Which muscle acts as the antagonist to the biceps brachii?

The triceps brachii is considered the antagonist to the biceps brachii because it performs the opposite action to the biceps during arm movements. When the biceps brachii contracts to flex the elbow, the triceps brachii must relax to allow this movement to occur. Conversely, when the triceps brachii contracts, it extends the elbow, which involves opposing the action of the biceps. This antagonistic relationship is a fundamental principle in muscle function, where opposing muscle groups work together to create balanced movement and control at joints.

The other options do not directly oppose the biceps brachii's function at the elbow. The deltoids are primarily involved in shoulder movements, while the erector spinae and rectus abdominis are core stabilizers that influence trunk posture and movement, rather than elbow flexion or extension. Thus, the triceps brachii's role as the primary muscle that opposes the actions of the biceps brachii makes it the correct answer in this context.