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Humans have two types of skeletal muscle: red muscle and white muscle. Red muscles are dense with capillaries and rich in myoglobin and mitochondria, giving them a reddish appearance. In contrast, white muscles have fewer mitochondria, less myoglobin, and a resulting whitish appearance. White muscle fibres can contract faster and are considered fast-twitch fibres, while red muscle fibres are slow-twitch fibres.
| Characteristics | Values |
|---|---|
| Appearance | White muscles have a "whitish" appearance due to less mitochondria and myoglobin |
| Function | White muscles contract quickly and are used for fast swimming movements and escape reflexes |
| Energy source | White muscles use anaerobic respiration and do not depend on oxygen for energy |
| Fatigue | White muscles tire out easily and are activated by the body as a last resort |
| Types of movements | White muscles are used for large movements of the arms, legs, and back |
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What You'll Learn
Humans have both red and white muscle fibres
White muscle fibres, on the other hand, are fast-twitch fibres. They can contract faster and stronger than red muscle fibres. White muscles have a lower myoglobin and oxygen content and are therefore myoglobin-poor, giving them a whitish appearance. They do not depend on oxygen for energy but instead get it from glycogen through anaerobic respiration. This is a very efficient way of producing energy, but these fibres fatigue more quickly. An example of a white muscle is the eyeball muscle.
The difference in energy production between the two types of muscle is significant. Red and white skeletal muscles have different energetic demands and metabolic control. White muscle fibres are recruited for shorter durations, while red muscle mitochondria spend more time near State 4, where respiration is more controlled. The similarity in metabolic capacity between the two types suggests that nuclear programming of mitochondrial protein expression may be configured to meet maximal energy demands.
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White muscle fibres are fast-twitch fibres
Muscle fibres are broadly categorised into slow-twitch and fast-twitch fibres. Slow-twitch fibres, also called red muscle fibres, contract slowly but can sustain the contraction for a long time without fatigue. On the other hand, white muscle fibres are fast-twitch fibres that contract quickly and are designed to produce rapid, powerful movements.
White muscle fibres, also known as type II or type IIx fibres, are skeletal muscles that help with power performance for short periods. They are called fast-twitch fibres because they contract faster than slow-twitch fibres. White muscle fibres have low myoglobin and low oxygen content. They do not depend on oxygen for energy but derive it from an anaerobic process using glycogen. This anaerobic energy generation process is faster and helps white muscle fibres contract faster and stronger. However, the white muscle fibres tire out easily and are activated by the body only when required.
The number of slow and fast-twitch fibres in the body varies between individuals and is determined by genetics. People who excel in endurance sports tend to have a higher number of slow-twitch fibres, whereas sprinters tend to have a higher number of fast-twitch fibres. Both slow and fast-twitch fibres can be influenced by training. While sprint training can improve the power generated by slow-twitch fibres, endurance training can increase the endurance level of fast-twitch fibres.
White muscle fibres are found in fish and are made up of fast-twitch muscle fibres that are designed to contract quickly. They are grouped in a helical form rather than parallel to the body axis and lay deeper in the body than the red muscles used for slow swimming. White muscles in fish are used for fast swimming movements and escape reflexes.
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Red muscle fibres are slow-twitch fibres
Red muscle fibres, also known as slow-twitch fibres, are muscle fibres that contract slowly and help the body move or stay still for longer periods. They are called "red" fibres due to their high myoglobin content, which gives them a red appearance. These fibres are rich in myoglobin and mitochondria and have a high tolerance for fatigue, making them ideal for strenuous activities like exercising. They get their energy from fat and glycogen by using oxygen, which is an aerobic energy generation process. This process is longer, causing the muscles to contract slowly but consistently over an extended period.
Slow-twitch fibres are essential for endurance and long-lasting energy. They are the body's primary source of power for muscles, as they can maintain their function without fatiguing quickly. These fibres are commonly found in muscles responsible for maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small, frequent movements that require endurance rather than high amounts of energy. Examples of muscles that contain a high proportion of slow-twitch fibres include the extensor muscles and the soleus muscle in the leg.
