In the birds that flyare attached to the keel of the sternum, which also holds the vital organs of the abdominal cavity.
Muscle development has increased through genetic selection, as is evident in modern breeds of chickens, broilers, and turkeys raised for meat, although these domestic birds can only fly short distances.
In flying birds, the pectoral muscles are very red, indicating a large number of muscle fibers rich in myoglobin (aerobic oxidative metabolism). Birds with white and red muscles produce white and dark meat, respectively. Red meat contains higher amounts of fat and myoglobin; the latter is a compound that transports oxygen and iron, similar to hemoglobin.
While in the birds that have lost the ability to fly, the musculature is pale, due to the prevalence of muscle fibers white glycolytic (anaerobic metabolism).
It is worth noting the muscles involved in flight; these are the pectoral muscles, which make up the cut of meat commonly known as the “breast.” Generally, these are described as the superficial pectoral muscle and the deep pectoral muscle, also known as the pectoralis major and pectoralis minor, respectively.
The pectoralis major originates from most of the surface of the sternal body, the clavicle, and the sternoclavicular membrane, and inserts into the proximal end of the humerus (pectoral crest). It acts as a wing-lowering muscle during flight.
The pectoral muscle deep originates in the part of the keel sternal, covered by the previous one; it develops a tendon that passes through the channel trióseo to be inserted into the dorsal surface of the proximal end of the humerus. Acts as an elevator of the wing during the flight.
This tendon can break due to trauma (shock or strike it against a window, for example) and then it is characteristic to note that the bird cannot pick up the wing.
In general, the color of a muscle is determined by its activity; red-fiber muscles are the most heavily used; they receive more blood and contain more fat.
White-fiber muscles are rich in glycogen (a white, amorphous substance found in abundance in muscles that can be converted into glucose when the body needs it), which is easily broken down under anaerobic conditions (physical exercise involving short bursts of strength-based activity).
In poultry, the muscles in a chicken's legs are darker than those in the breast, due to the constant effort these muscles exert to keep the body upright when the bird is standing.
In contrast, in non-domesticated flying birds, the breast muscle is darker and serves as support during flight. Broiler chickens have muscle fibers that are larger in diameter and lighter in color than those of our laying hens.
The pectoralis muscles should always be explored, since it indicates the nutritional status of the bird, and can be used for the application of intramuscular injections. It is recommended to introduce the needle into the caudal part of the pectoral muscle surface, by the fact that the head presents a greater blood supply and there is a risk of introducing substances into the bloodstream.
Among the muscles of the forearm, the radial wrist extensor deserves special mention. Originating at the medial epicondyle of the humerus, its short tendon extends over the dorsal surface of the carpal joint and inserts into the extensor process of the metacarpal bone. This tendon can be severed on one wing to prevent flight, as this limits wing extension.
In the hen’s forelimb, in addition to the muscles, it is worth noting the presence of the propatagium or patagium. This is a triangular fold of skin on the cranial side of the wing, extending between the shoulder and carpal joints. Its cranial edge is traversed by the elastic propatagial ligament. To prevent flight, a triangular section of this fold is sometimes removed. It is also used to attach the staples that secure leg bands on waterfowl.
The ulnar cutaneous vein may be visible on the ventral surface; it is suitable for venipuncture (injections and punctures) as it runs over the elbow joint.
