Critic Henry Pleasants has stated that it was a loss of physical strength and breath-support that led to Callas's vocal problems, saying,Singing, and especially opera singing, requires physical strength. Without it, the singer's respiratory functions can no longer support the steady emissions of breath essential to sustaining the production of focused tone. The breath escapes, but it is no longer the power behind the tone, or is only partially and intermittently. The result is a breathy sound—tolerable but hardly beautiful—when the singer sings lightly, and a voice spread and squally when under pressure.