Physiology, Podcast, Science

Musculoskeletal adaptations, “train low” strategies, and muscle fiber types with prof. John Hawley | EP#248

 August 24, 2020

By  Mikael Eriksson


Professor John Hawley is one of the world's leading researchers on the cellular and molecular basis of endurance sports adaptations within skeletal muscle. Additionally, his area of expertise covers the interaction of exercise and diet, and practical implications of this such as "train low" (with low carbohydrate availability) strategies to improve performance.

In this Episode you'll learn about:

  • The adaptations in response to endurance training that take place in skeletal muscle
  • Using nutritional interventions to enhance adaptations: the "train low" strategy
  • Fiber types (fast-twitch and slow-twitch muscle fibers) and how muscle fiber profile may influence training strategies
  • The relative importance of musculoskeletal (peripheral) adaptations vs. cardiovascular (central) adaptations for endurance athletes

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Skeletal muscle adaptations

04:50 -

  • There are many muscular adaptions that occur in respons to training, and these also occur quite quickly.

    Adaptions include increased mitochondria density and function, better capability to utilize fat as a fuel, capillary growth within the muscle as well as the quantity and quality of the muscle itself.

    As these adaptations occur quite quickly, they also reverse quickly as well, but this is not very well studied, especially in very well trained individuals.

Targeting specific adaptions in training

07:15 -

  • Any training that increase the metabolic load on the muscle will yield some kind of adaptations.

    Metabolic load could be training in hot conditions, at altitude with lesser availability of oxygen or to deliberately train with a low availability of glycogen (i.e. ”train low”), which seems to emphasize certain adaption signals and therefore lead to greater adaptation.

    What we have been able to demonstrate in our research is that some of the most important signals for mitochondria biogenesis are raised quite significantly when training low on glycogen, which in turn should yield more potent mitochondria adaptations.

    However, what needs to be pointed out is that athletes tend to put out 5-6 % lower power when training low compared to training during a normal glycogen state, and this needs to be taken into account so it doesn’t lead to psychological stress.

    It is also important to say that this type of training should probably only be done at the early aerobic building phase and quite far away from competition as it is quite risky to get too big of calorie deficits close to race date.

    And this aspect needs to to be taken into account no matter the time of the year, it is easy to ”dig a too deep of a whole” in terms of calorie deficit conducting these types of session and the recovery also is longer.
  • Another interesting strategy is ”sleep low”, which means that you deplete your glycogen stores the night before by conducting a high intensity session and after the session you take in as little carbohydrates as possible, then you go out for an endurance session in the morning in a fasted state.

    Athletes witness that this is really uncomfortable and hard the first few times it is conducted but then after 2-3 weeks it starts to feel much more manageable.

    Hence, the adaptations occur fairly quickly (2-3 weeks).

    Ironman athletes have done this for decades and it is only in recent years that we scientists have been able to find the explanation to why it seems to work.

    These type of sessions, however, tend to take rather long to recover from so you should probably strive for doing them as little as possible.

    One good idea could be to periodize them so that the adaptations are staying present all the time, this can be achieved by doing these sessions for instance once a week during three weeks and then have a gap of these sessions for another couple of weeks, whereafter they are implemented in the training again (when the adaptation starts to fade away).

    In order to get that strong stimulus needed for these kind of adaptations, you will probably need to have emptied around 50 % of the glycogen stores in the muscles, this makes the high intensity session the night before an important part of the strategy, and should therefore not be skipped.

Intensity and duration’s impact on muscular adaptations

22:30 -

  • There is no doubt that high intensity training is a very potent stimulus to skeletal muscles, the metabolic burden that high intensity training impose to skeletal muscles is extremely big.

    However, almost all studies conducted on this subject have been done on semi trained individuals for a fairly short period of time (normally 4-6 weeks), and the difference between these people and highly trained athletes is enormous.
  • From a scientific perspective it is very difficult to say what is happening when highly trained individuals throw in high intensity intervals in their training since almost all well trained athletes do both endurance training and high intensity training and therefore it is difficult to say what effect is due to what training.

    My personal opinion is that I think that high intensity training impose a highly potent muscular response among well trained athletes as well but since they are so well trained, the stress this kind of training puts on the body (not only the muscle but tendons, bones and ligaments etc.) is so high that the injury risk associated with these sessions is so high that it should be done as little as possible.

    Therefore the high intensity training is pretty suitable to do in rather close proximity to the race (for instance during the taper period) as you then don’t need to do that much of it in order to impose an adaptation that is still around for the race.

    Hence, the classic periodization strategy is in my opinion the way to go since you by endurance training and shifting different metabolic burdens (by for instance training with low glycogen stores) still can achieve great muscle adaptations in the building phase of the season but without the high risk of injury.

Different muscle fibre types

34:20 -

  • There are three main types of muscle fibre types, the slow twitch muscles fibers (type I) have a slower contraction rate and they are highly oxidized, meaning they prefer the aerobic metabolic pathways.

    The fast twitch muscle fibers (type II) kan be divided into to categories, the first one (type IIA) can express both similarities with type I and the other type IIB fibre, which have a high contraction rate and also have a higher glycolytic capacity.
  • The type IIA fibers can hence be trained to resemble the type I fibers but they require certain stimuli to be recruited and hence to be trained.

    These stimuli can be either intensity (one would probably need to get close to 80 % of VO2max to start recruiting them) or use different modalities such as low cadence when cycling or hills while running.
  • One very important part of performance is fatigue resistance and one should always strive to train your muscles to get more fatigue resistance, but in order to increase the level of fatigue resistance across all muscle fibers types, you need to train them all!

    Therefore you need to be creative in your training, and here I should also point out that one way of training all fibre types is to do really high intensity work since that requires a full recruitment of all muscle fibers to be able to produce that kind of power!

Practically applying the adaptations

56:40 -

  • Generally speaking, in order to generate a response or adaptation in the muscles, you need to expose your muscles for different stimuli all the time.

    Consequently, one big take home message from me would be to challenge yourself in different ways and this can be in terms of intensity, different terrain or cadence, then you will stimulate the whole spectrum of muscle fibre types and make the muscles more fatigue resistant.
  • I would also strongly recommend to be as professional as possible in regards to your training and keep a very strict training diary.

Where is the limiting factor within endurance performance?

1:00:45 -

  • The debate of what the limiting step for endurance performance is, whether it is the central cardiovascular aspect or the peripheral muscular component, has been around for many decades.

    My quick thought on the subject is that a very novis athlete will typically be limited by the cardio respiratory step while a better trained athlete will probably be limited by his or hers muscular peripheral system in a much higher degree.

    However, what needs to be pointed out here is that there is also a very large biomechanics factor that may very well play the most important part for endurance performance.

    This is where good coaching comes in as an extremely important tool in order to build a great athlete, they can see what needs to be changed biomechanically and have the expertise to initiate that change as well.

    So my final words would be to really recommend people investing in a good coach, I as a scientist truly respect the great coaches and have learnt as much from them as I have from science.


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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