One of the most important and generally overlooked aspects concerning the two vocal registers is that they are antagonistic toward each other. This antagonism remains in operation from the beginning phase of vocal training (when the voice is unstructured), until the final phase of development. It is, therefore, difficult and time-consuming to subjugate the registers to a training program of muscular structuring. The accomplishment is generally known as “harmonizing the registers”. It is this same antagonism which, when properly understood and used to advantage, gives the superior singing instrument its remarkable controls and qualities. These two major muscle systems, in their antagonistic state, strongly resist the singer’s efforts to maintain control over their separate, dynamic actions, denying him his wish to unify all the tones of his singing range. Correct structuring imposes rules upon these two antagonistic muscular systems, so they will come to function as a single unit throughout the vocal range, thereby granting the singer necessary muscular controls over his instrument. In this way, the singing instrument functions as a synergism.
Frisell, Anthony. The Tenor Voice: A Personal Guide to Acquiring a Superior Singing Technique. Branden Publishing Company, 2007.