I once listened to a talk by a vocal coach (Peter Trump - who had performed baritone at La Scala and the Met) who claimed that the secret to a powerful singing or speaking voice had a lot to do with balance. I remember old records of "Sing Along With Mitch" (Mitch Miller) where the men stood in a semi-circle looking almost ape-like balanced nearly on the tips of the toes that suggested the Trump method. of course, once learned the principle may be performed more gracefully as in opera, where acting is also required.
Basically, when your body is well balanced while standing on your feet, all your muscles can relax to the greatest extent possible, because they are not needed to maintain balance. This will include muscles in or near the throat that can constrict free passage of air.
Mr. Trump explained that singers who break wine glasses with the power of their voices do not need to use great force. They select the pitch at which the glass will resonate the add energy to the glass by singing that exact pitch (like pushing a swing higher and higher until it goes over the bar!). But he indicated that you could place a lit candle between the singer's lips and the glass and it would not flicker. By properly balancing and relaxing the throat muscles, a column of resonating air is produced that reaches from the glass back to the lips through the oral cavity and down the windpipe past the vocal cords. Strained muscles would pinch off the air column and cause vibrations to weaken and vary.
Interestingly, he also coached brass players who could produce better tones if their instruments participated in a resonating air column that reached both sides of the mouthpiece, just like a good singer. Of course the brass players had to learn how to balance sitting down, using good posture at all times. Hope that helps. Good luck.
BTW, good speakers (and singers) think "speak through the mask" where they try to project their voice as though their face were a mask, allowing their voice to resonate behind it including perhaps just enough nasal resonance to make the voice pleasant and not distorted, strained or harsh. The voice is also controlled for speaking and singing by raising and lowering the diaphragm rather than expanding or contracting the rib cage; which would produce muscular strain.