Economy of movement is achieved when, during a given act, those muscles which have nothing affirmative to contribute to its execution remain passive, while those upon which the burden of effort rightfully belongs become active. When muscles which should be passive become active, interfering tensions are introduced. As a result, the body responds awkwardly and inefficiently. Regardless of the quality of the response made, however, the response itself is the coordinative process. Efficiency of response in all bodily activity depends upon a condition whereby there is little or no muscular interference.
Reid, Cornelius L. The free voice: a guide to natural singing. Joseph Patelson Music House, 1978.