Blood Lactate in Training, Part 2: LT1 and Zone 2 Training For Performance and Longevity

By Renee Eastman,
CTS Premier Coach & Performance Lab Director

Blood lactate plays a key role in exercise and fitness, offering valuable insights into how our bodies produce energy, build endurance, and perform. Part 1 of this series covered the basics of blood lactate’s role in exercise and setting training zones. In Part 2, we’ll explore the concept of the Aerobic Threshold (LT1), its relationship to Zone 2 training, and how this foundational approach enhances endurance, fat oxidation, and long-term health.

The Aerobic Threshold (LT1) and Zone 2

The Aerobic Threshold (LT1) represents the lowest exercise intensity at which blood lactate levels begin to rise above baseline during incremental exercise. This subtle increase indicates a greater reliance on carbohydrate metabolism while you are still primarily oxidizing fat as an energy source. LT1 serves as the boundary between light and moderate exercise intensity and defines the upper limit of Zone 2 intensity.

Determining LT1 and setting Zone 2 intensity is most accurately done through blood lactate measurements from a graded exercise test in a physiology lab. However, athletes can estimate Zone 2 boundaries using field tests that determine Lactate Threshold (LT2) heart rate or power. Zone 2 intensity is typically estimated as 55–75% of LT2/FTP power or 68–83% of LT2 heart rate.

In contrast, the Lactate Threshold (LT2), or anaerobic threshold, reflects a significant increase in blood lactate, signaling the onset of net lactate accumulation. Closely associated with Functional Threshold Power (FTP), LT2 marks the boundary between the heavy and severe intensity domains, setting the upper limit of Zone 4 intensity.

How To Tell You’re Really in Zone 2

Lab or field testing provides concrete numbers to set Zone 2 intensity, but athletes can also rely on perceived effort, breathing rate, and heart rate response to self-guide Zone 2 training.

• RPE (Rate of Perceived Exertion): Zone 2 typically corresponds to an RPE of 3–4 on a 10-point scale. This level feels light to moderate and should be comfortable and easy to maintain for extended periods.
• Breathing: Breathing in Zone 2 is controlled and steady, falling within a comfortable conversational level. You should be able to talk in complete sentences without gasping for air.
• Heart Rate Drift (Aerobic Decoupling): This refers to a gradual increase in heart rate during prolonged exercise at a constant effort or power. While factors like heat, hydration, and caffeine can influence heart rate, significant drift may indicate you’re exceeding Zone 2 intensity. When this happens, you’ll likely also notice increased RPE and breathing rate.

Monitoring heart rate, breathing, and perceived effort not only ensures you stay within Zone 2 but also highlights fitness improvements. A lower heart rate or reduced perceived effort at a given pace or power indicates that your LT1 has improved, signaling it might be time to reassess your training zones. In TrainingPeaks, you’ll see a metric known as Efficiency Factor (EF), which is calculated as Normalized Power divided by Average Heart Rate for a given duration. As you gain fitness, you should be able to produce the same NP at a lower average heart rate, or a higher NP at the same average heart rate. This indicates you have adapted to training and can now achieve a greater output for the same oxygen consumption.

Performance Benefits of LT1 and Zone 2 Training

Zone 2 training is highly effective for a lot more than just increasing fat oxidation. All zones matter and there are good reasons to train Zones 3, 4, and higher. But here are the main benefits you’re after as you accumulate time in Zone 2:

• Improved Fat Oxidation: Zone 2 training enhances the efficiency of mitochondria in slow-twitch muscle fibers, optimizing fat usage as a fuel source.
• Enhanced Lactate Clearance: Long-duration Zone 2 sessions train the body to shuttle lactate from fast-twitch to slow-twitch fibers, where it can be reused as energy. This prevents early fatigue.
• Glycogen Preservation: Adaptation to Zone 2 reduces reliance on glycogen during submaximal efforts, which is critical for endurance in long events due to our limited glycogen stores.
• Improved Aerobic Capacity: Regular Zone 2 training boosts capillary density, enhances blood flow to muscles, and improves oxygen delivery and utilization.
• Increased Training Tolerance: A strong aerobic base built through Zone 2 allows athletes to handle higher intensity workouts without overtraining.
• Recovery and Injury Prevention: Low-to-moderate intensity efforts in Zone 2 are less taxing on the body, making them ideal for active recovery while reducing injury risk.

Health Benefits of Zone 2 Training

Beyond performance, Zone 2 training delivers substantial health benefits:

• Cardiovascular Health: Improves stroke volume, capillary density, and heart rate variability, promoting heart efficiency and balancing the autonomic nervous system.
• Metabolic Health: Enhances fat oxidation, improves lipid profiles, and lowers the risk of heart disease by reducing blood pressure, arterial stiffness, and systemic inflammation.
• Cellular Function: Increases mitochondrial density, promotes autophagy, and reduces oxidative stress, delaying aging and supporting cellular health.
• Brain Health: Boosts blood flow to the brain, improving cognitive function and emotional well-being while reducing the risk of chronic diseases.

How Much Zone 2 Training Is Best?

The popular answer for endurance athletes is that about 80% of your training volume should be in Zones 1 and 2, with the remaining 20% consisting of harder intervals. Some people think of 80/20 on a session basis, meaning 80% of your training sessions for a given period (week or month, for instance) should be easy aerobic intensity. You can also think of it on a training hours standpoint. The former creates a more distinct demarcation between “easy days” and “hard days”, whereas the latter accounts for Zone 2 training time accumulated during rides that include intervals (e.g., a 3-hour ride that’s 2 hours of Zone 2 and an hour of intervals and recovery periods).

Both ways can be effective, and your choice may come down to your schedule (rides per week) and total available training time. Generally speaking, if your goal is to go out for an “endurance ride” then the vast majority of that ride should be in Zones 1 and 2.

Conclusion

The Aerobic Threshold (LT1) and Zone 2 training are the foundation of effective endurance programs. By maximizing fat oxidation and building a robust aerobic base, athletes not only enhance endurance but also develop the metabolic flexibility required for varied race conditions. These adaptations come from consistent, methodical training, embodying the principle of “slow to go fast” as a mantra for endurance success.

References

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Borszcz FK, Tramontin AF, Bossi AH, Carminatti LJ, Costa VP. Functional Threshold Power in Cyclists: Validity of the Concept and Physiological Responses. Int J Sports Med. 2018 Oct;39(10):737-742. doi: 10.1055/s-0044-101546. Epub 2018 May 25. PMID: 29801189.

Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metab. 2018 Apr 3;27(4):757-785. doi: 10.1016/j.cmet.2018.03.008. PMID: 29617642.

Cheng B, Kuipers H, Snyder AC, Keizer HA, Jeukendrup A, Hesselink M. A new approach for the determination of ventilatory and lactate thresholds. Int J Sports Med. 1992 Oct;13(7):518-22. doi: 10.1055/s-2007-1021309. PMID: 1459746.

Jeukendrup, A., & Achten, J. (2001). Fatmax : A new concept to optimize fat oxidation during exercise? European Journal of Sport Science, 1(5), 1–5.

Author

Renee Eastman is a Premier coach for Carmichael Training Systems and has been with the company since 2001.  She has a bachelor’s degree and master’s degree in exercise physiology. Renee is certified as a USA level 1 coach, NSCA Strength and Conditioning Specialist, and NASM Nutrition Coach.  She is a former competitive cyclist and a 6-time masters’ national champion.