The tongue consists of 35 muscles, two matched sets and one in the middle. The larynx is suspended from the muscles in the front, under the chin, and attached to the side walls of the throat in the back (the upper constrictors). The “at rest” position of the tongue determines the acoustic possibilities of the shape of the vocal tract as it adjusts into various vowel shapes. While the front of the tongue determines which vowel we hear, the back determines how much the larynx can move and how much the muscles of the soft palate can stretch and lift. This has an effect on overall mobility of the larynx, and of the responsiveness of the laryngeal musculature to the crico-thryoid to stretch and thin the folds to raise pitch.
BUT
Muscles are muscles and they can, over time, stretch and adjust quite a bit. A trained dancer learns, over many years, to get the entire body to do things it doesn’t normally need to do. A ballet dancer, in particular, is doing a great number of things that bodies were never meant to do, but with enough time and attention, do quite well and without constant pain. The stretches that dancers and gymnasts do help give the muscles flexibility. The resistance training gives them strength.
Why should this not be true of the muscles in the throat and mouth that effect the sound? The muscles of the tongue can learn to lift more, stretch more, contract more and move more than they ever need to in normal speech, or even in theatrical speech. The muscles of the face, mouth/lips, and jaw can do the same. Even the vocal folds can learn, over time, to stretch to higher pitches, contract into lower pitches, and close more powerfully providing the sound with more “body” or “fullness” (all of this, of course, taking place indirectly). The muscles of the ribs (intercostals) and the abdominal muscles and, yes, even the diaphragm inside, can get more flexible, stretching further and stronger.
I cannot prove this theory, but I believe, based upon my own singing, that the “at rest” or “default” position of the back of the tongue is paramount in the way the vocal folds can react to the stimulus to make sound. Once these muscles are free to move, independently of the swallowing muscles and of the muscles in the back of the mouth, which takes time, the tongue can rest in the back of the mouth/top of the throat in a number of places and can also make a number of shapes and configurations, also in the back, that change both the feeling of freedom of phonation as well as the stability. There is also a corresponding release of the muscles directly under the jawbone in the front (the genio-glossus and genio-hyoid are included also), that allows the larynx to descend by “hanging” in the throat (which is not the same as having it be parked deliberately in a low position from which it cannot move). Further, the inner muscles of the tongue can contract, giving the back of the tongue a role in shaping the vowel. This shaping can be deleterious or advantageous, depending on the degree of contraction, the kind of sound being made and many other mitigating factors.
All of these changes can be accessed through registration and vowel sound correction but it speeds things up if the person singing can feel the interior changes. In the beginning, this is patently impossible, even in talented singers. But, over time, (over many years, actually) one can develop the capacity to feel things that are not normally felt and the capacity to feel them can be very precise, vivid and deliberate. (You can’t teach someone, however, to do that. It happens on its own over time if you pay attention to what you feel and where you feel it). You can actually learn to “let go” in a way that doesn’t happen at first. This is a kind of bio-feedback between the mind and the body.
Of course, some people who sing develop these capacities on their own and then try to teach them with the idea that they are teaching something that you “just do” and this ties the student up in knots. Teaching a sensation you have as if your students should also have that sensation is pointless. Understanding where you feel something, however, is extremely valuable as long as you understand that your student won’t feel the same thing, even if their vocal behavior exactly replicates yours, for a very long time.
If I can organize the back of my throat, the back of my mouth, the shape and position of my tongue and my soft palate, as well as my jaw opening and mouth/lip position and COUPLE THAT WITH REGISTER BALANCE, I can literally choose almost any sound I want to make and be 98% sure it will come out, before I sing it. Nothing, however, will substitute for register work, as this is how to get to the vocal folds themselves, and you must do that if you are to truly change the output of the mechanism.
Perhaps someday there will be a way to see or measure the individual muscles in the tongue and how they effect the laryngeal and pharyngeal behavior of a singer. Right now the only measures available are invasive or possibly harmful (EKGs and X-rays). Until more is known, this is only my anecdotal experience, but it is not random, I do not believe I am making up delusional theories and I do not believe the effects upon the sound are purely subjective or imaginary. If you relate to any of this, let me know, as I do believe I’m not alone in my perceptions and I would like to hear from others who have similar experiences.