Line of Effort

In Body and Mature Behaviour, Moshe writes about the "fixing" of the trunk for the movement of the limbs--not, as we imagine, the general immobility of "core stability," but finely calibrated to the direction of action, and with the least possible sum total of work in the muscles:

"The trunk by itself is normally not rigid. It consists of two smaller parts, the almost rigid thorax and the pelvis. Thus, before any significant movement can be made, it as necessary that the thorax and pelvis should be more rigidly connected [so that, as a unit, they will be the heavy part and the action of the muscles joining limbs to trunk will move not the trunk but the limbs]. And the stability of the whole body relative to the ground should be increased in the plane in which work is to be done. Among all the numerous possible configurations of the segments of the body in each case there is a group in which the total amount of pull in all the muscles of the body is the smallest." (p. 54, beginning of Chapter 7)

You can think of this lesson as an exploration of that idea.