Do you ever watch top-level athletes and wonder whether they were anatomically designed specifically for their chosen sport? Looking at the build and stride of marathon world record holder Eliud Kipchoge, or the explosive power generated by Sir Chris Hoy’s enormous thighs and hamstrings, their success almost feels predetermined. Surely they were built to win gold medals.

While there are undeniably a multitude of interrelated physiological, psychological, and sociological factors that determine success in a particular pursuit, there may be some genetically-influenced traits that set you up on the right path. One of these is your dominant muscle fibre type.

Type I vs. Type II muscle fibres

Muscle fibres are classified into two main types: slow-twitch (Type I) and fast-twitch (Type II). Slow-twitch muscle fibres are characterized by their high endurance capacity and low force production, while fast-twitch muscle fibres are characterized by their high force production and low endurance capacity. 

Slow-twitch muscle fibres are optimized for endurance activities such as long-distance running, cycling, and swimming. They are highly resistant to fatigue and can contract repeatedly for long periods of time without tiring. Slow-twitch muscle fibres also have a high capacity for aerobic metabolism, meaning they can generate energy from oxygen and sustain activity for extended periods.

On the other hand, fast-twitch muscle fibres are optimized for high-intensity, explosive activities such as sprinting, jumping, and weightlifting. They have a high capacity for anaerobic metabolism, meaning they can generate energy without oxygen, but they fatigue more quickly than slow-twitch muscle fibres. Fast-twitch muscle fibres also have a greater potential for muscle hypertrophy (growth) than slow-twitch muscle fibres, making them important for strength and power.

For a more detailed summary of the difference between muscle fibre types, I highly recommend taking a look at Geraldine Campbell’s blog: What's The Difference Between Fast And Slow Twitch Muscle fibres?

The influences of genetics and training

As you can imagine, not everyone carries the same proportion of slow-twitch to fast-twitch muscle fibres. People with a greater amount of slow-twitch fibres are generally more predisposed to endurance-based activity, while those with fast-twitch fibres carry an advantage for explosive performance. Your genetic profile strongly influences whether you have a higher proportion of slow-twitch or fast-twitch muscle fibres, with one study of twins finding that about 45% of the differences in muscle fibre composition are due to genetic factors. 

While an individual's muscle fibre composition is heavily influenced by their genetics, muscle fibres can shift from one type to another in response to changes in exercise patterns. Endurance-based training can increase the proportion of slow-twitch muscle fibres, while resistance training can increase the proportion of fast-twitch muscle fibres. This adaptation allows individuals to improve their performance in a variety of activities by targeting muscle fibre types through specific forms of training. 

Identifying your dominant muscle-fibre type

At FitnessGenes we combine the results of a multitude of genes, including ACE, ACTN3, AMPD1, NOS3 and VEGFA, with self reported data on training status to determine whether you are likely to have a higher proportion of slow- or fast-twitch muscle fibres, or a roughly equal balance of the two. As a result, our recently updated Fast Twitch Muscle Fibre report categorises all members between three distinct results:

1. Slow
2. Fast & Slow
3. Fast

‍Members’ result distribution

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While having a higher proportion of one type or the other may present certain athletic advantages, as you can see from the above graph, most FitnessGenes members have a relatively balanced distribution of slow- and fast-twitch muscle fibres.

Only 16% of members are calculated to have a higher proportion of the fatigue-resistant, endurance favouring Type I fibres, while even fewer carry genes associated with a greater amount of Type II fibres.

Although the 74% of members categorised in the middle group are not strongly predisposed to endurance or power based on their dominant muscle fibre type, they are likely to respond well to a variety of training protocols, rather than necessarily being restricted to a particular style of training.

Muscle building recommendations

As we've learnt, fast-twitch Type II muscle fibres are advantageous for building strength and muscle size. So is all hope lost for the 16% of members that are predominantly Type I (slow-twitch)? Not at all. They just need to know how to structure their training to best complement their muscle fibre composition. Here are a handful of recommended actions for those who are slow-twitch dominant. 

1. Use lower weights but higher reps, aiming to perform your reps close to failure. 

This will help to recruit the slow-twitch (Type I) muscle fibres and strengthen them further. This will help increase muscle strength and size. 

2. Complete 1 minute each of 5-8 exercises, rest for 2-3 minutes and then repeat the circuit 2 more times. 

Circuit training has been shown to be a great way to activate slow-twitch (Type I) muscle fibres and help increase muscular strength.

3. Get jumping in your workouts.

If you are looking to increase your expression of fast-twitch (Type II) muscle fibres, include some box jumps or lateral hops in your training. These exercises predominantly recruit fast-twitch muscle fibres, which will help to increase your muscle strength and power. 

Understand your personal muscle fibre type

If you’re a full FitnessGenes member, your report is now live in the Members’ Area. Login to view your personal result, insights, and actions. 

Not yet a member? Get started completely free of charge by unlocking your lifestyle-based reports with three simple steps. Create your free member account at fitnessgenes.com/signup.