Your skeletal muscles develop tension, or contract, to produce movement. A muscle can also contract to prevent movement that would otherwise be taking place. In both cases, muscles can only exert these influences at the joints they cross. The biceps brachii muscle of your upper arm crosses and acts at three different joints.
Origin and Insertion
The biceps brachii, or biceps for short, is a large muscle located on the front of your arm. It has two heads that originate from different parts of your shoulder blade. From these points of attachment, the biceps spans your shoulder, elbow and the joint formed by the two bones of your forearm, which are called the ulna and radius. The biceps ends at its insertion on the front of the radius.
Shoulder Function
Your biceps is active in a number of movements of your upper arm. Both heads are involved when you raise your arm in front of you and when you raise it to the side against opposition with your elbow straight. It is also active when you draw you arm toward the midline of your body once it is raised at the shoulder. The short head participates when you pull your arm down to your side against resistance and when you rotate your arm toward your body. Your biceps also acts as a stabilizer of the shoulder joint.
Elbow Function
Your biceps acts with two other muscles to bend your elbow. This is called flexion. The degree to which your biceps contributes to elbow flexion depends on the position of your hand. For example, when your hand is not facing forward, the biceps shows little activity unless external resistance is applied.
Radioulnar Function
Your radius and ulna articulate at two points on the forearm. At the elbow, the head of the radius articulates with the ulna's radial notch and at the wrist, the head of the ulna articulates with the radius' ulnar notch. Only the radius articulates with the hand at the wrist. This means that movement at the radioulnar joints causes the radius and ulna to rotate around one another. Your biceps helps to perform this rotation when it occurs away from your body. This movement is called supination.
References
- "Principles of Anatomy and Physiology Ninth Edition"; G. Tortora and S. Grabowski; 2000
- "Kinesiology: Scientific Basis of Human Motion Twelfth Edition"; N. Hamilton, W. Weimar and K. Luttgens; 2008
