the muscles which bring about these movements are called biceps and triceps
biceps are attached to scapula and radius for bending
triceps are attached to scapula, humerus and ulna for stretching
when the biceps contracts, it pulls the radius (forearm) and the hand bends the triceps relaxes at the same time
when the triceps contracts and biceps relaxes(extends) the arm is stretched biceps flexes the arm (flexor) and triceps extend(extensor muscle) the arm