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Birds have a unique musculature, with approximately 175 different muscles, mainly controlling the wings, skin, and legs. The largest muscles in the bird are the pectorals, which control the wings and make up about 15-25% of a flighted bird's body weight. Birds have a complete separation between pulmonary and systemic circulation, large hearts for their body weight, and lungs connected to nonvascular air sacs. The colour of meat comes from a high concentration of myoglobin in the fibres, which binds oxygen that can be released as needed to the muscle fibres to allow them to contract. Birds have white muscle fibres, also called fast-twitch fibres or anaerobic fibres, which are used for rapid, short-term activities like fleeing from danger.
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What You'll Learn
White muscle fibres are used for rapid, short-term activities
Muscle fibres are classified based on two criteria: how quickly they contract relative to others, and how they regenerate adenosine triphosphate (ATP), an energy-carrying molecule. There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
White muscle fibres, also known as fast-twitch fibres, are a type of fast glycolytic fibre. They are used for rapid, short-term activities like fleeing from danger. These fibres are able to contract very quickly, producing rapid, forceful contractions associated with quick, powerful movements. However, they fatigue quickly and are not suitable for long-term activities.
White muscle fibres primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria, a limited capillary supply, or significant amounts of myoglobin, resulting in a white coloration.
In contrast, dark muscle fibres, also known as slow-twitch or aerobic fibres, require a constant supply of oxygen to function. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Dark muscle fibres have a robust supply of blood vessels and myoglobin, which gives them a darker appearance. They are useful for activities requiring endurance, such as walking and running.
In birds, the breast or flight muscles are typically composed of white muscle fibres. The size of the breast muscles in a bird is related to its flying ability. Birds that need to take evasive action in flight or fly through forested areas benefit from the powerful but short-duration contractions of white muscle fibres. On the other hand, birds that migrate long distances have breast muscles made mostly of dark muscle fibres to enable long bouts of strenuous flight.
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Red muscle fibres are used for endurance activities
Muscle fibres are classified based on two criteria: how fast they contract and how they regenerate adenosine triphosphate (ATP). There are three main types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Red muscle fibres, also known as slow-twitch fibres, are rich in myoglobin and mitochondria, giving them their characteristic red colour. They contract slowly but can maintain activity for longer durations, making them ideal for endurance activities. The high myoglobin content in red muscle fibres increases the entry of oxygen into the muscle fibres, allowing them to contract and perform sustained work without fatiguing easily. These fibres are often referred to as aerobic fibres as they require a constant supply of oxygen to function. They are well-suited for activities such as marathon running, cycling, and maintaining posture.
In contrast, white muscle fibres, or fast-twitch fibres, have lower myoglobin and mitochondria content, resulting in a lighter colour. They contract quickly and generate rapid power but fatigue more quickly due to the accumulation of lactic acid. White muscle fibres are adapted for short, intense bouts of activity and are ideal for explosive movements like sprinting or weightlifting.
The ratio of slow-twitch to fast-twitch fibres is largely determined by genetics, but it can also be influenced by training. Individuals who excel at endurance sports tend to have a higher number of slow-twitch fibres, while those who perform better at sprint events have a higher number of fast-twitch fibres. Endurance training can modify slow-twitch fibres to improve their endurance and oxidative capacity, while high-intensity interval or resistance training can increase the strength and size of fast-twitch fibres.
In birds, the breast or flight muscles are mostly composed of red muscle fibres, which enable them to sustain long bouts of flight. White muscle fibres, on the other hand, are suited for birds that need to take evasive action or fly through forested areas.
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The appearance of white meat when cooked
White muscle fibres differ from dark muscle fibres in their metabolism and contractile properties. Dark muscle fibres, also known as slow-twitch or aerobic fibres, require a constant supply of oxygen to function and are used for endurance activities like walking, running, and sustained flight. The colour of dark meat is due to the presence of myoglobin, a molecule that binds to oxygen and facilitates its transfer to muscle fibres. Myoglobin is red in colour, giving dark meat its distinct appearance.
When cooked, the proteins in the white muscle fibres of birds denature and coagulate, resulting in the white, opaque appearance commonly associated with chicken or turkey breast. The myoglobin in dark muscles also breaks down during cooking, imparting a brownish colour to the meat. This breakdown of myoglobin is also responsible for the brown colour of cooked steak.
