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It’s a slightly odd title for an article and doesn't really hint at the topic enclosed but once you read it you will understand just why I chose it!

You may recall recently (see blog post here) that I listed my top 10 training tips and that the #1 was having a purpose for each training session.  Well today’s article goes one step (and maybe more) behind this to answer just why we do the specific types of training sessions.

Now the subject matter here can get heavy, pretty quickly and I could actually write a whole book on it (others have!).  My aim here is to cut through as much of the mumbo jumbo as possible and just lay it out as simple as possible.   With that in mind I am going to kick things off with a phrase that summarises the entire article, “fitness is in the muscles, not the cardiovascular system”.   This is a phrase I came across some time ago and it summed up my thinking quite nicely.

Why training is about muscles and not the heart and lungs?

Perhaps the best place to start is by asking a simple question, “what came first, the chicken or the egg?”

This is appropriate as it aptly described the dilemma that is physical conditioning.  So often I see people, even experienced coaches (inc myself previously) get confused about what exactly the training is trying to achieve and how to best approach it.  What you need to understand is that the stress applied to the muscle fibers is what causes the body to adapt and improve and that the cardiovascular system is simple responding to these increased requirements.

A somewhat odd example to expand on this:  Lets say you normally fill your car up with unleaded fuel and it happily chugs along at 100kmh.   After reading some promo material you decide to use a higher octane fuel, say 95ron, thinking it will give you a power boost.  What you have just done is provided more energy in the fuel supply but your engine hasn't changed, it will still go along at 100kmh but it simply does it using less fuel.  This provides for greater overall mileage/endurance.  The top speed of the car will most likely go up too but the engine isn’t capable of huge increases so it will only be a small increase.  Next we upgrade the engine to a more powerful model.  The fuel stays the same but the engine doesn’t have to work as hard to generate the same power so the engine will last longer, but it’s also capable of higher absolute power output so it can go faster.

To relate this to the human body, this describes the relationship between the cardiovascular system, fuel supply and muscles.  With improved fitness the cardiovascular system can deliver more blood to the working muscles and it does so more easily (less fatigue), however we are still limited by the fuel supply and the muscles ability to utilise that fuel.   If we train our bodies to be more efficient at oxidising fats then our endurance will increase and we generate less lactate for a given power.  But, as with the car example, despite improved endurance our top sustainable speed doesn’t improve very much.  The solution is to train our muscles to work more efficiently at using the fuel our body provides and this provides that supercharged speed that we need on race day.  To understand this more we need to consider the different types of muscle fibers in our body and how they work during exercise at different intensities.

Fiber types

The human body has three types of muscle: cardiac, smooth and skeletal.   Cardiac muscle is found in the heart, while smooth muscle is in the digestive system.  As the name suggests the skeletal muscle is found throughout the skeletal framework and makes up the greater proportion of muscles in the human body.  Most importantly there are three primary types of skeletal muscle:

  • Slow Twitch Oxidative (ST or SO) fibers (or type I)
  • Fast Twitch Oxidative Glycolytic (FOG  or type IIa)
  • Fast Twitch Glycolytic (FG or type IIb)
Characteristics of the Three Muscle Fiber Types
CharacteristicType I (SO)Type IIa (FOG)Type IIb (FG)
Contraction time Slow Fast Very fast
Size of motor neuron Small Large Very large
Resistance to fatigue High Intermediate Low
Activity used for Aerobic Long term anaerobic Short term anaerobic
Force production Low High Very high
Mitochondrial density High High Low
Capillary density High Intermediate Low
Oxidative capacity High High Low
Glycolytic capacity Low High High
Major storage fuel Triglycerides CP, Glycogen CP, Glycogen

(table from

While the different characteristics of these muscle fibers lend themselves to specific types of physical efforts, what we are primarily concerned with is their ability to produce power from a specific fuel source.    For road cycling the SO fibers do the lions share of the work with a solid contribution from the FOG fibers.  Most importantly, both of these fiber types can work in an aerobic (oxidative) state – that is they convert fuel to energy with the use of oxygen (breathing).    They do this via the mitochondria in each muscle fiber and therein lies the most important fact – the higher the mitochondrial density the greater the capacity of the muscle to produce power aerobically!  Slow twitch fibers are also highly resistant to fatigue so the more we can train them to produce power the better our endurance will be.

For the trackies, in particular the sprinters, it’s all about the type IIb FG fibers.  These babies can generate massive amounts of power extremely quickly (1500-2000+ watts), however they also fatigue just as fast so the sustainability of efforts is quite low (15-20 sec max).  For the slightly longer track events, such as the 500-1000m time trial or pursuit then the FOG fibers come into play a lot more and even the SO fibers need to pitch in for a significant amount of the work.

So, now knowing this, the next thing you need to understand about muscle fibers is the order in which, and how, they are activated. Then from this, how we can train their abilities.

