Slow-twitch vs fast-twitch muscle fibres and genetic predispositions

Genetic predisposition and muscle type

Types of muscle fibres in human body

There are three major types of muscle fibres in the human body: Type I, Type IIa and Type IIb. They are distinguishable through the level of activity of the enzyme Myosin Adenosine Triphosphate (ATPase) as well as through other characteristics. Your Type I fibres are slow-twitch while both Type II fibres are classed under the term of fast-twitch. The percentage of each found in your body is determined by both your genetics (40% for slow-twitch and 60% for fast-twitch) and environmental factors such as training experience.

Slow-Twitch Fibres

These muscle fibres are an endurance athlete’s best friend. They are plentiful in mitochondria, myoglobin (oxygen binding protein), and capillaries which allow for oxygen to be used to produce energy via oxidative phosphorylation. The capillaries transport the oxygen bound to the myoglobin into the muscles where it can be utilised by the mitochondria to synthesise ATP; allowing you to exercise longer. Slow twitch muscle fibres are therefore highly fatigue-resistant which is why muscles such as your gastrocnemius (calf) contain more slow-twitch fibres innately; allowing you to keep walking, standing and doing all those everyday activities without fatiguing quickly. The major fuel for slow twitch muscle fibres are triglycerides (fats) as this energy source can provide an abundance of ATP.
While slow twitch muscle fibres power you through long runs or rides, they won’t be ideal for explosive lifting or sprints as they are slower to contract.

Fast-Twitch Fibres

The two types of fast-twitch muscle fibres differ from the slow-twitch fibres because they can produce energy in the absence of oxygen (glycolytic oxidation). This allows them to create energy quicker using phosphocreatine and glycogen to fuel those quick explosive movements such as jumping and sprinting. This means they have faster contraction speeds than slow-twitch fibres. These two types of fast-twitch fibres also differ from each other, with Type IIa fibres displaying some similarities with the slow-twitch fibres.
Type IIa fibres can produce energy through both glycolytic AND oxidative pathways which make them slightly more fatigue resistant than the Type IIb fibres. Type IIa also has more capillaries than IIb; aiding their ability to produce energy using oxygen.

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 Sports and genetic predispositions

Your Type I fibres are perfect for long duration exercise performed at a low to moderate intensity. If you enjoy marathons, half marathons, 10ks, long walks or cycles; these are the muscle fibres powering you through.
Type IIa fibres are for moderate duration exercise at a higher intensity. Events like the 400m and using moderately heavy weights in an 8-12 rep range during your resistance training will predominantly use these fibres.
Type IIb fibres are the experts at short exercise bouts at a high intensity that require that explosive production of force by the muscles. Sprinting the 100m or powerlifting are examples of the kind of sports these fibres thrive in.
Understanding how the physiology of the body adapts to exercise can help you develop more effective training programs for your specific needs. Genetics determines how much of each muscle-fibre type you possess. An exercise program that applies the right training strategies for your muscle fibres can help you to maximise the efficiency and enjoyment of your workout time.
But your genetics only have partial control over your muscle fibre type composition. If you want to train to be a sprinter but have the genetic advantage for endurance, the training you put in to become a sprinter will help your muscle fibres to adapt to higher intensity stimuli and aid your performance!

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