The type of muscle fibres present in birds depends on their flight capabilities and habits. Birds that migrate long distances or exhibit powerful flight tend to have a higher proportion of dark muscle fibres, while birds that fly short distances or take evasive action during flight have a higher proportion of white muscle fibres. Domesticated turkeys, for example, have larger breast muscles due to selective breeding, resulting in a higher percentage of white meat compared to wild turkeys.
The distinction between white and dark meat in birds is an interesting aspect of muscle physiology, influencing the culinary preferences of many individuals during festive gatherings such as Thanksgiving. Understanding the unique characteristics of these muscle fibres provides valuable insights into the evolution and adaptations of different bird species.
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The difference in muscle composition between bird species
Birds have a unique musculature that sets them apart from other animals. Their muscle composition varies between species and even within families. Birds have approximately 175 different muscles, with the largest being the pectorals, which control the wings and make up about 15-25% of a flighted bird's body weight. These muscles provide the powerful wing strokes essential for flight. The muscle underneath the pectorals is the supracoracoideus, which raises the wing between wingbeats. Together, these two muscle groups make up about 25-40% of the bird's full body weight.
The muscles of bird wings work to extend or flex the elbow, move the wing as a whole, or extend or flex particular digits. These muscles adjust the wings for flight and other actions. Birds also have abdominal muscles that expand and contract the chest and hold the ribcage. The skin muscles help birds in flight by adjusting the feathers, which are attached to the skin muscle and aid in flight maneuvers and mating rituals.
The breast or flight muscles of birds consist of a mixture of two types of muscle fibers: white and dark (or red). White muscle fibers, also known as fast-twitch fibers, are used for rapid, short-term activities like evading predators or flying through forests. They contract very quickly but fatigue rapidly. On the other hand, dark muscle fibers, or slow-twitch fibers, are ideal for endurance activities and can sustain long bouts of strenuous flight. Migratory birds like ducks and geese have breast muscles made mostly of dark muscle fibers.
The difference in colour between white and dark muscle fibers is due to the presence or absence of myoglobin, a molecule that binds to oxygen. Dark muscle fibers have a high concentration of myoglobin, which gives them their red colour and increases the entry of oxygen into the muscle fibers. White muscle fibers, in contrast, do not contain myoglobin and are therefore paler in colour.
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White muscle disease in chickens
White muscle disease (WMD), or nutritional muscular dystrophy, is a condition in chickens that results in the degeneration of skeletal and cardiac muscles. It is caused by a deficiency in vitamin E or selenium, with selenium deficiency being the most common cause. These deficiencies can lead to progressive weakness and degeneration of the muscles that control movement, including the heart, diaphragm, tongue, and esophagus. Birds affected by WMD may experience difficulty walking and may even become paralyzed. In some cases, sudden death from heart failure can occur due to a scare or stressful event.
The treatment and prevention of WMD involve ensuring adequate levels of vitamin E and selenium in the chicken's diet. It is important to provide a nutritionally balanced feed and supplement with good sources of these vitamins, especially in regions where they may be deficient in the soil. However, it is crucial to consult a veterinarian before administering any injections of selenium or vitamin E, as high amounts of selenium can be toxic and fatal.
The muscle fibers in birds are composed of both white and dark (or red) fibers, which differ in metabolism and contractile properties. Dark muscle fibers, also known as slow-twitch or aerobic fibers, require a constant supply of oxygen and are suitable for endurance activities. On the other hand, white muscle fibers, or fast-twitch fibers, are used for rapid, short-term activities and do not require oxygen to function. Instead, they utilize glycogen for fuel, which allows for quick contractions but also leads to rapid fatigue.
The distinction between white and dark meat in birds, such as chickens and turkeys, is due to the presence of these different types of muscle fibers. Dark meat is composed of red muscle fibers, which contain high concentrations of myoglobin, giving them their color and aiding in oxygen storage. White meat, on the other hand, has fewer dark fibers and a lower concentration of myoglobin, resulting in a lighter color.
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Frequently asked questions
Bird muscles appear white due to a lack of capillaries and myoglobin, which is responsible for the muscle's red colour. Myoglobin stores oxygen for muscles to use.
White muscles, also known as fast-twitch muscles, are used for quick, short-term activities such as fleeing from danger. They can contract very rapidly but fatigue quickly.
Red muscles, also known as slow-twitch muscles, are used for endurance activities. They contract slowly but do not fatigue as quickly as white muscles.
When cooked, the proteins in the muscle fibres of white meat denature and coagulate, resulting in the white, opaque appearance we associate with chicken or turkey breast. The myoglobin in the dark muscles breaks down during cooking, imparting a brownish colour to the meat.