Force vs Intensity

Muscle fibers are activated in a sequential order starting with the SO, then FOG and finally the FG fibers.  The muscle fibers are grouped into motor-units of the same fiber type and each of these motor units is activated at FULL POWER every time it is called into action.  This is important as it means some of our muscle fibers are essentially dormant when exercising below certain intensities.   At lower intensity, for example at rest, only a small proportion of the ST fibers are required.  As intensity increases then more SO fibers are recruited.   As more and more of the SO fibers get activated the FOG fibers start to come on line – almost like a backup for a power plant.  Now there is a joint increase in fiber activation across both fiber types.  However, as the intensity increases further the body starts to recruit the FG fibers and continues to do so until all fibers are activated.  This relationship can be seen more clearing in the image below.


Image from: Training Fast Twitch Muscle Fibers: Why and How.  Ernest W. Maglischo

(nb: ST refers to Slow Twitch, FTa refers to Fast Oxidative & FTx refers to Fast Glycolitic fibers)

As you can see from the image above, there are several key points where fiber activation levels increase and if you look more closely you may note the relationship between these points and the aerobic and anaerobic thresholds (LT1 and LT2 respectively).

NB: It was once though that this activation sequence was driven by the force or weight being overcome but more recently it has been determined that the intensity of the activity is the key determinant of muscle fiber activation and this works nicely for developing training programs with heart rate monitors or power meters.

Muscle Fiber Activation and Training Responses

Now on to the important stuff.   

Muscle fibers will only adapt if stressed beyond their normal comfort levels.   The degree of stress then dictates the level of response from the body.    So, if we want to train the aerobic abilities we need to concentrate on the fibers that do work in an oxidative state, these being the SO and FOG fibers.  Basically what this means is that we need to train at intensities that sufficiently activate (and stress) these fiber types and force an adaptation.   That’s the easy part, the tricky part is adding in the fatigue aspects and recovery time and then deciding what type of session best meets the developmental needs. With that in mind lets take a look at training in the most common format – training zones.

  • Recovery

Training below the aerobic threshold/LT1 is typically below 40-50% of your power at Vo2.  At this intensity it’s primarily the SO fibers that are activated (see image above) and even then perhaps only half (or less) of them.  Training at this intensity is very low stress and as minimal, to no, fast twitch fibers are activated it provides a good recovery session.  However, if one were to ride at this intensity for extensive periods, for example 6-7 hours, the activated SO fibers may start to fatigue and there would be some, albeit a small training affect.  Spending too much time in this training zone is a common mistake and should be avoided as much as possible.

  • E1 – Endurance/Aerobic Development

Training in the E1 zone involves activating an increasing amount of SO fibers as well as a small amount of FOG fibers.  Depending on fitness levels, training for periods of between 1 and 6 hours provides sufficient stress to force a training adaptation, with shorter session (1-3hrs) typically towards the top of this zone.  Importantly, this training impact is primarily limited to the SLOW TWITCH fibers and has a strong positive influence on developing aerobic endurance for road cycling.  Recovery from these sessions is also typically faster than higher intensity sessions, therefore an increasing volume of training can be accumulated with good training outcomes.  Professional cyclists can spend upward of 80-90% of their training time in this zone – especially when completing training weeks of 20+ hours.

(nb: In some circles training at this intensity is referred to as Extensive Endurance)

  • E2 – Tempo

As the intensity increases beyond E1, more FOG fibers get activated and SO fiber activation also continues to increase and peaks at approximately the anaerobic threshold (LT2 – time trial pace, etc).  The E2/tempo zone is an important training zone as it not only activates sufficient amount of FOG fibers to elicit and adaptation, but it provides close to maximal stress for the ST fibers.  The down side to training in this zone is that it carries with it a higher residual fatigue.  As a result sessions need to be either shorter or require more recover time.  

Examples of E2 sessions includes 2 x 30min at E2 during a longer E1 ride, 1 x 1hr or even 2 x 1hrs at E2, maxing out at approx 2.5-3hrs of E2 for a full on session that might test the limits of a rider.  

Training in this zone can also be referred to as Intensive Endurance.  More recently the term “Sweet spot training” has also been used and if you understand why this is the case then you’re well on your way to understanding not only the benefit but also the detriments of training too much in this zone.

  • E3 – Threshold

Most people think of threshold training as time trial training or hill reps – however truth be told these are not actually as closely linked as you may think.    Training above the anaerobic threshold/LT2 point continues to maximally activate the SO fibers but FOG fiber activation also approaches maximal levels.  So like tempo training, this is a potent training zone for improving aerobic abilities. However, and it’s a big however, training at these intensities also activates FG fibers and these do not work aerobically.  So by definition any training above threshold is both aerobic and anaerobic.   How anaerobic depends on two important variables; how far over threshold and how long over threshold.  An increase in either (or both) of these variables will result in the activation of more FG fibers and therefore increase the anaerobic component of the training.

