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A Word on Slow and Fast-Twitch Muscle Fibres

Slow and fast-twitch muscle fibres are some terms you may have come across in your fitness journey. A short recap of what they are and what kinds of exercises suit each kind of fibre should serve any fitness enthusiast well. 

To a large extent, athletic success depends on slow-twitch and fast-twitch muscle fibre composition. A combination of each kind is utilised in every athletic endeavour. However, Kiwi certified personal trainer David Robson says one type will usually dominate among hardcore athletes depending on their sport. 

To give one a clearer picture, skeletal muscle is made up of individual muscle fibres known as myocytes. These contain many myofibrils, strands of protein than hold on to and pull each other. This shortens the muscle and results in muscle contraction. 

These muscle fibres are further classified into two kinds, which the average person has an equal proportion of. 

Furthermore, this classification should give an idea how certain muscles respond to a particular kind of training. 

1. Slow-twitch muscle fibres—also known as Type I muscle fibres, they are more effective in using oxygen to generate more adenosine triphosphate (ATP) which is used as fuel for continuous muscle contractions.     

They possess more mitochondria, an organelle found in large numbers in most cells, than their fast-twitch counterparts. They come into play or dominate during the endurance sports such as long-distance marathons or rowing competitions. 

Muscles which are slow-twitch dominant include the shoulders, forearms and calves. 

For these muscles, focus on higher repetitions anywhere in the 10 to 15 range. Studies show these muscle groups react better to high volume, high frequency, short rest periods and low-intensity training. 

2. Fast-twitch muscle fibres—these are the Type II muscle fibres which are much better at generating short bursts of strength or speed than slow muscles. The downside is they fatigue more quickly. 

These muscle fibres exert their efforts due to peak-tension facilitation. Robson draws up an example of a sprinter reaching top speed because of the force production this peak-tension allows. 

The reason why they differ from slow-twitch muscle fibres is their ability to generate a high amount of force in a short period. Robson cites the examples of the take-off motion in a sprit or long jump.  

Fast-twitch muscle fibres are further classified into two sub-categories:

Type IIA fibres—they are also known as intermediate fast-twitch fibres which use both aerobic and anaerobic metabolism equally to create energy. 

Type IIB fibres—they use anaerobic metabolism to create energy. It has the highest contraction rate among all muscle fibre types. 

Muscle groups which are fast-twitch dominant are the chest, triceps, biceps and hamstrings. 

These muscles should be on the receiving end of lower repetitions which number around five to seven. Resistance should be increased. This is because these muscles react better to high intensity, low-volume, low-frequency training. Make sure to also make your rest periods between sets a bit longer. 

Now that you have a better idea of what slow and fast-twitch muscle fibres are, you have a clearer picture of how your fitness programme should be structured. Make this one of your considerations together with your personal fitness goal, age and gender.

For best results, consult with your certified personal trainer today. 

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