The terms, tension, rigidity, interference, resistance, all mean essentially the same thing. They mean the various forms of contraction in the vocal instrument which prevents its involuntary action. If we follow these things back far enough we shall find that they all have their origin in some degree of fear. This fear, of which anxiety is a mild form, begins to show itself whenever the singer attempts tones above the compass of his speaking voice. Here is undeveloped territory. The tone lacks power, quality and freedom, and as power is what the untrained singer always seeks first, he begins to force it. In a short time he has a rigid throat, and the longer he sings the more rigid it becomes. By the time he decides to go to a teacher his voice is in such a condition that he must take his upper tones with a thick, throaty quality or with a light falsetto. Among female voices I have seen many that could sing nothing but a full tone in the upper register, and that only with an unsteady, unsympathetic quality.
Now a point upon which all voice teachers can agree is that the upper voice is not properly trained until it has a perfect messa di voce, that is, until the singer can swell the tone from the lightest pianissimo to full voice and return, on any tone in his compass, without a break and without sacrificing the pure singing quality. How shall this be accomplished? If the singer is forcing the upper voice it is safe to say in the beginning that it never can be done by practicing with full voice. Such practice will only fasten the habit of resistance more firmly upon the singer. To argue in the affirmative is equivalent to saying that the continued practice of a bad tone will eventually produce a good tone.
Clippinger, David Alva. The head voice and other problems: Practical talks on singing. Oliver Ditson Company, 1917.