Podcast, Science and Physiology, Training

Training, testing and physiology with coach/scientist Iñigo San Millán, PhD | EP#262

 November 30, 2020

By  Mikael Eriksson


Iñigo San Millán, PhD, is the head coach of the World Tour cycling team UAE Team Emirates, and the personal coach of Tour de France winner Tadej Pogačar. He is also a professor at the University of Colorado School of Medicine where he does clinical and research work in cellular metabolism, not only in exercise science but also in diabetes, cardiometabolic disease and cancer.

In this Episode you'll learn about:

  • Iñigo's training approach: periodisation, intensity targets, balance of training and rest, and much more
  • Metabolic testing: the protocol's Iñigo uses, and how the test results inform training decisions
  • Differences and similarities between multisport like triathlon and sports like cycling and running
  • Physiological underpinnings of endurance performance: lactate metabolism, central vs. peripheral adaptations, and more
  • Top tips for age-group athletes

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04:00 -

  • My name is Inigo San Millan and I am a professor at the School of Medicine at Colorado University and besides being active in academia I am also active as a coach, mainly for elite athletes.

    Within academia, I am both conducting research within sports and physiology as well as within the cancer and general health field, one can believe that these two areas stand quite far from each other but they are both very much about metabolism.

    Among athletes I have coached, Tadej Pogacar, the winner of this year’s Tour de France, is probably among the most recognizable.

Training principles for endurance events

06:00 -

  • I use to start by looking at things from the perspective of different energy systems.

    The human body have three different ways of producing mechanical energy: the oxidative system (oxidation of fatty acids), the glycolytic system (dependent on carbohydrates as substrate and resulting in lactate as the end product) and the phosphate creatinine system.

    In all endurance events, I think it is essential to be metabolically efficient, i.e. have a great oxidative capacity (fat metabolism), this is true even for sprinters in cycling as they need to get to the final kilometers of the race as fresh as possible.

    The main place for the fat oxidation process in the body is the mitochondria within the slow twitch muscle fibers.

    The glycolytic system is crucial for being able to produce high power outputs for an extended period of time (from a couple of mins up to 1h), and hence it also needs to be well developed (especially for GC riders).

    As many of the listeners probably already are familiar with, lactate (the end product of the glycolytic system) is no longer regarded as a ”waste product” but can be utilized within the mitochondria of the slow twitch muscle fibers as fuel.

    This is the reason for why it is very important for athletes competing in longer endurance events such as full distance triathlons to also have an efficient glycolytic system.

    A good ability to clear/utilize lactate in this way is also highly associated with a strong neuromuscular fatigue resistance, which obviously is a very important component in long endurance events as well.

    The lactate, which is mainly produced within the fast twitch muscle fibers, needs to be shuttled to the mitochondria of the slow twitch muscle fibers where the energy from the lactate molecule can be recovered.

    The effectiveness of this process is highly trainable (developed/less developed glycolytic system).

    Accordingly, for both the oxidative and the glycolytic system, the mitochondria within the slow twitch muscle fibers have a crucial role.
  • In conclusion, in my opinion all energy systems need to be adressed (trained), with the exception of pure sprint training for athletes competing in events that have no such demands (for instance middle and long distance triathlons).

Testing and the guidance of testing for training

14:10 -

  • When I do physiological testing of athletes I use to do a ramped test where every step is quite long, 10mins, in order to be able to really evaluate what is happening on that specific intensity level/power output.

    If you’re going to do a lactate test at a facility that is close to where you live, then make sure that every step is at least 4 or 5mins.

    I measure lactate concentration in the blood during these tests as I think lactate is a really great surrogate marker for what is going on physiologically in the muscle.

    It is a common misunderstanding that highly trained endurance athletes do not produce much lactate, in fact, they produce tremendous amount of lactate when they are riding at intensities that require a high contribution of energy from glycogen/carbohydrates (high intensities) but as they have a very well developed glycolytic system they are also extremely efficient at clearing/combusting the lactate so that the blood concentration of lactate remains low.
  • During my tests I always use to take my athletes to exhaustion and measure the maximal lactate concentration in that athlete, but I don’t mind too much about what that concentration is for an endurance athlete as these are not race specific intensities.

