During the bending of the arm, the biceps muscles contract and the triceps muscles relax. The biceps muscle is located in the front of the upper arm and is responsible for bending the elbow, while the triceps muscle is located at the back of the upper arm and is responsible for straightening the arm. When you bend your arm, the biceps muscle contracts and pulls on the forearm bone, causing it to move towards the upper arm bone, while the triceps muscle relaxes and allows the movement to occur smoothly.