We control our muscles with messages we transmit from our brain. These messages are quickly transmitted to the muscles through nerves and the spinal cord. When the muscle receives the message, it begins to contract.

If we want to bend our hand towards our shoulder, our brain sends a message to the biceps muslce through the nervous system. The biceps muscle is connected to the radius and the shoulder. When the cells of the biceps muscle contract, the hand begins to move towards the shoulder. 

If we want to stretch our hand back to its regular position, our brain must send a message to the triceps muscle, which is located on the other side of the arm as the biceps. When the muscles of the triceps muscle contract, the arm straightens.