It is important for this discussion to define the transition point between “Heavy Mechanism” and “Light Mechanism”. There is a point in the vocal range whereby the voice is dominated by a sensation of “stretching” of “leaning out”. The “two-register model” would have that point at middle C (C4) for men +/- a perfect 4th with very low basses at the lower extreme and the tenorino at the higher extreme. One octave higher for women of comparable voice types. Basses sense a change around G3 (contraltos at G4), Baritones feel a little struggle at Bb3 (mezzos at B4b) while tenors seem to feel it around C4# or D4 (dramatic sopranos at C5# and lyrics at D5). Tenorinos can sense the beginning of instabilities around F4 and even as high as G4 (Very high coloraturas at F5). These points of change, these passaggi, do not take into account voices that are over produced in the low and causing a need for change earlier in the voice.
Indeed the influence of the Thyromuscularis (TM) may be crucial to balance and efficiency of the folds during phonation. Unfortunately the literature on this muscle is at times contradictory. Because TM is a part of the Thyroarytenoid muscle system (TA), it is considered in tandem with its complement Thyrovocalis (vocalis muscle) to thicken and relax the vocal folds. Some believe that the TM contributes to the same action. Others observe a branch of the TM that supports the functions of the Lateral Crico-Arytenoid, which is to adduct and lengthen the vocal folds. It is this latter action that I find interesting. See following sources:
Experts agree that when the folds lengthen, they tend to approximate more because of this additional medial action by Thyromuscularis. Yet the lengthening of folds does not mean that they will be adducted ideally. It seems there is an extra lengthening and adducting that is essentially a function of TM. It is also my belief that the contraction of TM is driven by vowel quality. If the singer intends to produce a vowel quality that includes strong high overtones, the TM probably contracts to create those conditions. TM however is a refining muscle. Its action contributes positively to a situation where there is already a relatively good balance between fold mass (substance) and glottal closure (adduction). The additional contraction of TM (again probably driven by vowel quality) can make all the difference in the acoustic passaggi (where the voice necessitates a change as to which vowel formants will dominate the spectrum of harmonics).
When there is adequate substance and adequate fold closure, the additional stretch and adduction of the TM produces a level of efficiency that results in a source tone richer in high overtones. These overtones can be further enhanced by acoustic adjustments making the entire phonation process less resistent at the glottal juncture.
In plain terms, a voice that is substantial and well adducted will benefit from a brighter concepts of vowels as long as the natural laryngeal depth is not compromised.
Achieving the combination of “open throat“, clean and clear adduction and a brighter concept of the vowel will result in sensations of high harmonics toward “the mask.” If this balance is achieved in the lower range, the transition from low to high (heavy mechanism to light mechanism) becomes fluid and dynamic. A lower voice that does not experience this leanness leads to the necessity of an abrupt change from heavy to light.
It is important to note that this is not about singing “lightly” in the low range. The nature of the low and lower middle ranges is such that sufficient vocal mass is imperative. Within the natural heavier construct of the lower voice there is room for stretching and leaning. Singing too lightly in the low will only force tightening at the glottal juncture, elevating sub-glottal pressure and raising the larynx. It is important to make the difference between the desirable “substantial and lean” lower and lower middle voice to a “light” lower voice lacking in fundamental substance.