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What Are Fast Twitch & Slow Twitch Muscle Fibres

Each type of athlete usually has a predominant muscle fibre type, that being the type that best suits their sport. Bodies are composed of many different groups of muscles. Each individual has a unique muscle tissue composition. The Three Different Types Of Muscle Fibres are Type I fibres, Type IIa fibres and Type IIb fibres. These fibre types can also be classified by their contractile and metabolic properties, thus dividing them into slow and fast twitch fibres.

  • Type I: Slow Twitch Muscle Fibres - These are slow to contract, and can sustain muscular contractions for an extended period of time. This factor makes them ideal for endurance exercise. Physically, these fibres are red in appearance, due to their iron containing cytochromes, Endurance athletes often possess up to 90% slow twitch fibres.
  • Type IIa: Fast Twitch Muscle Fibres - Fast twitch fibres generate an explosive burst of power for a short period of time. This makes them most suitable to stop and go activities such as weightlifting. Type IIa fibres are in the middle of the muscle fibre spectrum, as they are less fatigue resistant, produce more muscular force, and contract at a faster speed than slow twitch fibres.
  • Type IIb: Fast Twitch Muscle Fibres - These are the most fatigable out of all the fibres but also generate the most power and force, and therefore are the fastest twitch muscle fibres. These types of fibres are recruited in activities that require an all out burst of power and only act for an extremely short period of time. They are also the last to be recruited. During normal activities, slow twitch fibres are recruited first, then type IIa when the type I is no longer sufficient, and finally the type IIb type is recruited for maximal strength.

Difference Between Fast & Slow Twitch Fibres

In regards to physical appearance, type IIa are pink in colour, have an intermediate diameter, capillary level and mitochondria volume. The type IIb fibres are white, have the largest diameter and have a low capillary and mitochondrial volume. Most strength athletes possess a higher percent of fast twitch fibres. Fast twitch fibres are not resistant to fatigue because they rely on anaerobic glycolysis to produce ATP, whereby lactic acid accumulates and a acidosis occurs bringing about muscle fatigue.

Your Muscle Fibre Type & Training

Knowing what types of muscle fibre you predominantly have determines what type of training you will benefit most from. Understanding your genetics helps you determine your sets and reps and training weight. If you have a large number of red fibres, you are geared for endurance training. High numbers of white fibres mean that you are geared for strength training. You can find out what type you are through a muscle biopsy. In Russia, children had muscle biopsies at a young age to determine which sports they best suited.

Muscle Type Tips

  • Training Body Parts And Frequency. Muscle responds to changes in training. Consider diversity for maximum muscle stimulation.
  • Recovery Speed. The more intense the training the longer it takes to recover. If you can recover fast enough, then you will need to train a lot more.

How to Train Fast Twitch Muscle Fibres

In the weight room try to lift in excess of 60% of your 1 Rep Max. The heavier you lift, the greater the recruitment of Fast Twitch Fibres. Use maximum speed with all movements, as short bursts of speed will cause the positive muscle adaptation you are seeking. Use eccentric training – i.e. emphasis on the lowering phase of an exercise. Use plyometrics to activate and create more powerful contractions. Contrast your training loads. For example you could do a set of heavy squats, then rest, then perform a lighter but more explosive movement such as unweighted jump squats. This technique is known to achieve a higher recruitment of fast twitch fibres. Over-speed training involves adding weight to a movement, for example via a partner or bands. This means you recruit more fast twitch fibres to achieve the extra speed.

Muscle Types & Athletes

For recreational athletes, commitment to practice and training plays a more significant role in performance level than muscle fibre type alone. So, the three types of muscle fibres are slow twitch (Type I) and fast twitch (Types IIa and IIb). Slow twitch have long contraction rates, being resistant to fatigue, relying on oxygen as their main source of metabolism, and are used primarily in endurance type of activities that do not require a great deal of force. Fast twitch fibres have short but powerful contraction rates, are highly fatigable due to their reliance on anaerobic metabolism that produces lactic acid, and are more suited to activities that are powerful and quick in duration. The objective is to stimulate muscle fibre and then let it recover as much as possible to then be able to achieve maximal stimulation again. This allows for optimal gains. Use some of the creative techniques mentioned in this article to help you engage, and build more fast twitch fibres.

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