Muscle Fibres Part 2 : Usain Bolt Has Never Run A Mile


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

  1. Type II A
  2. 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

Muscle Fibres Part 2 : Usain Bolt Has Never Run A Mile
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.

Training Strategy

Muscle Fibres Part 2 : Usain Bolt Has Never Run A Mile

  • 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 IType II AType II B
Contractile (twitch)STF TF T
Structural aspects
Muscle fiber diameterSmallIntermediateLarge
Mitochondrail densityHighIntermediateLow
Capillary densityHighIntermediateLow
Myoglobin contentHighIntermediateLow
Functional aspects
Twitch (Contraction) timeSlowFastFast
Relaxation timeSlowFastFast
Force productionLowIntermediateHigh
Metabolic aspects
Phosphocreatine storesLowHighHigh
Glycogen storesLowIntermediateHigh
Triglyceride storesHighIntermediateLow
Myosin-ATPase activityLowIntermediateHigh
Glycolytic enzyme activity
Oxidative enzyme activityHighIntermediateLow
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