Perhaps one of the most well-known muscles of the body, the Biceps Brachii, or biceps for short, is a dual-headed muscle pair on the upper arm, which originates at the scapula and joins into a single belly that continues along to connect with the upper forearm. Contrary to popular belief, the biceps isn’t the arm’s strongest flexor. That title belongs instead to the brachialis muscle. Indeed, Biceps Brachii is actually one of the most variable of all the body’s muscles, and in one out of ten cases, a third head can arise. In fact, there are known cases (albeit extremely rare), where as many as seven supernumerary heads have appeared.
As a strong supinator, the biceps turns the palm upwards. It also functions as a key flexor of the forearm, principally when the forearm is supinated. A great example of this function is in the action of opening a wine bottle where we see supination while unscrewing the cork, and flexion when extracting it. Biceps Brachii also slightly supports the forward flexion of the shoulder joint (bringing the arm out in front and raising it), lateral adduction (pulling the arm across the body), and finally, leveraging its connection with to the scapula, it helps stabilize the shoulder joint when carrying a heavy object in the arm, such as a mother would with her baby. Innervation comes from the musculocutaneous nerve, and is shared with both the coracobrachialis and brachialis muscles.