Anything which induces rigidity, however slight, during the act of singing, produces wrong condition, and is fatal not only to good tone, but also to the development of the voice. There are many earnest students who are having trouble with their voices because they fail to perceive the contraction of the throat and the rigidity at the base of the tongue and the pressure on the larynx, which accompany their efforts to sing.

Manchester, Arthur Livingston. Twelve lessons in the fundamentals of voice production. O. Ditson, 1908.