The infraspinatus muscle is attached along the broad inner edge of the |scapula| (shoulder blade). It narrows into a strong ~tendon~ that joins firmly to the top of the |humerus| (upper arm bone). The infraspinatus muscle sits on the back of the shoulder blade below the |spine| and under the |deltoid| muscle. It works with the |supraspinatus| to help lift and twist the upper arm. These two muscles also help support and protect the |shoulder joint| by keeping the head of the |humerus| in place.