The longest bone in the human arm is the humerus. It extends from the shoulder to the elbow and plays a critical role in the arm's mobility and strength. The humerus forms the ball-and-socket joint at the shoulder, allowing for a wide range of arm movements. It also connects with two bones in the forearm—the radius and ulna—at the elbow joint. Injuries to the humerus can significantly affect arm function, highlighting its importance in the upper limb's anatomy and biomechanics.