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Running Basics: How Muscle Fiber Types Affect Runners

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As you practice running, you might have noticed your inclination towards a particular running style. In general, people either like to run fast over a short distance, like sprinters; or they like to run consistently over long distances, like marathoners. While you might think that this propensity depends on the liking or disliking of a particular running style, but this is not the case. This difference in running styles is actually due to different muscle fiber types in the body. It is not your inherent bent of mind towards a particular running style but the type of muscle fibers that your body possesses.

Different types of muscles possess different types of muscle fibers. We will brush up the scientific facts behind the various types of muscle fibers and their function. Key differences in the various kinds of muscle fibers are responsible for different running abilities among athletes. Knowing the type of dominating muscle fibers in a runner will be tremendously useful in improving performance. This information is also vital for planning and executing a personalized training program. 

Know more here:

Type of Muscle Fibers in the Human Body

Human beings have three distinct types of muscle fibers. They have a different structure, different physiology, and different function. The details of each of these are discussed below.

Type I Muscle Fibers

These muscle fibers are also known as ‘slow twitch’ muscle fibers. They consume oxygen and take some time to get charged up for performance, but once they are activated, they keep firing for a very long time. These fibers are present in a relatively higher proportion in marathoners and ultra runners. A typical result of activation of these fibers is an enhancement of muscle endurance.

Structurally, these muscle fibers have a smaller motor neuron and fiber diameter and high stores of fat. They don’t possess very high amounts of creatine phosphate, that is required for a sudden burst of energy generation. Most of our daily activities like walking and maintenance of our posture involve type 1 muscle fibers. Slow-twitch muscle fibers have a high resistance to fatigue.

Type II Muscle Fibers

The second type of muscle fibers present in humans is type II. They are characterized by a very quick contraction time and low resistance to fatigue.

This category is further divided into two kinds - Type IIa and Type IIb.

Type IIa muscle fibers have properties of both the other muscle fiber types and can function in the presence or absence of oxygen. These are middle players that are used to different extents in long and short races. They have moderate resistance to fatigue and are characterized by a large motor neuron and fiber diameter. They have limited stores of fats but are rich in creatine phosphate.

Type IIb muscle fibers are known as ‘fast twitch’ muscle fibers and don’t consume oxygen for firing up. As their name suggests, they are activated very fast and give a great speed for a short period of time. These are more prominent in the muscles of sprinters and short distance runners. This muscle fiber is also great for weight training due to a large amount of force it generates in a short time. These fibers are very susceptible to fatigue and are characterized by a large motor neuron and fiber diameter.

Different persons have these three fibers in different proportions, depending on the type of muscle. The relative abundance of each of these fibers is determined by the genes. Generally, the majority of the population has 50% type I and 50% type II (IIa and IIb) muscle fibers. However, it is postulated that it might be possible to increase a particular muscle fiber type by training.

muscle fiber types
Different type of muscle fiber (source)

How to Determine Muscle Fiber Type

Even though the human body contains both slow twitch and fast twitch muscle fibers, the relative proportion of the two vary from person to person. This is why some runners are good at short and fast races while others perform better in long races. Accordingly, they cannot be made to deliver similar performances if their format is changed.

The conventional method of determining the dominant fiber type in a runner involved establishing the value of the greatest weight they could lift just once. Then they were asked to perform a repetition of lifting 80% of this weight value and listed below were the findings:

  • If they could manage to lift this weight only up to seven times, it was believed that they have more fast-twitch fibers.
  • If they could perform more than twelve repetitions, they were believed to have more slow-twitch fibers.
  • A performance of repetition between seven and twelve times was considered due to an equal proportion of these two muscle fiber types.
muscle fiber type
Both muscle fibers have their own importance (source)

If you want to know the answer to the question ‘what is my muscle fiber type’, you can opt for muscle fiber testing involving a biopsy of your muscles and find out. However, if you are not too keen on needlework, then there are few other ways of finding out. These ways are discussed below:

  • During your regular work out sessions, if you feel more comfortable while running for long distances, you are more likely to have an abundance of type 1 muscle fibers.
  • If, however, you feel that short and fast intervals are easier for you, then you have a predominance of type 2 or the fast twitch muscle fibers in your body. This is because you display a trait of having more endurance.
  • During your races, if you are able to sustain your pace in the beginning but tend to get outrun by other runners towards the end of the race, it is likely that you have more type 1 or slow twitch fibers.
  • Conversely, if you feel you cannot maintain your pace in the middle of the run but can outrun other runners towards the end of the race, you have more type 2 or fast twitch fibers. This is because you display the trait of more power.

Finally, your mental framework before your workouts can also give an insight into the proportion of the two muscle fiber types in your body. If you look forward to long interval workouts, then you probably have a predominance of type 1 muscle fibers. If on the other hand, you can’t wait for short and fast interval training, you are more likely to have more type 2 muscle fibers in your body.

It is a practice by most sports coaches who train young athletes/ runner to encourage them to try different events. This way, they come to know the tendency of the budding athlete to perform better at one type of events. This happens because the muscle fiber type that is dominant will show its effect. An athlete with more type 1 fibers will continuously fare better in endurance events than in power events. The converse is true for an athlete having more type 2 fibers. The training of the young athlete can then be directed accordingly.

Training for Different Muscle Fiber Types

The inherent composition of the runner’s muscles will determine their performance in different sports events and training. The dominant muscle fiber type will show its effect on the athlete’s performance as the amount of weight that they lift, a number of repetitions they can do, increase in endurance or speed etc.

For example, if a runner has a higher proportion of type 1 fibers, he/she will not be able to run as many fast intervals as an athlete who has more type 2 fibers. Similarly, an athlete with a higher percentage of type 2 fibers will never be able to match the endurance of the athlete with more type 1 muscle fibers.

Therefore, training of both these runners has to be different. This will ensure maximal performance by each of them. It is the duty of the coach to identify athletes who experience fatigue earlier than others during training. These athletes should be asked to have more intervals at faster speeds, followed by longer rest periods.

Between type IIa and type IIb muscle fibers, proper training can result in a mutual increase in endurance and power. But there is no way to convert one muscle fiber type into another. What an athlete is born with, he/she has to live with. However, selective training can increase the relative percentage of one of the muscle fiber types. In other words, selective hypertrophy of a particular muscle fiber type is possible by directed training.

If the athletes train regularly for muscular endurance, for example, slow twitch muscle exercises, it leads to a gradual reduction of fast twitch fibers. Similarly, heavy weight lifting with fewer repeats will increase muscle strength and result in an increase in fast twitch fibers. However, it has to be always kept in mind that the genetic make-up of an athlete will determine the result of such training.

barbell behind the shrug
Weightlifting training to increase fast twitch fibers


Different muscle fiber types have different characteristics. Accordingly, the dominant type of muscle fiber will manifest its effect in terms of either endurance or strength. Knowing about your dominant fiber type will be very useful for you to focus your training on the kind of exercises that will complement your inherent body structure.

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