    However, maximum lactate concentration is highly associated with fatigue and a glycogen depleted state (in the same way as maximum HR, these parameters almost always go hand in hand), so it is a way for me to evaluate whether the athlete is fatigued and/or glycogen depleted or not when conducting the test).
  • I also use to measure carbohydrate and fat oxidation rates a these are really telling in regards to understand what systems are more or less developed as well as can be used to dial in nutrition during training and races.
  • I have been measuring carbohydrate combustion rates for 15 years now, and what I found in the beginning was that the carbohydrate combustion was much higher than first anticipated, which made me suggested that athletes during long endurance events should aim for a carbohydrate intake of 80-100g/h instead of the previously suggested target of 30-50g/h.

    To start with, I was heavily criticized for this thesis, but today, everybody knows that one needs to take in around 80-100g carbohydrates per hour rather than 30-50g in order to replace as much as possible of the glycogen spent for energy production.

Prescribing training

26:00 -

  • I am a big fan of the polarized 80/20 principle, as it in my experience works really well for most athletes.

    I use to divide the intensities in different zones, where zone 1 = recovery, zone 2 and 3 = base endurance and FatMax (where the oxidative capacity is at its peak, spending a lot of time here can be tremendously effective for improving mitochondria function), zone 4 = threshold, zone 5 = VO2max and zone 6 = anaerobic work.
  • When planning a week, I use to prescribe one day completely off training and one day of only zone 1 training (recovery), then I use to have (at least in the pre season training) two-three days of zone 2 training and typically one day of zone 5 training.
  • The specific design of the intervals/high intensity training depends on what the race looks like (i.e. the specific demands of the targeted race), but in general I like to do slightly longer intervals than 3-5mins for cyclists (maybe build up towards 40-50mins of total high intensity work) as this represents the demands of the races, the intensity of the work then is close to threshold.
  • I think that the same principles apply really well to amateur athletes as well, this I have seen many times over the years.
  • In regards to coaching triathletes specifically, I try and prescribe as specific training as possible since I believe specificity is really important and as you must train three disciplines, then there has to be a lot of specific work to fit that in.
  • In terms of what modalities I use when prescribing training, I like to combine RPE, power and HR as much as possible, I think they all have their place.

    Combining the different factors can generate even more value, like looking at HR:power decoupling, which is a great indicator of metabolic inefficiencies.

Central v.s. peripheral capacity

42:00 -

  • In my experience, for well trained athletes, the main limiter is the peripheral capacity and hence I don’t pay too much attention to what the central capacity (stroke volume) is.

    I have seen so many examples of people with the same VO2max that are performing massively different due to differences in peripheral aspects (metabolic efficiency, lactate clearance capacity etc.), so to me VO2max does not seem to be especially predictive for performance.

    Consequently, I don’t mind too much about what the VO2max is (or spend a lot of time trying to drive it up in my athletes).

Fueling in training

55:25 -

  • The discussion about limiting carbohydrate intake for some of the sessions have been going on for quite some time, but my advice is to not go down this path.

    By training, you are already putting a lot of metabolic stress on the body and that is why I would say that it is unwise to add additional stress.

    Moreover, one has never been able to show scientifically that it is beneficial to restrict carbohydrate intake in any way.

Low and high cadence work

58:10 -

  • I’m not an expert within this field and therefore I avoid answer the question on how to use different cadences in training.

Athlete phenotype/muscle fibre distribution

59:35 -

  • I don’t pay too much attention anymore to what the phenotype/muscle fibre distribution of the athlete is.

    Nowadays, I pay much more attention to how the athlete/the muscle fibers of the athlete reacts to different types of stimuli, i.e. the plasticity of the athlete rather than the ”default” mode of the athlete.

Rapid fire questions

1:03:45 -

  • What is your favorite book, blog or resource related to endurance sport? ”Exercise Physiology” by George Brooks and ”Exercise Metabolism” by Mark Hargreaves.
  • What is a personal habit that as helped you achieve success? Perseverance, resilience and having long terms goal.
  • Who is someone you have looked up to and admired? George Brooks, he was 25-30 years ahead of his time and a great researcher.


Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

Please contact me if you have feedback on the podcast or want to make suggestions for improvement or send in a question for a Q&A episode.

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