I was asked to look at this "opening the joints" model in the course of another discussion on the learning of natural movement in aikido. Working toward a mechanical interpretation of some aikido principles, I have placed what Gernot describes as the "opening of joints" and extension in all directions in the context of a catenary (hanging chain) force profile.
I have also discussed some other thoughts on the issues of ki/kokyu training, aikido principles and and the mechanics of it all.
It is here for those who care to look:
The shape of one half of a catenary or inverted catenary arch is basically that of tegatana. The involute of a catenary is a tractrix, which has basically the shape of two tegatana meeting in shomenuchi. Both can be seen here :
One would know if a catenary profile is achieved by the sense of equal linear forces (compression/tension) along the the limb running across the joint and evenly distributed radially around the joint, This would mean that the line of thrust or tension is running within the middle-third of the arm's cross-section. If it runs closer to the surface there would be a sense of differential across the joint on one side or the other (a hinging pressure) that one could "open" to even out and put the line of force back into the center of the limb.