Zone 3: Why, When, and How Leverage Zone 3 Training

By Renee Eastman,
CTS Premier Coach

Part 3 of a series on leveraging blood lactate response in training

How did Zone 3, a challenging aerobic intensity below lactate threshold, or between LT1 and LT2, get demoted to being a “grey zone” and considered a waste of training time? Well, there’s good evidence that athlete performance improves when we polarize training between “easy days” and “hard days”.  For athletes to have the capacity for 1-2 truly difficult workouts per week, the other days needed to be considerably easier. To keep the messaging simple and understandable, athletes were told to stay in Zone 1 or Zone 2 for easy days, and to make sure that hard days were Zone 4 and above. This left the middle ground – Zone 3 – as a misunderstood wasteland. But it’s not. Zone 3 training can be very effective and important when it is utilized properly.

Background

We know from Part 1 in our series that blood lactate provides critical insights into energy system engagement and helps establish zones using the Aerobic Threshold (LT1) and Lactate Threshold (LT2).

Part 2 highlighted the significant benefits of low-intensity training in Zone 2, including aerobic development and fat oxidation. However, endurance athletes must also leverage training at higher lactate levels in Zone 3 and Zone 4 to achieve optimized performance.

In this article (Part 3), we’ll show you how incorporating Zone 3 and Zone 4 training into a structured program targets higher lactate levels, improving your capacity to perform at higher intensities, to delay fatigue, and to excel in endurance competition.

What are Zone 3 and Zone 4 Training?

Zone 3 and Zone 4 in a 5-zone system represent the intensity range between LT1 and LT2. Exercising in this range is a moderate to hard effort, characterized by rising but manageable blood lactate levels. Above LT1, lactate accumulates beyond baseline, while LT2 marks the threshold where clearance cannot keep up with production, leading to rapid accumulation and fatigue. In this range, fuel utilization shifts from predominantly fat to increasing carbohydrate reliance as LT2 approaches.

Performance Benefits of Zone 3 and Zone 4 Training

Before we can dispel the myth that Zone 3 training is a waste of effort, or even an inefficient use of training time, it’s important to understand what’s happening when you’re exercising at an intensity between LT1 and LT2. Accumulating time-at-intensity in Zone 3 and Zone 4 can help you:  

1. Increase Lactate Clearance and Buffering Capacity
   Regular training at or near the Lactate Threshold (LT2) enhances the body’s ability to clear and use lactate as a fuel source. This adaptation delays fatigue and allows athletes to sustain higher intensities for longer.
2. Improve Anaerobic and Aerobic Power
   Training in Zones 3 and 4 boosts mitochondrial density and oxidative enzyme activity, enhancing aerobic energy production and tolerance to anaerobic efforts.
3. Enhance Cardiovascular Efficiency
   High-intensity efforts improve cardiac output and stroke volume, increasing oxygen delivery to working muscles.
4. Improve Glycogen Storage and Utilization
   Training at higher carbohydrate utilization improves glycogen storage and usage efficiency, delaying depletion and enabling prolonged performance.
5. Improve Mental Toughness and Pacing Strategies
   Zone 3 and Zone 4 training develops resilience and focus during challenging conditions, fine-tuning pacing strategies for sustained high-intensity efforts.
6. Achieve Race-Specific Adaptations
   These zones mimic competition demands, ensuring readiness for varying intensities.
7. Improve Recovery
   Exposure to higher lactate levels conditions the body to recover more effectively between hard efforts.

Differences Between Zones 3 and 4

If the benefits listed above apply to Zone 3 and Zone 4, then what’s the difference between them and why are they separate training intensities? Well, the differences are nuanced, but important:

Zone 3 (Tempo)

• Intensity: 76-90% of LT2 power/FTP or 84-94% LTHR.
• Effort Level: Moderate to moderately hard. RPE between 5 and 6 on a 10-point scale. This effort is sustainable with focus. Breathing is elevated but short sentences are possible between deep breaths.
• Time Duration: 1-3 hours (This is how long athletes can sustain this intensity. Less experienced athletes may complete 4 x 15-minute Zone 3 intervals to accumulate 60 minutes of time-at-intensity. An elite amateur or pro may hold this intensity for 2-3 hours at a time.)
• Lactate Dynamics: Elevated but manageable lactate (~2-3.5 mmol/L).
• Purpose: Builds steady-state endurance for prolonged moderate efforts.
• Example: A steady group ride or 50-mile road race/ group ride or half-marathon running race.

Zone 4 (Threshold)

• Intensity: 91-105% of LT2 power/FTP or 95-105% LTHR (Lactate Threshold Heart Rate).
• Effort Level: Hard to Very Hard. RPE (Rating of Perceived Exertion) between 7 and 8 on a 10-point scale.  At this intensity breathing is labored and it becomes difficult to maintain a conversation.

