your lactate threshold occurs in Zone 4.
Zone 2 training is essential. When training in zone 2, you should target a heart rate or effort that allows you to stay 8-10 bpm below your AeT. You should think of your AeT as a ceiling and not a goal.
High effort domain work
It is important to provide a hard stimulus to our bodies on occasion. Whether that’s 10% or 20% of your runs/rides is up to you, your trainer, and what you want to optimize for.
High heart rate domain work stimulates many advantageous cardiovascular adaptations, such as VO2 max, and it plays a very important role in avoiding type 2 (fast twitch) muscle fiber loss. Type 2 muscle fibers are glycolytic. They use glucose for fuel. Type 1 fibers are mostly oxidative and use fat and lactate. Therefore, we need these harder efforts to maintain our fast twitch fibers to help us maintain our power quickness and minimize the risks of muscle atrophy– which occurs predominantly from the loss of type 2 muscle fibers.
The Lactate Shuttle Mechanism
The lactate shuttle mechanism is a process where lactate is produced by Type 2 (fast twitch) muscle fibers) which is transported to and utilized in Type 1 fibers as a fuel. This mechanism is facilitated by MCT1 and MCT4 transporters. MCT1 aids in lactate uptake by oxidative fibers, while MCT4 is involved in its efflux from glycolytic fibers.
Training in Zone 2 is one way to enhance this lactate shuttle mechanism. The body becomes more efficient at using lactate as a fuel source, which is crucial for prolonged exercise and overall metabolic flexibility.
Training that targets more MCT1 and MCT4 receptors typically involves exercises that enhance the lactate shuttle mechanism. MCT1 is more active in oxidative (Type 1) muscle fibers, while MCT4 is predominant in glycolytic (Type 2) fibers. Therefore, training that stimulates both fiber types effectively can enhance these transporters:
- Endurance Training: Engaging in prolonged, moderate-intensity exercises can increase MCT1 activity, improving lactate uptake and utilization in oxidative fibers.
- High-Intensity Interval Training (HIIT): This type of training involves bursts of high-intensity efforts, stimulating MCT4 activity in glycolytic fibers, which helps in lactate release.
Improving your lactate threshold
Several controversies exist regarding the best methods to improve lactate threshold (LT) in the training world. I’m not a trainer… so I’ll leave this to the experts.
- Intensity of Training: One major debate centers around the optimal intensity for improving LT. Some advocate for training just below the LT to gradually increase it, while others suggest training at or slightly above the LT for more significant improvements.
- Volume vs. Intensity: There’s an ongoing discussion about the balance between training volume (distance or duration) and intensity (effort level). Some trainers emphasize longer, less intense workouts to build endurance (and build out our lactate shuttle), while others prioritize shorter, high-intensity sessions.
- Specificity of Training: Another debate involves how specific LT training should be to an athlete’s sport. While some argue for highly sport-specific training, others promote a more generalized approach to improve overall fitness and LT.
- Role of Cross-Training: The efficacy of cross-training in improving LT in a primary sport is also debated. Some believe that cross-training can be beneficial, while others argue it’s less effective than discipline-specific training.
- Use of Technology and Data: The reliance on technology, such as lactate analyzers or heart rate monitors, for LT training is another point of contention. Some trainers emphasize a data-driven approach, while others advocate for a more intuitive, perceived effort-based training method.
These controversies reflect differing philosophies and approaches in training, and the most effective method may vary depending on an individual’s unique physiology, goals, and the sport in question.
Physiological Adaptations in Each Zone
Broadly speaking, each training zone elicits specific adaptations. It’s more nuanced than this, but it gives you a good idea of what adaptations we’re targeting.
- Zone 1: Primarily focuses on fat oxidation and building an aerobic base.
- Zone 2: Optimizes fat oxidation, increases mitochondrial density, and enhances lactate shuttle efficiency. That may seem counterintuitive. However, to utilize lactate, you need a multitude of properly functioning mitochondria. Low heart rate training does that.
- Zone 3: Balanced energy utilization from fats and carbohydrates.
- Zone 4: Improvements in lactate threshold and glycolytic capacity.
- Zone 5: Adaptations in anaerobic power and speed.
Many people get the concept of zone 2 training wrong. They target a heart rate using formulas that don’t work for many. Most folks have no idea how their level of training affects what zone they can and should train in. For example. If you are a 50-year-old, you are overweight, and haven’t exercised in years; your AeT will occur walking. Your fat max will occur as soon as you start walking. If you used a formula and tried to run at that pace the formula gave you, you would be in zone 4. But your AeT might be higher than predicted if you’re a trained athlete.
There are many ways to determine your zones and your target heart rates. By far, the most important and useful is to be tested. But there’s a fair amount of research out there that shows that a perceived effort approach can work, too. The talk test might work somewhat, but many of us can talk well into zone 3. Getting tested might be your best approach if you’re genuinely curious.
Low heart rate training offers significant benefits to athletes and others. It targets unique physiological adaptations that help us improve our endurance and improve our chances of living a healthier and longer life. That being said, high heart rate domain work is also important. I try to ensure that approximately 10% of my workouts are in the high heart rate domain. Either zones 4 or 5. You will read about an 80/20 mix, too. Whatever floats your boat. But, avoid thinking that the 20% is time-based. If you’re doing hours of long runs or rides, 20% of that volume at a high heart rate might not be your friend.
My buddy Paul and I cofounded Simplavida. At Simplavida, we published a 4 part series on Zone 2 alone. In addition, I have a long-form post here that goes into much more detail about Zone 2.
