The Oral Cavity can produce phonemes with a minimal expenditure of movement and energy.
So relatively minimal is the movements for speech, that I have never heard of a single professor coming down with Carpal Tunnel Syndrome of the Tongue.
To the contrary, the tongue is the strongest muscle in the body (inch for inch), and has a large quantity of neural tissue in the brain devoted to it's control.
It can assume a number of different shapes in the oral cavity with a minimum expenditure of energy.
For example it can arch in the front, middle or back; and can at the same time be raised or lowered.
As the air masses around the tongue in the oral cavity are changed (thus changing the natural frequencies) their screening potential for overtones changes. Each change in the pattern of overtones creates a new vowel sound.
Actually, that is only partly true, because we do not really produce discrete phonemes. What do we do?