• Time Duration: 30-60 minutes (This is how long athletes can sustain this intensity. Less experienced athletes may complete 3 x10-minute Zone 4 intervals to accumulate 30 minutes of time-at-intensity. An elite amateur or pro may complete 3 x 20-minute, 2×30-minute, or even 1×60-minute intervals in Zone 4.)
• Lactate Dynamics: At or just above LT2, lactate clearance equals production; levels quickly accumulate (~4.0 mmol/L).
• Purpose: Improves the ability to sustain higher-intensity efforts, increases lactate tolerance, and enhances the capacity to perform near maximum steady-state effort.
• Example: Comparable to a 40-km time trial or 10-km running race.

Why Zone 3 is Called the “Grey Zone”

The idea that Zone 3 is a “grey zone” in training has been debated extensively in endurance sports. Some coaches and athletes label it as such because of its position between low-intensity (Zone 1–2) and high-intensity (Zone 4 and higher) efforts, but this characterization depends on context. Here are some of the valid criticisms of Zone 3 training.

1. Moderate Stress Without Specific Adaptations: Zone 3 doesn’t stimulate as much aerobic base development (like zone 2) or significant gains in anaerobic power and VO₂ max (like zones 4–5).

2. Higher Fatigue for Limited Benefit: Zone 3 efforts are harder to recover from than zone 2 training because of increased carbohydrate utilization and lactate production. Over-reliance on zone 3 may lead to fatigue without offering the same aerobic or anaerobic rewards as lower or higher intensities.

3. The “Sugar Burner” Phenomenon: Over time consistently training in zone 3 conditions the body to rely more on glycogen and less on fat during aerobic exercise. That decreases the ability to oxidize fat effectively, even at low-intensity efforts.

4. Risk of Overtraining: Athletes who unknowingly spend too much time in zone 3 can overtrain because it feels challenging but sustainable, leading to excessive cumulative stress. This often results in “junk miles” that don’t effectively improve either endurance or power.

When is Zone 3 Useful and Recommended?

All training zones matter because you’re always producing energy for muscular work through all available channels. Except for at the very extreme ends of effort you rarely ever reach, you’re always burning fat and carbohydrate, there are always contributions from your aerobic and anaerobic systems, and training at any intensity level affects performance at all other intensity levels. It’s all connected. Manipulating time-at-intensity within these zones is just a matter of applying stress to focused areas along the entire spectrum. With that said, when does it make sense to utilize Zone 3 training?

1. Threshold Development: Zone 3 overlaps is critical for improving lactate clearance and sustainable power output. Training in Zone 3 improves the body’s ability to work efficiently at moderate-high intensities over long durations

2. Race-Specific Preparation: Zone 3 mirrors the intensity of many steady-state efforts required in races, such as long climbs, breakaways, or sustained efforts on flats. For many endurance events (e.g. medium distance road races, group rides, or running marathons), being able to sustain zone 3 efforts is a key performance determinant.

3. Building Resilience: For athletes with a solid aerobic base, controlled Zone 3 efforts can provide a bridge between endurance (Zone 2) and high-intensity interval training (Zone 4–5). This can be particularly useful during the transition from base to build phases of training.

4. Time-Crunched Athletes: For those with limited time, Zone 3 provides a good balance of intensity and duration, allowing for significant training stimulus without requiring extensive hours.

Conclusion

Zone 3 is only a “grey area” if it is misused or over-relied upon. Thoughtfully incorporated, it is a powerful tool for building endurance, improving lactate threshold, and preparing for race-specific demands. Balance is key to ensuring metabolic flexibility and optimizing performance.

Zone 3 and Zone 4 training is essential for endurance athletes aiming to push their limits. By strategically incorporating tempo and threshold efforts, athletes can improve their ability to perform at higher intensities, delay fatigue, and excel in competitive settings. A well-rounded program progresses these efforts from a solid Zone 2 base and ensures adequate recovery for sustainable, impactful progress.

References

• Dantas, J.L., et al. Determination of Blood Lactate Training Zone Boundaries With Rating of Perceived Exertion in Runners. Journal of Strength and Conditioning Research, 29(2), 315-320, 2015.
• Esteve-Lanao, J., et al. Impact of Training Intensity Distribution on Performance in Endurance Athletes. Journal of Strength and Conditioning Research, 21(3), 943-949, 2007.
• Seiler, S. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? International Journal of Sports Physiology and Performance, 5(3), 276-291, 2010.

About the Author

Renee Eastman is a Premier Coach for Carmichael Training Systems with a bachelor’s and master’s degree in exercise physiology. She is certified as a USA Level 1 Coach, NSCA Strength and Conditioning Specialist, and NASM Nutrition Coach. A former competitive cyclist and six-time masters’ national champion, Renee has been with CTS since 2001.