Every human being has about 656 muscles, which in turn consist of thousands of thin and very resistant muscle fibers. However, muscle fibers are not all the same, so you can read here about the differences between muscle fibers, how you can influence them through your training, and exactly what "Fast-Twitch" and "Slow-Twitch" are all about.
As mentioned earlier, each of your muscles consists of individual thin muscle fibers. These have a diameter of 10 - 200 nanometers. For comparison, 1 millimeter corresponds to 1,000,000 nanometers. Muscle fibres are incredibly thin, but can carry 1000 times their own weight and are therefore extremely resilient. The length of the muscle fibres depends strongly on the length of the respective muscle to which they belong and can measure from a few millimetres to a few centimetres. The individual muscle fibres are in turn combined into fibre bundles. Such a fibre bundle can then be 0.1 - 1mm thick and is attached to tendons and bones at its ends.
The basic principle of muscle function is muscle contraction. This means that a muscle contracts and creates the movements of the body through its connections to the skeleton and tendons. In this context one also speaks of the skeletal muscles, i.e. muscles which are connected to your skeleton and which you can move at will. When a muscle contracts, it is also called concentric muscle work. This shortens the muscles and transfers the movement via the tendons to your bones, allowing you to move your body parts. When muscles work, there are three different types of muscle work.
You can illustrate all three ways of muscular work using the example of bench pressing:
The three ways of working the muscles can be found not only in the gym, but also in almost all other movements in everyday life. In eccentric movements, the muscle is usually put under the greatest tension and the risk of injury is greatest there.
You know the feeling that after a hard workout you get up the next morning and your muscles ache with every movement. The famous sore muscles. But what actually causes it? As you have learned, your muscles are always under tension during movements. If this tension becomes too great, for example when you move very high weights, some of these muscle fibres can tear. These small injuries are also called micro traumas because they are very small and occur at different points in the muscle. Especially when training with a very high weight or movements with a very high percentage of eccentric muscle work, such as running downhill, the muscle is under a lot of tension. If the muscle is not used to this tension, individual muscle fibres tear and cause the soreness with which you have to fight your way out of bed the next morning.
If a muscle becomes too tense, there is a risk of a muscle fibre rupture or a muscle bundle rupture. The main difference to sore muscles is the extent of the injuries. While in sore muscles very small injuries of the muscle tissue occur in various places, a torn muscle fibre usually tears a lot of muscle fibres in a certain place. This in turn can lead to bleeding or even an acute loss of muscle strength. As a consequence, the muscle should be spared as much as possible over a longer period of time in order not to disturb the healing process. A torn muscle bundle, on the other hand, exists when entire muscle bundles tear under excessive tension.
If a whole muscle tears, then one speaks of muscle tear or muscle rupture. This is the most severe form of muscle injury and in many cases an operation is necessary to achieve complete healing or to restore full muscle strength.
Within each muscle there are different types of muscle fibers, the distribution and extent of which vary from person to person and are mainly determined by the training condition. These different fibre types can be easily divided into the following three categories:
Slow twitch fibres are often referred to as red musculature or type 1 fibres. This type of muscle fiber is characterized by slow contraction, but is very resistant to fatigue. Slow-Twitch fibers gain their energy through oxidative metabolism, i.e. energy is generated with the aid of oxygen, glycogen and fat. Glycogen is an energy store that is primarily obtained from carbohydrates and stored in your muscles after eating the food. This process is commonly referred to as aerobic metabolism.
Fast-Twitch fibers are often referred to as white muscle fibers or type 2 fibers. In contrast to type 1 fibers, they are characterized by very fast contraction and are able to deliver very high power outputs at short intervals. Fast-Twitch fibers obtain their contraction energy mainly from glycogen, without the influence of oxygen. This process is therefore also referred to as anaerobic metabolism.
You can imagine the intermediary type of muscle fiber as a kind of hybrid of the other two fibers. It is capable of fast power output and is relatively fatigue-resistant.
ST and FT fibres therefore differ particularly in their energy metabolism. ST fibers are constantly supplied with new oxygen by respiration and can therefore work for very long periods of time. FT-fibers, on the other hand, are dependent on quickly available stored energy whose reserves are exhausted much faster. The energy consumption of an FT-fiber is on average even 3 - 4 times as high as that of an ST-fiber.
If you look at the distribution of ST and FT fibres in muscles, their ratio is determined by a person's genetics on the one hand and by a person's training condition on the other. This means that we tend to have a certain ratio of muscle fibers, but we can influence this significantly through our training.
As you might imagine, endurance athletes therefore have more ST fibres due to the long and continuous stress, as they are able to use oxygen for energy metabolism and withstand stress for a long time. In contrast, athletes who only perform fast, short movements with a high input of strength have to use significantly more FT-fibers in their muscles. These include above all sprinters and weight lifters. For example, Usain Bolt and other sprinters avoid longer jogging rounds in order not to run the risk of increasing the proportion of ST fibres and reducing their sprintability. If you want to be on the 100m as fast as possible, you should remove endurance units as much as possible. With genetic predisposition and appropriate training, a sprinter can achieve a proportion of up to 85% FT-fibres and a marathon runner the same proportion of ST-fibres.
But since there are not only marathon runners and sprinters, athletes also fall into the middle of the spectrum and have a rather balanced ratio. A good example for this are martial artists, who need on the one hand sufficient speed and power, on the other hand should be able to hold the high speed over a period of up to 40 minutes. Of course, individual muscles can also have different proportions of a fibre type, depending on how they are trained.
As you can see, your training not only affects the size of your muscles, but also the composition changes with the training stimuli. So you should always be aware that it is very difficult to be extremely fast and extremely enduring at the same time. So if you want to get the maximum weight out of your squats, think about how often you do a 10km run and vice versa. Even if this doesn't mean that you can't make great progress in both disciplines at the same time, you should always include your knowledge of muscle fibers in your training planning.
Marc graduated from the DSHS Cologne and the KIT with a master's degree in sports science. His personal trainer background and more than 14 years of training experience contribute to his expertise.
