Muscle fibre types can be broken down into two main types. These are slow twitch (type I) muscle fibres and fast twitch (type II) muscle fibres. Fast twitch muscle fibres can be further categorized into (type IIa) and (type IIb) fibres.
Human muscles contain a mixture of both slow and fast twitch fibres. Here’s how the fibre types differ from one another:
Slow Twitch Type I
Type I muscle fibres are the endurance fibres in the body. They are more efficient at using oxygen to generate more fuel for continuous, extended muscle contractions over a long period of time. They are very slow to fatigue which makes them very useful for endurance activities. They have a slow speed of contraction which would make them useless for sprinting. They are the smallest fibres size wise and can generate the least amount of force. Type 1 fibres are useful for long duration, low intensity activities like cycling or running a marathon. When we are trying to build muscle these are not the fibre types we want to stimulate. Since they fatigue less easily. To stimulate these muscle fibres you would need to do a high amount of reps at a lower intensity. Most studies have found sets in the range of 12-20+ repetitions to be optimal.
Slow twitch Muscle fibres have low strength potential, a slow contraction speed and high endurance.
Fast Twitch Type IIa
These are the intermediate fibres. They are capable of both power and endurance but not at the level of type I or type IIb. You could say they are a combination of type I and type II muscle fibres. These fibres are mid range on size, the amount of force production and there potential for growth. Studies show the more endurance training you do, the more they will shift towards the endurance end of the spectrum. The more strength/power training you do they will shift towards the power end of the spectrum. So the type of rep ranges you use will shift towards one type of fibre or the other depending on how they are stimulated.
Fast twitch A have a medium strength potential, medium contraction speed and medium endurance.
Fast Twitch Type IIb
These fibres are the power fibres in the body. They are very efficient at generating strength. They are capable of generating the most force but fatigue quickly. They excel at producing quick, powerful bursts of speed. This muscle fibre has the highest rate of contraction, but it also has a much faster rate of fatigue so cant last as long before it needs rest. Fast twitch B fibres have high strength potential, fast contraction speed and low endurance.
These Fibres are the largest and have the highest potential for growth out of the three. So as you might of guessed these are the muscle fibres we want to target for optimal muscle growth. Studies show these type IIb fibres seem to respond best to sets in the 4-8 repetition range. This is roughly 80-85% of your 1REP Max.
Training within the 4-8 repetition range will help you achieve a great foundation of strength and dense muscle growth when combined with simple load progression, or better known as progressive tension overload, which I will cover in my next article. Thank you very much for reading.