We all know that we use our muscles to exercise and to keep our bodies functioning. But, did you know that you’re born with certain types of muscle fibres in specific amounts?
By understanding how our muscles are constructed, you can better understand why specific exercises and workouts work well and why some people are genetically predisposed to building muscle.
Overview of muscles
It is estimated that there are up to 650 skeletal muscles in the human body, making up about 30-50% of the body weight. As well as skeletal muscles, there are also types of muscles within the body which are just as essential to keep the shape.
Types of muscles
Involuntary muscle is also known as visceral or smooth muscle; similar to cardiac muscle. It contracts under unconscious control. It contracts in response to stimulation by nerves or chemicals, such as hormones, which circulate around the body. These would be found I’m urinary and respiratory systems, for example.
This is the type of muscle we encounter most frequently during exercise. This is also known as striated, striped or skeletal muscle because it is attached to the skeleton. This type of muscle is under our conscious control, hence the term voluntary muscle. It is stimulated by the nervous system and when the muscles contract, they shorten and create movement on the bones to which they are attached. Examples would be biceps and quadriceps.
Cardiac muscle is only found in the heart. It is also known as an involuntary muscle as it not under voluntary control. It contracts when stimulated by electrical patterns. Although it is involuntary, it has a different structure to involuntary muscle.
A muscle is a highly organised structure consisting of
Fascia – the entire muscle is surrounded by a broad protective fibrous sheath called the epimysium.
Bundles of muscle fibres(fascicles) – muscle is composed of bundles of muscles fibres that are surrounded by another layer of fascia called the perimysium. If one muscle fibre were extracted from the bundle, it would also be surrounded by fascia called the endomysium.
Muscle fibre – a single muscle fibre is composed of groups of myofibrils surrounded by a membrane called the sarcolemma consisting of different types of proteins.
These proteins are grouped into thin and thick portions called filaments. Your muscles can contract when these thick and thin portions slide along each other. When skeletal muscles contract, they cause a movement at a joint. They can do so because they are attached to bones by tendons. While all skeletal muscles share these properties, they can be further categorised by muscle fibre type.
What are the Different Muscle Fiber Types?
Skeletal muscles are made from several different types of fibre and vary in two ways:
Speed of contraction
For this reason, muscle fibres tend to be divided into two descriptions:
- Slow-twitch fibres (red coloured) – these are also known as type I or slow oxidative fibres
- Fast-twitch fibres (white coloured) – these are also known as type II or fast glycolytic fibres
Type I (slow oxidative)
These fibres are red because they contain a large amount of myoglobin (a substance that can store oxygen) and red blood cells. They also contain large numbers of mitochondria (the principal sites of energy generation within cells, capillaries (tiny blood vessels) and endurance enzymes. The fibres are characterised as being slow to contract and slow to fatigue. They are recruited for low intensity, more extended duration types of activity. Thus, they are best equipped for endurance or aerobic work. Type I fibres are sometimes referred to as slow-twitch muscle fibres.
2. Type IIb (fast glycolytic)
These fibres are white. They can contract rapidly and with high force but fatigue quickly. They tend to be recruited for high intensity, short-duration activity. They contain low numbers of mitochondria and capillaries and high levels of glycolytic enzymes, so they are best equipped for strength or anaerobic work. Type II muscle fibres are sometimes referred to as fast-twitch muscle fibres.
Generally, muscles tend to be composed of both types of fibre. The percentage of each fibre type varies from muscle to muscle and person to person. Research has shown that muscle fibre composition is, to a large extent, genetically determined.
Although there are two distinctive muscle fibres types, the human body has a fantastic ability to adapt to any additional stress placed upon it. For example, when the activity requires a rapid contraction but also needs a small supply of oxygen, an intermediary muscle fibre type is available for the job. This is known as a type IIa – fast oxidative glycolytic.
Type IIa (fast oxidative glycolytic)
These muscle fibre types can often be associated as intermediate muscle fibre. They are pink in colour and have a stronger and quicker contraction, using a higher force than the slow-twitch but less robust, rapid or intense than the fast-twitch muscle fibres. They also have more significant amounts of mitochondria and capillaries than the fast-twitch muscle fibres, so, therefore, can work under endurance for a small period. They are predominantly recruited to assist slow-twitch muscle fibres during the harder stages of endurance events or assist fast-twitch muscle fibres when the activity performed requires a few additional repetitions (for example during a muscular endurance activity)
Can Muscle Fibers Change?
All of us are born with a set percentage of these muscle fibres. However, some theories claim that you can change the properties of your muscle fibres based on what type of exercises you do.
For instance, someone born with a certain amount of fast twitch and slow twitch fibres can have slow-twitch muscle fibres that exhibit some characteristics of fast-twitch muscle fibres through training such as sprinting or heavy weightlifting.
So, while you may not be blessed with the slow-twitch muscle makeup of an Olympic marathon runner or the fast-twitch fibre makeup of a sprinter, it is possible to improve your performance through proper training and hard work.