Musculoskeletal adaptations, “train low” strategies and muscle fibre stimulus

Updated: Mar 23

Having recently spent a great deal of time explaining the different energy systems and the corresponding musculoskeletal relationship to several athletes who are very interested in developing their understanding further and therefore their performance, I thought I would take the time to explain it in more detail.



So, in this blog we explore the relationship between developing your skeletal muscle system, adaptations to that system and the recruitment of the different muscle fibre types to maximise both your training and racing. Specifically what adaptations occur in response to endurance training and how to enhance that adaptation with nutritional interventions. We also look at the importance of musculoskeletal (peripheral) adaptations vs. cardiovascular (central) adaptations for endurance athletes.


Let’s get started with Skeletal muscle adaptations


There are many muscular adaptions that occur in response to training, and these also occur in some instances quite quickly. Adaptions include increased mitochondria (They make most of the cell's supply of adenosine triphosphate (ATP), a molecule that cells use as a source of energy. This means mitochondria are known as "the powerhouse of the cell") density and function, better capability to utilise 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.


How targeting specific adaptions in training is critical to your gains.


Any training that increases the metabolic load on the muscle will yield adaptations. An example of metabolic loading 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 emphasise certain adaption signals and therefore lead to greater adaptation. What has been seen in the latest research is that some of the most important signals for mitochondria biogenesis (i.e. the process by which cells increase mitochondrial numbers, which in simple terms more is better) are raised quite significantly when training low on glycogen, which in turn should yield more potent mitochondria adaptations. However, what needs to be accounted for is that athletes tend to produce 5-6 % lower power values when training low compared to training at a normal glycogen state, 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 there is a risk of getting too big of a calorie deficit close to race date. This element needs to be considered no matter the time of the year, it is easy to dig a big hole so to speak in terms of calorie deficit. So, when conducting these types of session, we must bear in mind that 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 also known as a primer, 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. Athlete’s feedback is that it can be uncomfortable/challenging the first few times but after 2-3 weeks it starts to feel much more manageable. Hence, the adaptations occur relatively quickly (2-3 weeks). Elite athletes have done this for quite some time, and it is only in recent years that sport scientists have been able to find the explanation to why it seems to work. One good idea could be to periodise them so that the adaptations are staying present all the time, this can be achieved by doing these sessions initially a couple of times a week and then for instance once a week during the season. In order to get that strong stimulus needed for these kind of adaptations, you will 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.


Next up is intensity and duration’s impact on muscular adaptations


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 sports science context 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. High intensity training imposes 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 big that the injury risk associated with these sessions is so high that it should be completed at very specific phases during the training cycle. Therefore high intensity training is best placed in close proximity to the race (for instance during the competition preparation phase) because you then don’t need to do that much of it in order to impose an adaptation that is still around for the race itself. Hence, the classic periodisation strategy is the way to go. By continuing 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 substantial risk of injury that high intensity sessions can produce.


Different muscle fibre types


There are contrary to popular belief three main types of muscle fibre types, the slow twitch muscles fibres (type I) have a slower contraction rate and they are highly oxidized, meaning they prefer the aerobic metabolic pathways. The fast twitch muscle fibres (type II) can be divided into two 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 fibres can hence be trained to resemble the type I fibres, 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 to increase the level of fatigue resistance across all muscle fibre types, you need to train them all.


How to practically apply the adaptations


To generate a response or adaptation in the muscles, you need to expose your muscles for different stimuli at different times. An example of this would be if you are in your build period then there is little to no benefit in exposing your muscular system to very high intensity levels, the reason is that you won’t maintain the gains that you have from those sessions into your race specific phases 6 months later. However, it is possible to expose your system to varying levels of intensity, cadence, terrain, timings of the on and off phases to gain a greater recruitment of muscle types which in turn will make the muscles more fatigue resistant.


What is the limiting factor within endurance performance?


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 decades. My initial thought on the subject is that a novice 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, but and it is a big but the better trained athletes (Elite level) know all too well that they need to maintain their aerobic capacity and develop this every season. You cannot rely on an aerobic base that you built years ago thinking that it will still be there if you haven’t devoted the time and training stimulus continually to improve it. 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.


Train well and stay safe.



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