Have you ever noticed that some people can cycle for a long time but are weaker in a sprint? Or that they can handle heavy strength training but can’t run for five minutes? The reason why person A quickly becomes broad while person B can seemingly effortlessly run dozens of kilometers without getting tired lies in the composition of their muscle fibers. This muscle fiber composition determines your strengths and weaknesses.
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Which types of muscle fibers exist?
Although there are three different types of muscle fibers, here we only consider the ‘slow-twitch’ and ‘fast-twitch’ fibers. The type of muscle fibers you have is largely genetically determined.
Slow-twitch or Type I muscle fibers are used during ‘aerobic activities’ that require little power production. ST fibers are also called red muscle fibers because they are well-vascularized and contain many mitochondria (“energy powerhouse” of the cell). To excel in aerobic activities, you mainly need a lot of ST muscle fibers.
Fast-twitch or Type II muscle fibers are used during short, intense efforts. These ‘white’ muscle fibers are less vascularized, and therefore tire more quickly. However, they can generate more energy for that shorter duration. If you can sprint very quickly, you probably have a lot of FT muscle fibers.
We are all born with a certain percentage of ST and FT muscle fibers, and the correct ratio can vary from muscle to muscle, and from person to person. The speed and strength you can develop in any given movement depend on your muscle fiber composition.
To optimize your results, your fitness schedule must be ‘tailor-made’ based on the composition of your muscle fibers. But how do you determine your type of muscle fibers?
How do you determine your type of muscle fibers?
The best way to determine the muscle fiber type of a particular muscle or muscle group is through a biopsy. A small piece of muscle tissue is taken with a slightly thicker needle and then examined under a microscope.
Research has repeatedly shown a clear correlation between the ratio of FT muscle fibers and muscle strength and speed. Since then, it has been possible to estimate muscle fiber type by measuring muscle strength and speed.
Determining muscle fiber type
Determine your ‘1RM’ (the heaviest weight someone can lift only once), for each muscle group (you want to test). Then do as many repetitions as possible at 80% of your 1RM:
- < 7 repetitions: > 50% FT muscle fibers
- > 12 repetitions: > 50% ST muscle fibers
- 7 to 12 repetitions: 50/50% FT/ST muscle fibers
Your type of muscle fibers determines your training
By now, you understand that muscle fiber type has a significant influence on training weight, the number of repetitions someone can perform, and the desired training effect. Someone with more FT fibers will never be able to perform as many repetitions as someone with more ST muscle fibers, and will therefore have less ‘muscle endurance’.
In contrast, someone with more ST muscle fibers will never be able to lift as heavy weights as someone with more FT muscle fibers.
Below is an overview of the differences between the types of muscle fibers:
| Property | Type I | Type IIa | Type IIx |
| Motor neuron | Small | Large | Large |
| Nerve conduction velocity | Slow | Fast | Fast |
| Contraction speed | Slow | Fast | Fast |
| Relaxation speed | Slow | Fast | Fast |
| Resistance to fatigue | Large | Medium/low | Low |
| Strength | Low | Medium | High |
| Power | Low | Medium/high | High |
| Endurance | Large | Medium/low | Low |
| Amount of aerobic enzymes | Large | Medium/low | Low |
| Amount of anaerobic enzymes | Small | Large | Large |
| Capillary density | High | Medium | Low |
| Amount of Myoglobin | Large | Small | Small |
| Mitochondria (size/density) | High | Medium | Low |
| Fiber diameter | Small | Medium | Large |
| Color | Red | White/red | White |
Which type of training do you choose?
While most people train for a specific goal, training must also accommodate their physiology. Someone with more ST muscle fibers is better suited for sports that require a lot of endurance, and the training should mainly consist of aerobic exercises or strength training with more repetitions and less resistance.
Someone with more FT muscle fibers is better suited for anaerobic activities, so the training should mainly include anaerobic exercises or strength training with fewer repetitions and more resistance.
Someone who wants to become stronger and faster should increase the intensity of his or her strength training and the speed during cardio workouts. Someone who wants more endurance should increase the duration of his or her cardio workouts along with the number of repetitions during strength training.
