Final answer:
The rotator cuff muscles, specifically the subscapularis, supraspinatus, infraspinatus, and teres minor, are likely responsible for projecting pain and tenderness to the middle deltoid and elbow due to overuse or injury, as seen in athletes like baseball pitchers.
Step-by-step explanation:
The muscle that projects pain and tenderness to the middle deltoid and to the elbow is likely to be one of the rotator cuff muscles, as these are known to get injured during repetitive movements such as throwing. The four muscles that make up the rotator cuff are the subscapularis, supraspinatus, infraspinatus, and teres minor. These muscles are all involved in the stabilization and movement of the shoulder joint and can refer pain to surrounding areas when inflamed or injured.
For instance, in athletes like baseball pitchers, the repetitive motion of bringing the arm overhead to throw a pitch can lead to inflammation or tears in the rotator cuff tendons, leading to symptoms such as pain and tenderness. In severe cases, surgical intervention such as Tommy John surgery may be necessary, particularly when there are injuries to ligaments such as the ulnar collateral ligament of the elbow, which is a separate but related condition known as 'tennis elbow'.
Conditions such as tendinitis can cause significant discomfort and may be the result of chronic overuse, leading to inflammation and pain that can radiate to various areas, depending on the specific tendons and muscles involved.