Many muscles act together to control motion at the elbow joint. The main elbow flexor muscles are the brachialis and the biceps (the "Popeye" muscle) (star). The action of the triceps in the back of the upper arm plays a main role in extending (straightening) the elbow. The forearm contains flexor and extensor muscles (cross and circle, respectively) that are responsible for flexing and extending the wrist/fingers.