Most skeletal muscles in the human body are arranged into pairs of antagonists. The biceps brachii forearm flexor versus the triceps brachii forearm extender is one example of an antagonistic muscle pair. ''Can the biceps brachii ever be considered the antagonist?'' Yes, it can, if the body movement in question is forearm extension, rather than forearm flexion. In that case, the triceps brachii is the agonist, while the biceps brachii is the antagonist. Therefore, the labels, agonist versus antagonist, are all relative. Their use depends upon what particular body movement is being considered at the time.
We can summarize the above information by stating the Yin-Yang Principle of Opposing Muscle Action:
Whenever a particular agonist and its synergist are excited and contract, its antagonist is inhibited and relaxed.
This simultaneous excitation of agonists on the one hand, but inhibition of their antagonists on the other hand, is an impressive feat performed by the nervous system. We know that the Yin-Yang Principle is important, because whenever both an agonist and its antagonist are excited and contract at the same time, they tend to produce a body-lock or rigidity of the affected part. In the case of the forearm, it will tend to lock into mid-position, neither completely flexing, nor completely extending.
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