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The Extensors
The triceps brachii is the major muscle involved in elbow extension (although the anaconeus muscle also plays a minor role). The triceps is composed of three heads: the lateral (outer) head, the medial (inner) head, and the long head, which runs above the medial head and is parallel to the lateral head. The long head of the triceps also comes into play when extending and adducting the shoulder, such as when doing pullovers or lat pulldowns.
Because the triceps has three distinct heads, you can increase the tension on a specific portion of the muscle by varying the way you perform an exercise, just as you can with the biceps. An exercise done with the elbows farther apart than the hands, for example, stresses the lateral head of the triceps, especially if the shoulders are internally rotated (elbows out). Conversely, if the hands are farther apart than the elbows, more stress is placed on the long and medial heads.
A supinated grip (palms up) also emphasizes the long and medial heads, and you can place even more tension on the long head by flexing the shoulders, as in overhead cable triceps extensions. Triceps extensions with the arms down (elbows pointing toward the ground) will put more emphasis on the medial head.