explain how muscles and bones work together to help bend the arm.

Muscles and bones work together to facilitate movement in the body, including bending the arm. This process involves the skeletal and muscular systems working in tandem, through a mechanism known as the musculoskeletal lever system.

1. Bone Structure - The human arm mainly consists of two bones in the forearm: the radius and the ulna, and one bone in the upper arm: the humerus. At the elbow, the humerus connects with the radius and ulna to form a hinge joint, which allows the arm to bend and straighten.

2. Muscle Contraction - To bend the arm, the brain sends a signal through the nervous system to the muscles in the arm. The key muscles involved in bending the arm are the biceps brachii, located at the front of the upper arm, and the brachialis muscle, which lies beneath the biceps.

3. Leverage - When the muscles receive the signal, the biceps and the brachialis muscles contract. Muscle contraction pulls on the tendons, which are strong connective tissues attached to the bones. Since muscles can only pull (not push), the biceps and brachialis pull on the forearm bones.

4. Joint Action - As the muscles contract, they pull on the radius and ulna, bringing them closer to the humerus. This movement occurs at the elbow joint, which acts as the pivot (fulcrum) in the lever system.

5. Movement Completion - As the forearm is pulled upwards, the angle between the ulna/radius and the humerus decreases, resulting in the bending of the arm. The elbow functions as a hinged joint, allowing this motion in one plane, similar to opening and closing a door.

It is important to note that when the arm bends, antagonistic muscles such as the triceps brachii (located at the back of the upper arm) relax to allow this motion. When the arm straightens again, the roles reverse: the triceps contracts and the biceps relax.