To stand still, completely motionless, for any length of time is considerably more difficult and fatiguing than it is to move freely. The opposed muscular system responsible for a condition of body ‘tone’ demands a constant release and pickup of tension to avoid tiring. Engineers acknowledge this principle when constructing a large bridge or building by making allowance for sway and ‘give’ in order to eliminate the strain engendered by absolute rigidity. When Leopold Mozart declared of the vibrato that ‘Nature herself suggested it to man,’ he arrived at the very essence of the phenomenon. For just as the vibrato is ‘natural’ to beautiful tone so, too, is flexibility and ease of execution natural to the vibrator.
Reid, Cornelius L. Bel canto: principles and practices. Coleman-Ross, 1950.