"Any voluntary or involuntary movement involves an effort. The state in which no effort is made is rest. Rest is something other than relaxation, as in rest all effort is absent, while in the activity of relaxing some effort is present.
In relaxation the degree of exertion contained in the effort might be reduced without, however, abolishing it altogether. Examples of this are the relaxed movements which alternate with movements of greater exertion and create thereby the simplest form of rhythm."
Quote from "Effort" by Laban & F. C. Lawrence, 1947