Final answer:
The antagonist to the extensor pollicis longus and brevis muscles is the flexor pollicis longus, which performs the opposite motion of thumb flexion.
Step-by-step explanation:
The antagonist to the extensor pollicis longus and brevis muscles is the flexor pollicis longus. These muscles are responsible for the movements of the thumb, allowing for a range of motions. The extensor pollicis longus and brevis extend the thumb, while their antagonists, the flexor muscles, produce the opposite action by flexing the thumb.