What is the function of the pectoralis major muscle?

Nowadays, more and more people like to go to the gym, with the aim of making their bodies stronger and fully showing their muscles. In the gym, many men prefer to exercise their pectoralis major muscles, believing that men with very full pectoralis major muscles look the most secure. So from a medical point of view, what function does the pectoralis major muscle play in the human body?

Overview

The pectoralis major is located in the superficial layer of the upper front part of the thorax.

Origin: clavicle (medial half of clavicle), sternocostal region (sternum and upper 5-6 costal cartilages) and abdomen (anterior wall of the rectus abdominis sheath).

Insertion point: crest of greater tuberosity of humerus (where clavicle and abdominal muscle bundles cross above and below).

The muscle belly is fan-shaped, converging outward and upward to form a "U"-shaped flat tendon that ends at the greater tuberosity ridge (lateral lip of the intertubercular groove).

Anatomy and Function

One of the structural characteristics of the pectoralis major muscle: fibers from different starting points have different endings. The fibers of the clavicular part form the anterior layer of the flat tendon outward, the fibers located on the outside end at the upper end of the outer lip, and the fibers of the medial part end at the lower end of the outer lip. The sternocostal and abdominal fibers extend upward and outward, and are located below and behind the clavicular fibers. This structure allows the upper and lower muscle bundles to stretch to the same degree when the upper arm is abducted. When the pectoralis major contracts, it can adduct and internally rotate the humerus, and the sternocostal part can extend the raised upper limb and help breathing; the contraction of the clavicle can flex the shoulder joint.

Auxiliary structures

1. There is a distinct and palpable deltoid groove between the pectoralis major and the deltoid muscle. The cephalic vein is in the groove and is located in front of the shoulder joint. It is a local area that should be paid attention to during the anteromedial cervical approach and the anteromedial shoulder approach in humeral surgery. It is also a useful landmark for the natural boundary between the deltoid and pectoralis major muscles. When performing surgical repair at the junction of the deltoid muscle and the pectoralis major muscle, attention should be paid to the cephalic vein that enters the subclavian vein along the groove between the deltoid pectoralis major muscles. Once damaged, its proximal end may retract, making it difficult to stop bleeding.

a) The blood supply to the pectoralis major muscle comes from multiple sources, among which the superior and inferior pectoralis branches of the thoracoacromial artery, the pectoralis branches of the axillary artery, and the anterior intercostal arteries and perforating branches of the internal thoracic artery are more important, as well as the pectoralis branch of the superior thoracic artery. There are abundant anastomoses of blood vessels from different origins within the muscle.

b) Innervating nerves: lateral thoracic nerve and medial thoracic nerve arising from the brachial plexus of the spinal nerves.

Clinical technology and application

1. In 1979, some scholars introduced the application of pectoralis major myocutaneous flap in the repair of head and neck tumor defects. Subsequently, this flap quickly replaced other pedicled flaps at the time and became the most commonly used tissue flap for repair after head and neck tumor resection. However, the blood supply of the pectoralis major myocutaneous flap is not very reliable. It is reported that the rate of necrosis and partial necrosis of the flap is as high as 7% to 20%. However, since the pectoralis major myocutaneous flap is easy to prepare, there is no need to change the body position during the operation, and no vascular anastomosis is required. The donor site can also be directly sutured. Therefore, for units that do not have microsurgical technology and patients who are not suitable for free flap transplantation, the pectoralis major myocutaneous flap is still a reliable repair method.

2. The improved method uses the medial pectoralis major approach. Since the medial pectoralis major is thinner than the lateral side and has a distinct layer between it and the chest wall, it is very easy to flip up. Without the interference of the pectoralis minor, the positioning of the vascular pedicle is faster, simpler and safer than the lateral approach. In addition, only part of the inner side of the pectoralis major muscle is removed during flap preparation, so that most of the muscle fibers on the outer side of the pectoralis major muscle are preserved, thereby maximally retaining the function of the pectoralis major muscle. The pedicle of the traditional pectoralis major myocutaneous flap is the muscle vascular pedicle. When it passes over the surface of the clavicle, the bloated muscle will cause compression on the vascular pedicle, thereby affecting the blood supply of the myocutaneous flap. In fact, many reasons for the failure of pectoralis major myocutaneous flap can be attributed to the compression of the root of the vascular pedicle. In addition, the bloated muscle vascular pedicle also limits the length of the vascular pedicle when it passes over the clavicle surface. In order to extend the length of the vascular pedicle, the traditional method is to design the skin island farther away from the distal end of the pectoralis major muscle, or even on the surface of the rectus abdominis sheath. However, since the blood supply of the thoracoacromial artery to the skin on the surface of the rectus sheath spans a vascular unit, according to Taylor's theory, the blood supply to this area is not very reliable, which results in the randomness and unreliability of the blood supply to the skin island, thereby increasing the chance of flap necrosis. In order to prevent compression of the vascular pedicle and extend the length of the vascular pedicle, our method is to prepare the pectoralis major myocutaneous flap into a true island flap, that is, the vascular pedicle 5 cm below the clavicle does not carry any muscle, so that the vascular pedicle is not easily compressed when crossing the surface of the clavicle, and the length of the vascular pedicle of the myocutaneous flap is greatly extended. At the same time, since the skin island is designed to be located on the surface of the pectoralis major muscle, its blood supply is very reliable, which improves the success rate of the pectoralis major myocutaneous flap. If a longer vascular pedicle is needed, we can also use the method of passing the vascular pedicle behind the clavicle, which allows the length of the vascular pedicle to be further extended by 4 cm. However, you should be very careful during the operation to avoid accidentally injuring the root of the vascular pedicle and the subclavian artery and vein, so as to avoid serious consequences.