Introduction

This article is part of a series on ‘Core Principles of Exercise Training’.  The idea is that this series will include the key background information that all other aspects of fitness training build upon.  These fundamentals of exercise training can be used by everyone to help define their approach to exercise, based upon their individual goals.

Muscle Fibres

The functional components of the body are the cells.  Within the cells are carried out all the basic functions for their own maintenance.  The cells themselves do not last long (days, weeks or months), so are continually replaced.  This process helps to ensure their health, as older and poorly functioning cells are removed.  Cells replicate and renew, whether components or the cell as a whole.  Each cell contains organelles, with are the organs of the cell, in the same way that we have organs in our body – each with specific functions that need to be performed for health.  A nucleus contains our genetic material, and mitochondria are for using oxygen to produce energy.  Enzymes are the proteins that perform reactions.

Muscle fibres are a specific type of cell.  In addition to the functions we expect of any cell in our body, their primary reason for existing is their ability to contract.  Muscle fibres do not contract individually, but as a mass of fibres, all connected to each other from end to end and running parallel to each other.  This means that, when activated by nerves, the muscles contract and pull on the tendons at their ends, which in turn attach to the bones around joints, and movement results.  How many fibres are recruited to perform movement depends upon how much force is required (such as lifting weights against gravity).

Muscle Fibre Types

Muscle fibres have unique properties depending upon their function.  There are broadly three types of muscle tissue: skeletal muscle, smooth muscle and cardiac muscle.  Cardiac muscle is in the heart.  Following activation via a single nerve channel the entire heart contracts in a precise, specific pattern.  Smooth muscle can be found in the digestive and cardiovascular systems, as well as various other body ‘systems’.  It’s role is to contract to move materials around (food and blood, for example).  The function of skeletal muscle is to contract to permit movement of bones around joints.

Skeletal muscle itself comes in three types: type 1, type 2a and type 2x (if you have come across these before, 2x is broadly the same as 2b, but for humans we consider 2x and in animals 2b, although they are often used interchangeably).  These three fibre types are specific to different types of body movement. 

Type 1 muscle fibres are for aerobic exercise.  Because aerobic exercise is performed at a relatively low exercise intensity for a long time, these fibres have evolved to be very efficient at using oxygen together with carbohydrates and fats to produce energy for exercise.  They have a rich blood supply to transport oxygen to these fibres, and contain a variety of components that aid in transporting oxygen and producing aerobic energy (aerobic enzymes and mitochondria).

Type 2x (or ‘2b’) fibres are for high intensity strength, power and explosive exercise.  Because this intensity of exercise is performed at a very high intensity over a short duration (before fatigue), these fibres do not have the time to use oxygen to break down carbohydrates and fats for energy. This process is called ‘anaerobic’ (without oxygen). Instead these use much more basic energy systems to utilise the energy already within the fibres, and to rapidly recycle that without the use of oxygen.  Hence, these fibres are not good for performing aerobic work (lower blood supply, fewer mitochondria and aerobic enzymes), but have developed to be very good at recycling energy during high-intensity exercise (more anaerobic enzymes). 

Type 2a fibres sit somewhere in the middle of the two fibre types.  They are okay with using oxygen to produce energy from carbohydrates and fats, and they are okay at going without oxygen to produce energy during higher intensity exercise.  They contain components of both type 1 and type 2x fibres, and are not specialised.  These are the ‘Jack-of-all-Trades’ fibres.

Exercise Specificity

We can see that we have some muscle fibres that are very effective during high intensity exercise, such as lifting weights in a gym, home gym equipment (free weights, gym rings, triceps ropes), jumping (including plyometrics and parkour) and sprinting.  We also have fibres that are very effective during aerobic exercise, such as long distance running, swimming, cycling and so on.  There are also fibres the lie in between, and can do a bit of everything.  When we are born we have a fairly even distribution of all three, with some of them not yet developed into a specific fibre type.

During growth and development the distribution of fibres within our muscles changes, as the physical stresses of daily activities help to shape our muscular system to perform more efficiently.  Muscles that tend to be involved in aerobic exercise – such as the legs for walking – develop to contain a higher proportion of type 1, aerobic fibres.  Muscles that are more involved in strength and power work – such as upper body muscles – develop to contain a higher amount of type 2x fibres.

In the same way as these repeated, natural activities of daily life shape our muscles during growth and development, so exercise training becomes another stimulus for change.  If someone does a lot of aerobic exercise, they will have a shift in fibre type, with more type 1 (aerobic) fibres, and their type 2a (intermediate) fibres becoming better suited to aerobic exercise than power.  If someone performs a lot of high intensity strength and power exercise, so they will gain more type 2x (anaerobic) fibres, and their type 2a (intermediate) fibres become more like 2x fibres.  How we target our training impacts the structure and function of our muscles, to the level of the individual muscle fibre types. 

As individuals, it is down to us to decide what we want to gain from our training.  If we want to train as specifically as possible – to be as well adapted and trained as physically possible for our chosen activity – it makes sense that we prioritise that style of training.  If we want to be as strong as possible, but include a lot of aerobic exercise, we will switch some fibres from strength-specific to more aerobic-specific, which reduces our capacity for strength, and vice versa.  If our focus is general health and fitness, we should aim to include a bit of everything, to develop all muscle fibres equally. 

Relevance to Massage

Whereas some types of massage – such as Swedish or non-therapeutic massage – are typically superficial, deep tissue massage is intended to influence muscle fibres that lie closer to the bones.  The distribution of type 1 and type 2 fibres can differ according to location, and within a muscle the type 1 fibres are often closer to the bone, and the type 2 fibres are closer to the surface.  Hence, to ensure massage and similar treatments target all areas of a muscle and all the fibre types, it is important that the technique(s) can be applied at depth.  Due to the interest in self-massage techniques, there has been development in recent years of a variety of tools to help people effectively reach deeper muscle fibres.  Most recently this has included the use of vibration rollers, intended to allow the user to vary weight application (limb weight / body weight) to permit these deeper fibres to be accessed, whilst using the vibration to perform the massage.

Summary

Muscle fibre type specificity is another fundamental component of exercise training.  As with all our cells, they have evolved to confer specific benefits according to how they are used.  How we exercise shapes the structure and function of our bodies, to the level of influencing those individual muscle fibres.  It is therefore down to us to consider exactly what we want to achieve through exercise, and to adopt training programmes that suit those goals.