The bicep and tricep muscles are located in the upper arm between the shoulder and elbow joint. The triceps are the posterior arm muscles, supplied by the radial nerve. The biceps, along with the brachialis and coracobrachialis, are the anterior arm muscles, and are supplied by the musculocutaneous nerve.
The biceps and triceps work together as flexors and extensors. The bulk of the posterior upper arm is taken up by the tricep muscle, which acts as the antagonist to the bicep and brachialis muscles. The two sets of muscles are separated by medial and lateral intermuscular septa. The anterior arm muscles are the flexors of the elbow joint and forearm, while the triceps, along with the anconeus to a lesser degree, are the extensors.
The process by which opposing muscles on either side of a joint work together, one contracting while the other relaxes, is called reciprocal inhibition. The two types of muscles, flexors and extensors, must work in synchrony so that the joint can move smoothly. The process is also a safeguard against injury that prevents one muscle from working against the other. When a contraction is activated in one muscle, the opposing muscle is stopped by the actions of an inhibitory interneuron originating in the spinal cord.