“Two sets of muscles in the anatomical structure of the larynx function in stretching the vocal cords and holding them in tension during their vibration – (1) the crico-thyroid group and (2) the arytenoid group. The second group is the heavier and stronger, and functions predominantly when tones of low pitch or loud intensity are being produced. All tones produced under this adjustment of the physical mechanism are identified as tones of Heavy Mechanism. Muscles of the first group (crico-thyroid) are of lighter texture and function predominantly as tensors of the vocal cords when tones of high pitch or soft intensity are being produced. All tones produced under this adjustment are identified as Light Mechanism tones. Note that these terms bear no fixed relationship to pitch range, but are used to designate certain adjustments in the muscular mechanism of the voice-producing organism.

[…] It should be obvious to the thoughtful student that use of the low pitch Heavy Mechanism tone as a practice medium is for the purpose of bringing the stronger group of tensor muscles into action and coordinating a habit that will insure their automatic response so that any given voice may fully achieve its inherent power and range.

[…] During the early stages of practice this Heavy Mechanism tone will probably not have a mellow, pleasant sound as it is carried to higher pitches of the medium range – particularly in the case of the soprano voice. So long as the larynx does not rise from the low position that it always takes when breath is deeply inhaled through a released throat, the tone may be continued with the assurance that its use is contributing to vocal development, even though a vigorous use of energy is required. A tone continued after the larynx definitely rises must inevitably be forced against throat constriction, and prove harmful to the vocalist. The student and the teacher must approach this problem of voice development with discriminating understanding and courage, remembering that no vocalist can harmfully strain his voice mechanism so long as his throat is freely open. Conversely, it must be realized that any tone, loud or soft, made when the throat is constricted, will cause strain and build up interfering muscle-reflexes.”

Wilcox, John C. The living voice: A study guide for song and speech. C. Fischer, inc., 1945.