冈下肌, or the infraspinatus muscle, is a key muscle located in the shoulder region. It is part of the rotator cuff, a group of muscles and tendons that stabilize the shoulder joint. The infraspinatus is responsible for the external rotation of the arm and helps in lifting the arm away from the body. This muscle originates from the infraspinous fossa of the scapula and attaches to the greater tubercle of the humerus. It plays a crucial role in various arm movements, making it essential for activities such as throwing and lifting. Proper functioning of the infraspinatus is vital for shoulder health and mobility.