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Body Basics - Muscle Fibre Facts

What are the different muscle fibre types? Understanding these will help you to recognise the different types of training we do and why – and how to incorporate into your own training.


Your body is an amazing and complex unit – comprised of multiple muscle groups that each have different uses.  No two of us have the exact same muscle composition.  And this makes some types of training and sports activities easier than others for us.


An athlete usually excels in ONE sport or type of sport, and this is because, just like you, they tend to have one predominant type of muscle fibre that allows them to do this.


Because of our natural ability in one type of training, it’s important to be aware of how cross-training can both improve your performance across the board and keep your body in balance. I notice the reluctance in clients who weight train when I recommend adding some running or yoga to their training programs – even though after adding these other disciplines to their regime they generally look and feel better.  The hesitation of clients to take up these different kinds of training tends to come from an inability to do these activities easily.


What are the different types of muscle fibres we use when training? 


Type I fibres: Slow Twitch


Type IIa fibres: Fast Twitch


Type IIb fibres: Fast Twitch


I like to think of each type as having its’ own characteristics because each tends to be used for certain types of movement.  And each fibre has slightly different contractile and metabolic properties too.


Type I: Slow Twitch Fibres.


These fibres are slow to contract which is why we call them slow twitch.  They are designed for extended muscular contractions and are perfect for sustained endurance events because they excel at being able to produce energy for long periods but conversely, provide low levels of force output.  If you love long distance sports and have a high V02 capacity (aerobic capacity), you will have a larger amount of these types of fibres.Marathon runners and long-distance cyclists and swimmers can have up to 90% Type 1 muscle fibres.

Type IIa: Fast Twitch Fibres.


There are two kinds of fast twitch fibres – Type IIa and Type IIb. Fast twitch fibres are best at generating explosive bursts of power for a short period.  And lots of different athletes use these in sports like basketball, netball, track and field events – and even weightlifting.  Type IIa muscle fibres sit in the middle of the spectrum – they are fatigue resistant like Type I but produce more force and can contract at much faster speeds than the slow twitch muscle fibres.


Type IIb: Fast Twitch Fibres.


Type IIb Fibres are by far the fastest to fatigue the fastest but produce the most power and force.  These fibres are recruited in activities that require an all-out burst of power and only act for an extremely short period, as the total length of their contractions usually lasts only 7.5 milliseconds. You don’t have them for long, but they will generate speed – which is why we tire quickly when sprinting.  Because of their design, we tend to recruit them last – and it goes in order of muscle fibre type.  The first Type I -slow twitch, then IIa takes over when slow twitch becomes overloaded.  And finally, when Type IIa gives out, IIb switches on for a final burst of speed, strength or power.


What Is The Difference?


Although each muscle fibre type has certain characteristics that make it more suited for certain activities, this does not mean that a person with a predominance of one type of muscle fibre can only participate in those activities that call for that type.  It just means you’d probably find one kind of training easier.  But you'll also reap the rewards from developing your other types of muscle fibres too - greater power, speed or endurance.


What do you do with this information now?


For us recreational athletes, as I mentioned in the beginning, commit to practising other sports and activities that challenge you and remember to cross-train.  Challenging your body at activities, it finds harder will keep your body balanced and improve your performance across the board – even in the activities, you excel in already.

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