Usain Bolt has never run a mile. One of the greatest runners of all time sucks at distance running. It didn’t surprise me at all when I heard about it. The guy is a sprinter. His body is composed of 80% fast-twitch muscle fibres. He can’t run for long distances. Even if he does, the timing won’t be impressive. You can’t have it all. That’s nature’s law. You can either be a good sprinter or a good distance runner, not both. Last time we discussed slow-twitch muscle fibres and endurance sports. This time, we are gonna focus on fast-twitch muscle fibres, and strength, power and speed sports. Fast-twitch muscle fibres (Type II Muscle Fibres) can be classified into
- Type II A
- Type II B
Characteristics of Type II A Fibres
- Contractile Speed : They have a fast contraction speed but not as fast as Type II B, so they are called intermediate fibres.
- Energy Production : These fibres can work under both oxidative and glycolytic conditions. They are also called fast oxidative glycolytic (FOG) fibres.
- Colour : Because of the presence of a large number of mitochondria and myoglobin, they are red in colour.
- Fatigability : Since they also use oxidative pathways to generate energy, they are fairly resistant to fatigue but not as much as slow-twitch muscle fibres.
- Force Production : They can generate high force output for a longer period.
- Hypertrophy : Muscle fibre diameter is larger than slow-twitch fibres.
- Major Storage Fuel : Fat and carbohydrate (glucose) both are the predominant fuel since they use both aerobic and anaerobic pathways.
- Threshold : They have higher threshold than slow-twitch muscle fibres and are recruited when the activity becomes intense, the force on muscle increases and the slow twitch fibres can’t meet the demands.
Characteristics of Type II B Fibres
- Contractile Speed : They have the fastest contraction speed.
- Energy Production : These fibres perform predominantly under glycolytic conditions. So they are also called fast glycolytic (FG) fibres.
- Colour : Since they don’t contain much blood (low myoglobin and mitochondria), they are white in colour.
- Fatigability : They fatigue easily.
- Force Production : They can produce maximum amount of force and can reach peak force in a very short period.
- Hypertrophy : They are the largest in diameter. Very responsive to hypertrophy training.
- Major Storage Fuel : They utilise the anaerobic pathway, meaning they don’t use oxygen to generate energy. That makes carbohydrates (glucose) their predominant fuel source.
- Threshold : Fast-twitch muscle fibres predominate when the contraction is maximal. They have the highest threshold.
Muscle Fibres Transition
In the last article, I told you that it’s not possible to transform slow-twitch muscle fibres to fast-twitch and vice versa. But, apparently, Type II B muscle fibres can transition into Type II A muscle fibres after years of strength endurance and maximum endurance training. It does so by developing mitochondria and capillaries in the muscles. Type II B fibres get more fatigue resistant and convert into Type II A fibres. But Type II A fibres can’t transition back into Type II B or Type I fibres.
- We need Type II B fibres to generate maximum strength, explosive strength and speed. So to recruit Type II B fibres, follow the same training program you do for developing maximum strength and power.
- To recruit Type II A fibres, follow the training routine you do for strength endurance.
- Fast-twitch muscle fibres are required for sports like weightlifting, sprinting, throwing events of track & field (shot-put, javelin, discus, hammer), jumping events (high jump, long jump, triple jump), powerlifting, and they are also recruited while striking in sports like MMA, boxing, tennis.
Characteristics of Muscle fibres
|Type I||Type II A||Type II B|
|Contractile (twitch)||ST||F T||F T|
|Muscle fiber diameter||Small||Intermediate||Large|
|Twitch (Contraction) time||Slow||Fast||Fast|
|Glycolytic enzyme activity||Low||Intermediate||High|
|Oxidative enzyme activity||High||Intermediate||Low|