Subscapularis muscle function

After sitting for a long time or maintaining a posture for a long time, you will feel particularly tired, especially on both sides of your shoulders will feel sore and weak. This is mainly due to the excessive tension of the subscapularis muscle, which causes lactic acid accumulation. At this time, you must do some appropriate exercise. If it is done for a long time, it will cause the subscapularis muscle function to deteriorate, and the scapular joint will also change, affecting the circulation of blood and lymphatic system.

The subscapularis muscle is a triangular muscle located in front of the shoulder blade. It originates from the infrascapular fossa, passes in front of the scapular joint, and ends at the lesser tubercle of the humerus. Its function is to retract and internally rotate the scapula joint. The subscapularis muscle is often injured suddenly due to sudden adduction and internal rotation of the upper limb, or due to long-term continuous adduction and internal rotation of the upper limb, which causes repeated contraction and relaxation of the subscapularis muscle, resulting in slight tearing of the tendon fibers at the origin and insertion point and damage to small blood vessels. Because there are no prominent symptoms, it is often not noticed in the early stage. The constant movement of the upper limbs pulls the injured area, making it unable to repair well, bleeding, and exudation, which eventually leads to adhesion and scarring, resulting in functional impairment. There is pain in adduction and internal rotation of the shoulder joint, and the range of motion of the back of the affected limb is reduced.

The subscapularis muscle is located in the subscapularis fossa and is triangular in shape. It is a multi-feathered muscle composed of multiple feathers. On the sagittal section of the muscle, 5 to 7 lamellar or C-shaped key plates can be seen in the muscle. The 2 to 3 key plates above the muscle are longer, extending from the middle of the muscle belly to the end key. Near the stop key, the key plate is bent and enlarged to form a semi-circular shape, thickened and connected to the stop key. The muscle fibers of the subscapularis muscle originate from the bone surface of the subscapularis fossa and the fascia on the surface of the subscapularis muscle, run towards each other, and end at the above-mentioned 5 to 7 health plates, so the subscapularis muscle is composed of 5 to 7 double-pinnate muscles. Its physiological cross-sectional area is 21.48±6.4cm², which is larger than the physiological cross-sectional area of ​​the middle sub-part of the deltoid muscle (20.52cm²). The function of the subscapularis is generally described in textbooks as adducting and internally rotating the shoulder joint and acting as a descending arm muscle to assist the deltoid in abduction. Basmiajan's study showed that the subscapularis plays an important role in the early stage of sacral abduction. The electromyographic activity of the subscapularis reaches its peak when the sacral abduction reaches 90°, and the maximum activity of the deltoid muscle occurs between 90° and 180°. Inman et al. estimated that when the sacrum is abducted to 90°, the strength of the deltoid muscle is 8 times the weight of the limb (assuming the weight of the limb is 9% of the body weight).

The combined force of the subscapularis, teres minor, and infraspinatus muscles is 9.6 times the weight of the limb, and the combined force is maximum when the lift reaches 60°. It is reasonable that the physiological cross-sectional area and maximum contraction tension of the subscapularis muscle are larger than those of the middle part of the deltoid muscle. The insertion of the subscapularis is wide and thick, interweaving with the fibers of the anterior wall of the shoulder joint capsule, thereby enhancing the stability of the shoulder joint. During the movement of the shoulder joint, especially rapid movement, the fat bone generates an accelerated centrifugal force at the shoulder joint. The subscapularis and glenoid cuff muscles pull the fat bone through eccentric contraction, so that the fat bone is close to the glenoid cavity to prevent the fat bone from shifting. The subscapularis then prevents the fat bone from dislocating forward. There are few reports on the morphology of the subscapularis muscle. The results of this paper are compared with those of Hin's study, which showed that the latter has a larger physiological cross-sectional area (26.3 cm²). In addition to slightly different calculation formulas, it may also be affected by aspects of a person's height, weight, etc.