If I hold a twenty pound weight, I cannot detect a fly landing on it because the least detectable difference in the stimulus is half a pound. On the other hand, if i hold a feather, a fly landing on it makes a great difference. Obviously then, in order to be able to tell the differences in exertion one must first reduce the exertion. Finer and finer performance is possible only if the sensitivity, that is, the ability to feel the difference is improved.
In order to recognize small changes in effort, the effort itself must first be reduced. More delicate and improved control of movement is possible only through the increase of sensitivity, through a greater ability to sense differences.
We have more experience of movement and more capacity for it than of feeling and thought... We know much more about movement than we do about anger, love, envy or even thought. It is relatively easy to learn to recognize the quality of movement than the quality of other factors.