Zone 2 Training Explained: Why Too Much of It Can Slow Down Your Performance

Zone 2 training has established itself as a popular concept for endurance athletes, especially runners who want to improve their aerobic capacity.

But how useful is it to train in this zone, and what intensity is really optimal for performance improvement?

In this article, I explore why Zone 2 is not always the best choice and which alternatives can help you improve your endurance more effectively.

What exactly is Zone 2 training for runners

Zone 2 training refers to a specific intensity level based on heart rate and is often associated with the aerobic threshold. For runners, training in Zone 2 means the effort is high enough to improve endurance, but not so high that glycolysis (using sugar as the main energy source) becomes the primary energy supplier. Instead, training occurs in the lower aerobic range, where the body primarily uses fat as fuel.

What does this mean in practice? Zone 2 is a heart rate range that feels moderately challenging. Runners can speak relatively comfortably in this zone but still feel some effort. It is at the lower end of aerobic endurance, making it ideal for long training sessions without significantly increasing fatigue. However, in many cases, the intensity is too low, which leads recreational runners to spend too much time in this zone.

What is the heart rate in Zone 2 training

The exact heart rate for Zone 2 varies depending on the source and definition. In most cases, it is about 60 to 70 percent of maximum heart rate, with some sources indicating up to 75 percent. For an athlete with a maximum heart rate of 190 bpm, Zone 2 would be between 114 and 142 bpm. However, individual fitness and physiology can cause slight variations.

The exact maximum heart rate can be determined through professional testing, such as a lactate test or spiroergometry. This allows Zone 2 training to be tailored to individual fitness levels, optimizing training effects. There are also simpler methods to estimate maximum heart rate.

What effect does Zone 2 training have

The effect of Zone 2 training is multifaceted. The intensity is in the lower endurance range, meaning the training mainly improves the body's aerobic capacity. The body learns to use oxygen more efficiently and derive energy from fat. This zone is especially important for long-distance runners to build endurance over extended periods.

However, there is a downside: intensity at 60 to 65 percent of maximum heart rate can be too low to induce significant endurance adaptations. At this low intensity, the training is often suboptimal, as the body is not sufficiently challenged to adapt. Training exclusively in this zone can slow progress. This is why running in this intensity range plays a minor role in effective training programming.

It is also important to combine Zone 2 training with higher-intensity sessions to achieve a balanced training spectrum.

Why we should use the traditional GA1 designation

In traditional endurance training theory, the aerobic range is often called GA1 (BE1), which stands for "basic endurance 1." This intensity zone typically ranges from 70 to 80 percent of maximum heart rate (Zone 3 in Garmin watches) or in some cases 65 to 80 percent. This intensity is slightly higher than Zone 2, leading to better aerobic adaptations. GA1 training is generally more intense and results in faster improvements in endurance, as the body is challenged to use oxygen efficiently and provide more energy.

Using GA1 as a reference instead of Zone 2 can lead to better training results, as it is closer to the individual’s capacity and promotes greater adaptation. Advanced runners should regularly train in GA1 to further build aerobic capacity and push performance limits.

Important: Even in GA1, the body remains in the aerobic range, making this intensity suitable for regular running training. From my experience, it is much more effective to orient training intensity to GA1.

How much of the total training volume should be in the GA1 range for endurance athletes

The GA1 (BE1)  range is crucial for building a stable aerobic base. The percentage of total training in this zone depends on training goals, current fitness, and sport. Generally, well-trained endurance athletes should spend about 60 to 80 percent of their training in GA1.

• Beginners – approx. 60 percent in GA1
• Intermediate – approx. 65 to 70 percent in GA1
• Elite athletes – approx. 70 percent in GA1, possibly more depending on season and goals

This ensures the body has enough time to adapt physiologically without overloading. The remaining training time can be spent at higher intensities to improve anaerobic capacity and speed. Beginners can also include some low-intensity training (<65 percent of max heart rate), but too much is ineffective.

Difference between Zone 2 and GA1

It is clear that Zone 3 better represents the GA1 range, as it is more intense but still aerobic. The zones overlap, but Zone 2 is often not intense enough for significant aerobic improvements.

Summary

Zone 2 training is popular, but not always the best choice for runners aiming to optimize aerobic capacity. Zone 2 has advantages for long, low-intensity sessions that promote fat burning. However, optimal intensity for endurance improvements lies in the GA1/BE1 zone, between 70 and 80 percent (Zone 3) of maximum heart rate. Endurance athletes should aim to spend about 65 to 75 percent of their training in GA1 to significantly increase aerobic performance.