As I mentioned in my first post on weights, I want to write a couple things about static and dynamic balance. They are my own thoughts and have not been documented, so you don't have to believe me. I'll be happy if anybody has any thoughts on this though...
Balance of bodies/objects in motion is different from that of static bodies/objects, because of inertia. My definition of static balance: natural tendency of a body/object to fight gravity by spreading its weight uniformly around a center of gravity (COG). In other words, if we just stand or sit, without moving, the weight is not really affected by inertia, so the body can balance its component parts in such a manner that we have equal weight around the COG (now that's theoretically impossible since we move at all times, so I'm talking about not moving from place to place or making too broad gestures - even though all human activity is virtually placed under 'dynamic balance', the effects of inertia on small movements exist, but are so subtle that we can forget about them, or maybe muscular movement even overrides inertia, so really, forget about them).
Weight is such an elusive concept folks, since it can't be touched or seen, etc, it's more or less... abstract. Weight in animation is nothing but dealing with the component parts in order to balance the whole. Which is heavier? How much 'body matter' we have up front, leaning forward or backwards, are the hands balancing or off-balancing the body, do I have a leg in the air?... All this stuff. It has to be said that raising a leg in the air shifts the weight on it, but it doesn't always happen: if the body leans to effectively counterbalance that raised leg, we still have static balance.
You can picture weight as a plastic bag half filled with water, so we have room to move that water around. The water will always move to stay level, but being fluid... it overshoots the movement. Now, the more movement, the more exaggerated the overshooting. Which takes us to chapter 2 in the book: dynamic balance.
If the body moves, as in a walk, run, jump, etc, the weight has to gain momentum (the water-bag moves ahead but the water stays behind at first, and then gains momentum and gets in line, and keeps level, as long as we keep the bag moving in the same direction), and then its inertia that takes over! If the movement is small, inertia will have little effect, so it doesn't even matter, and the old rule of static balance applies. But as the movement increases, as it becomes broader, inertia becomes pretty much the leader, and IT balances the body. Of course, we still need to use limbs to keep going, squash and stretch, contact the ground, etc. But my opinion is that balancing weight in movement is mainly (if not even 'only') due to inertia. So we fight gravity through constantly thrusting and keeping the energy level.
Take a bike for example: you have a real hard time balancing it while it's stopped, but if you start going, inertia will balance it and you have no problems if you just keep going and keep the direction more or less straight.
Last thing: is a walk balanced by our own body or by inertia? I think it's a mix, but I'm not sure which part plays a bigger role. But definitely inertia is really important: take the 'contact' position for example, the position where 'there is no weight yet' (of course there is weight, it's just that it's :) hovering in the air) - 'there is no weight' because of inertia. That position is absolutely impossible to hold, it only works in movement. So are many many others. I think a run, because of its thrusting that literally throws the weight up (where it hovers for a short while, because of inertia, again) is mainly balanced through inertia.