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Exploring Zone 2 Training and Lactate: Insights from DR.Iñigo San Millán

Updated: Sep 25, 2023


Dr. Iñigo San Millán

In recent times, there has been a growing interest in the advantages of 'Zone 2' training, largely fueled by the successes of athletes like Tadej Pogačar and the wisdom shared by his coach, Dr. Iñigo San Millán, who champions the benefits of Zone 2 training.


As the Head of Performance at UAE Team Emirates and a professor at the University of Colorado School of Medicine, San Millán offers valuable insights into this training method.


Here I will condense key takeaways from some of his notable interviews, addressing common questions stemming from these discussions.



Dr. Iñigo San Millán lactate testing Tadej Pogacar

Understanding Lactate

Before diving into San Millán's insights, it's essential to grasp the role of lactate. When the body breaks down carbohydrates for energy, they are initially converted into pyruvate. Some pyruvate is directly used for energy, while the rest is transformed into lactate and other metabolites. Contrary to the traditional view, lactate serves as a rapid energy source that can move between muscle fibers and circulate in the blood. However, other metabolites produced alongside lactate are associated with muscle fatigue, making blood lactate levels a marker for fatigue and metabolic stability.


Defining Zone 2

Fat and Carbohydrate combustion rates

Whilst most of us would define Zone 2 as our endurance zone, Dr. San Millán defines Zone 2 from a bioenergetics standpoint, considering the energy systems activated at different intensities. Zone 2 stimulates mitochondrial function, fat oxidation, and lactate clearance while primarily engaging Type I muscle fibers (slow twitch). This intensity is often referred to as 'FatMax' because fat oxidation rates are near maximal. At the upper end of Zone 2, glucose metabolism and blood lactate levels increase, as lactate is a byproduct of carbohydrate metabolism. INSCYD's interpretation aligns with Dr. San Millán's definition of Zone 2, where the top end marks a significant rise in blood lactate levels. The midpoint of Zone 2 is where FatMax is usually achieved. Using an INSCYD metabolic test will allow the coach and rider to better understand the various bioenergetics occurring within Zone 2. For example a riders may have a Zone 2 prescribed: 51% - 80% 157 - 249 w (FatMax 174 - 200w).

Tyme wear vest

Zone 2 Definition in Practice

Defining Zone 2 without laboratory testing can be challenging. Common approaches use fixed percentages of FTP or heart rate. Dr. San Millán supports using the talk test, the point at which one is able to hold a conversation indicates Zone 2. Technology like real-time ventilation rate measurement (see Tyme Wear vest opposite) have emerged to personalise training zones.


Benefits of Zone 2 Training

Fat and carbohydrate combustion rates

Dr. San Millán lists three key benefits of Zone 2 training: improving fat oxidation capacity, enhancing mitochondrial function, and facilitating lactate transport between muscle fibers. However, we acknowledge that optimising these attributes may require a combination of different intensities.


While Zone 2 is valuable for aerobic adaptations, it may not maximise these attributes individually. As research suggests, different intensities might be necessary for optimal development.

the lactate shuttle

Dr. San Millán emphasises the importance of lactate transport between muscle fibers, noting its role in performance for endurance cyclists. Two key transporters, MCT1 and MCT4, facilitate lactate movement. Lactate transport is often overlooked but is crucial for sustained power and recovery. It relies on aerobic capacity, emphasising the significance of VO2max.


Using Lactate for Training

lack of pyruvate and lactate accumulation

Just like Dr. San Millán, INSCYD incorporates lactate testing into coaching, highlighting its predictive value for performance and the variability across athletes. He stresses the importance of understanding one's lactate profile.


INSCYD allows the coach to individually interpret their athletes lactate values for a given intensity or pace. Individuals lactate levels will vary significantly, combining subjective feelings with objective data is essential for effective training.


Targeting Zone 2 Intensity

Dr. San Millán suggests focusing on the upper end of Zone 2 for optimal benefits. However, you need to consider other factors like fitness level and training phase before focusing the large portions of your volume on upper Zone 2. Riding at the upper end can be demanding, and a nuanced approach should consider training priorities and recovery.


Avoiding Too Many Power Surges

Dr. San Millán advises against power surges during Zone 2 rides to prevent lactate buildup. I'm one who will advocate that a rider should control their power within endurance rides, but exceptions exist for specific training goals.


Including intervals within longer rides can be beneficial, as they stimulate adaptations related to fat oxidation. Check out my other blog for more details.


Importance of Subjective Feelings

Dr. San Millán emphasises the significance of subjective sensations in training. This is a skill set that is often over looked by endurance athletes, you should really get into the habit of paying attention to how you feel as it's essential for adjusting training effectively.


Subjective data, combined with objective metrics, offers a comprehensive view of recovery and training intensity.

garmin cycle computer

Using Power or Heart Rate in Endurance Rides


San Millán addresses the debate of using power or heart rate during longer rides. He suggests that heart rate can be more reliable for staying in Zone 2. Heart rate is useful for monitoring Zone 2 intensity, but power can still provide valuable information, especially to prevent power spikes.


INSCYD training zones

In conclusion, Zone 2 training, lactate management, and individualised approaches are key aspects of effective endurance coaching. Understanding these concepts and applying them judiciously can lead to improved performance for cyclists and triathletes. To find out how INSCYD metabolic testing can help your training, book your free consultation here.


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