The muscles responsible for increasing the angle between the humerus and the radius/ulna are the triceps brachii and the anconeus. The triceps brachii is a three-headed muscle located on the back of the upper arm, while the anconeus is a small muscle located on the back of the elbow joint. When these muscles contract, they extend the elbow joint and increase the angle between the humerus and the radius/ulna.
