What is an antagonist to the supinator muscle?

According to Know Your Body, the antagonist to the supinator muscle is the tricep extensor, also known as the triceps brachii muscle. The triceps extensor is a large, thick muscle that spans the back of the upper arm on most vertebrates, humans included.

Movement of the triceps extensor allows the organism to move the elbow, by either extending or contracting the limb, while movement of the supinator muscle allows rotation of the same limb, according to the University of Washington. Together, the two muscles combine to produce most of the range of movement allowed by the upper limbs in the majority of all vertebrates.