The brachialis muscle is located in the upper arm. It lies underneath the biceps muscle. It acts as a structural bridge between the humerus, which is the bone of the upper arm, and the ulna, which is one of the forearm bones. The muscle is innervated by both the musculoskeletal nerve and the radial nerve. In some people, the muscle may appear doubled. Also called the brachialis anticus, its primary action is to flex the forearm muscles at the elbow. Due to its high contractile strength, the branchialis makes many arm and elbow movements possible. Such movements are important for the activities of daily life. Because movements involving the arms and elbows are almost always continuous, injuries to the brachialis muscle are quite common. They can be as subtle as muscle aches or as serious as muscle rupture and hematoma. The muscle can tear, lacerate or rupture with overextension of the elbow or when extreme force is exerted against surrounding structures.