As the intensity of this type of training is quite demanding it is customary for it to be structured as intervals and this provides a positive environment for structuring a workout to meet the core training requirement.   In essence this means: to keep the training effect more aerobic the interval duration should be shortened as the intensity increases.  Some examples of intervals in this zone include: 2 x 20min, 2-3 x 15 min, 4-6 x 10min, 6 x 8min and even 8 x 6min.   You can also go for micro intervals, such as 10 x 1min or if you are desperate for a hard session that taxes the mind as well as the body you could do 40 x 1min or 60 x 30sec.    It’s also worth recognising that as the duration drops so does the lactate accumulation, therefore the recovery period can be shortened.  A typical recovery for a 2 x 20 session is 10 minutes between efforts but comes down to the 1 min or 30 seconds with a 10 x 1min session completed as 1min at E3 with 1 min recovery, repeated 10 times.

Additionally, the recovery period can be customised to elicit a change in the training effect, for example a session with 10 x 1min may start out more aerobic in nature but by the end there could be considerable FG activation and therefore there is an increased anaerobic training effect.  If more FG activation is required then the work duration can be increased or the recovery decreased, so that same 10 x 1min workout could be changed to 10 x 1min with 30sec recoveries – ouch!

  • E4 – VO2/Maximal Aerobic

Training near or above the maximal aerobic power (Vo2) point ensures maximal activation of all aerobic muscle fibers (SO and FOG) and therefore training at this intensity is a very potent stimulus.  However, there is considerable fatigue associated with these sessions and the structure of the intervals themselves are quite important.    In the early days these intervals were typically completed as 5-6 x 3min efforts with 3 min recovery.  More recently a 5 x 5min with 5 min recovery protocol has become more common.  Closer to home, an Australian study looked at a slightly different approach which was 2.5min at the Vo2 power with a 5 minute recovery and these were found to provide an excellent stimulus for aerobic improvement.   If you look at the different structures of these intervals and keep in mind the point from E3/threshold training above you will begin to understand why.  The longer intervals require the recruitment of an increasing amount of FG fibers (nb: as SO and FOG fibers fatigue the power demands can only be met by activating FG fibers!) and the efforts become more anaerobic in nature.  This can be quite bad where the FOG fibers are concerned as they can operate both aerobically and anaerobically.  Training for too long at this intensity pushes the balance away from aerobic to the anaerobic side and the aerobic gains are lost in favour of anaerobic gains. That’s OK if you’re a track rider but not for roadies - unless anaerobic power is a limiting factor (which is quite rare).  So as with the threshold intervals, keeping the Vo2 intervals to shorter duration helps ensure the benefits are aerobic in nature.

  • Anaerobic and Neuromuscular Power

Once you get over the Vo2 max then SO activation can actually decrease (due to fatigue and some specific training effects) but FOG and FG fibers are at maximal activation.  This intensity is typical of sprinting and prolonged ‘lactate’ efforts.  Track sprinters spend 99% of their time training at these intensities and when they are not doing their ‘efforts’ they are either sitting on their butts (they don't want to use their slow twitch) or in the gym doing heavy weight lifts.   This last point is an important one as heavier weights create higher intensity lifts and greater FOG and FG activation, while lower weights activate more ST fibers and are better for endurance training purposes.

On the road training efforts in this zone are typically split into two parts, alactic and lactic.  These are the two energy systems responsible for anaerobic energy production.  The alactic system is a high power but short term energy system lasting between 6 – 15 seconds and is used to power short, explosive efforts.   As the name suggests, the lactic system results in the production of lactate.  This energy system is predominantly used in efforts from 15 seconds to approx 4 minutes – although after approx 75 seconds the effort is becoming more aerobic with approximate contributions of 50% anaerobic and 50% aerobic.


So why is all this important?  It comes back to understanding the purpose of a training session.  If you’re completing an E1 session then the purpose is to stress your SO fibers and aerobic system.  In turn the body increases mitochondrial density and there is an increase in the amount of power these fibers can produce.  Training too much at a low intensity (recovery or low E1) only activates a certain proportion of these fibers and this means that when intensity increases during a race the remaining SO fibers (the ones that have been ignored in training) can’t cope as easily and the rate of FOG fiber activation increases more rapidly.  Lactate levels also start to increase and fatigue starts to accumulate more rapidly (*).  For this very reason it is important to train at a variety of intensities to ensure all muscle fibers are developed appropriately. 

Putting all of this together is the job of a good coach (that would be me).  However, once you have your program you need to ensure you are completing the training sessions correctly and that gets back to answering the question posed in the title, why?  


Having an understanding of the purpose of every training session will help you complete it more accurately, maximising the efficiency of your training and helping you obtain better results on race day.

* Footnote: lactate itself does not cause fatigue but the chemical reaction that produces lactate has several other outputs and these are thought to lead to fatigue.

Jason Mahoney Tuesday 08 October 2013 at 09:47 am | | Tips

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