Final answer:
The muscle that attaches to the middle shaft of the radius is the pronator teres.
Step-by-step explanation:
The muscle called the pronator teres attaches to the middle shaft of the radius. It is located on the anterior side of the forearm and is responsible for pronation of the forearm and hand. The pronator teres originates from the medial epicondyle of the humerus and inserts onto the lateral surface of the radius.