In contrast, fast-twitch fibres contract quickly and are responsible for sudden, powerful movements. They have low myoglobin and oxygen content, relying on glycogen for energy through anaerobic energy generation. This process is faster but leads to quicker fatigue, which is why the body activates them only when needed. Examples of muscles composed primarily of fast-twitch fibres include the eyeball muscles and the muscles in the eyelids.
The ratio of slow-twitch to fast-twitch fibres varies between individuals and is influenced by genetics. People who excel at endurance sports tend to have a higher proportion of slow-twitch fibres, while those better at sprinting or power events have a higher number of fast-twitch fibres. Training can also impact the performance of these fibres, with sprint training improving the power of slow-twitch fibres and endurance training enhancing the endurance of fast-twitch fibres. However, training cannot completely change the inherent characteristics of each fibre type.
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White muscle fibres have less myoglobin and mitochondria
Red muscle and white muscle are both types of skeletal muscle, which perform different functions in the body. Red muscles get their name from their high density of capillaries and rich supply of myoglobin and mitochondria, which gives them a reddish appearance. White muscles, on the other hand, have a lower concentration of myoglobin and mitochondria, resulting in a whitish colour.
Myoglobin is a protein found in muscle cells that stores oxygen. It is particularly abundant in red muscles, which have a high oxygen content and rely on aerobic respiration to generate energy. This type of respiration allows red muscles to contract slowly and for extended periods without fatigue. The slow contraction speed of red muscles makes them ideal for strenuous activities like exercising.
White muscles, in contrast, have a lower oxygen content and rely on anaerobic respiration. This leads to a faster contraction speed and higher power output in white muscles compared to red muscles. However, the anaerobic respiration also causes a quicker buildup of lactic acid, resulting in early fatigue. White muscles are, therefore, better suited for short-term labour and are activated by the body only when necessary.
The difference in myoglobin and mitochondria content between red and white muscles is a key factor in their distinct characteristics and functions. White muscles, with their lower myoglobin and mitochondria levels, are designed for rapid, powerful contractions but tire quickly, while red muscles can sustain longer periods of activity without fatigue.
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Red muscle fibres have a higher tolerance for fatigue
Red muscle fibres, or slow-twitch fibres, have a higher tolerance for fatigue compared to white muscle fibres. This is because red muscle fibres contract slowly over long periods, making them ideal for strenuous activities like exercising. They derive their energy from fat and glycogen, using oxygen in a process known as aerobic energy generation. As this process is lengthier, the muscles contract more slowly.
Red muscle fibres are rich in myoglobin and mitochondria, giving them their characteristic red appearance. Myoglobin, an O2-binding molecule, enhances the delivery of oxygen to the muscle fibres. Additionally, the abundance of mitochondria and aerobic respiratory enzymes in red muscle fibres contributes to their endurance.
In contrast, white muscle fibres, or fast-twitch fibres, contract quickly and are designed for rapid, powerful movements. They have lower myoglobin and mitochondria levels, resulting in a whitish appearance. White muscle fibres rely on anaerobic energy generation, deriving their energy from glycogen without depending on oxygen. This faster energy generation process enables white muscle fibres to contract with greater force but leads to quicker fatigue.
The difference in fatigue resistance between red and white muscle fibres is crucial for various physical activities. For example, marathon runners have a higher percentage of slow-twitch fibres in their leg muscles, enabling them to run long distances without tiring easily. On the other hand, sprinters have a lower percentage of slow-twitch fibres, relying more on fast-twitch fibres for explosive speed.
Overall, the higher tolerance for fatigue in red muscle fibres makes them essential for sustained physical activities, while white muscle fibres are better suited for short, intense movements.
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Frequently asked questions
Yes, humans have white muscle.
White muscle appears whitish due to having less mitochondria and myoglobin.
Red muscle is dense with capillaries, mitochondria and myoglobin. It is slow-twitch and contracts slowly over a long period without fatigue. White muscle, on the other hand, is a fast-twitch muscle that contracts quickly but tires easily.
The eyeball muscle is an example of white muscle. White muscle is also found in fish, used for fast swimming movements and escape reflexes.
White muscles produce energy anaerobically and do not depend on oxygen.
