There are two heads to the biceps brachii muscle: the short head (pictured at right in green) and the long head (pictured at right in red). Both heads originate at the scapula (shoulder blade), and end (insert) on the forearm.
The common belief is that the biceps only flex (decrease angle) the elbow, but on top of flexing at the elbow, the biceps also play a relatively small role in:
Supination of the forearm (turning hand palm up)
Flexion (when palm is supinated) and abduction of the shoulder
By knowing and understanding exactly what the biceps do, you can then target them more effectively!
My Favourite Biceps Exercises
A studied completed by the American Council of Exercise (ACE) has found that concentration curls are the most effective at recruiting the most muscle fibers of the biceps. Keep in mind that the biceps assist in supinating the hand and forearm so any exercise where the palms are supinated appear to be superior for biceps brachii recruitment.
I am also a big fan of chin-ups, and believe that they have helped me to build my biceps to where they are today. Sure, there is some back engagement involved when doing a chin-up but compound movements where multiple muscles and joints are involved can still be extremely effective, especially when working on assistance or accessory muscles such as the biceps.
Overrated Biceps Exercises
Hammer curls are a great way to increase the size of your arms, but primarily target the brachialis and brachioradialis muscles rather than the biceps brachii. Hammer curls can be great for developing the forearms, and I personally still use them, but for pure biceps engagement, they are not the best.
Also keep that in mind when doing curls with an E-Z curl bar. Although only partially supinated, E-Z curl bars are promoting more engagement through the brachialis and brachioradialis.
In the picture on the right, you can see the difference in “short” and “long” biceps. Phil Heath on the left has longer biceps, which can be seen by how close his biceps are to his forearms. Kai Greene on the other hand has shorter biceps, which can be seen through the gap between his forearm and bicep. This also allows Kai to have a greater “peak” to his biceps than Phil.
Biceps training can elicit strength and size increases, however the appearance of your biceps is largely due to genetics. The length of your humerus, the exact points of attachment of the biceps, and the actual length of the muscle itself will determine whether your biceps are longer in appearance or shorter with more of a "peak."