Animation, just like real life, is a continuous squash and stretch (s&s). Is there anything outside s&s? This is the question I'm asking myself right now, so here's what I'm thinking...
Squash is tension. Stretch is tension. Life is a struggle between tensions, a continuous activity, like that of a piston, up and down, on and on. Until we die and there's complete relaxation (we become minerals). So while we're not minerals yet... we s&s continuously. The question comes, naturally: do we ever relax? What about when we're sleeping? What is relaxation? Isn't relaxation a squash or stretch too? But it's not tensioned though.
So both s&s can be tensioned or relaxed states. But when we do something we fade from tensioned squash into tensioned stretch, continuously. And I think there is no relaxed moment until we lie down and just... do nothing. So, can you relax into a squash? I think you definitely can. I have 2 cats at home: Dixi & Moti. When Dixi is relaxed he relaxes into a squash, something like this (I can't find a better image, I'd have to go to Brasov to take pictures or browse through my parent's photos):
You see, Dixi is an introvert (he's 17 years old btw, but he's always been like this, Stinky-Pete-old-cat). So for him squashing is a state of mind :)
Here's a rare view, I guess - Moti sleeping is almost always exaggeratedly stretched, insanely so (oh, no, he's not stretching, he's just sleeping, I'm not kidding you - if you don't believe me just ask for a video):
Moti is an extrovert, a real nuclear missile, he's always up to some nastiness, his state of mind is totally different. Believe it or not, these 2 are my cats, and they just happen to be perfect illustrators of relaxed s&s. :D
Humans too... can s&s in relaxation. But I'm more interested in tensioned s&s. A walk is a subtle s&s, a run is a more pronounced s&s (the 'down' position being a squash and the 'up' a stretch). There are actions where the entire body goes into a squash to prepare the action and then into a stretch to... make it happen. Like the walk or run, or the famous baseball pitcher throwing the ball, or the golf player, or you name it. But you can break it down to small actions too, at 'component level'. The same piston-work of the s&s can be found in fingers, hand, arm, etc, for any/every action imaginable. A bit different when you get hit by a Lamborghini. :) If there's an external force at work then the body doesn't run the game anymore, it's the external force that creates the tension. Still, even in that case, all action is nothing but s&s. Like with the Lamborghini - it hits you, you fold into a squash and then you fall/fade into a stretch. It's like with relaxed s&s - if you faint, and therefore no longer control your muscles, all the s&s happens nevertheless.
S&s is like anticipation & action. The anticipation/squash explodes into action, or, as I like to see it: the squash fades into a stretch (it can be a quick or a slow fade, the idea of fade is just that it goes gradually, so there's nothing else BUT s&s at all times: more squash, less stretch, more stretch, less squash. So, in a... limited sense, there is nothing outside s&s...). The reaction is just a prolonging of the stretch OR coming back, fading back into squash. The tension of the squash gives energy to the stretch, the tension of the stretch gives energy to the squash - elasticity. Muscles indeed are the true engine, but their work, their influence on the body can be seen as s&s.
As Milt Kahl was saying (see my previous post), you need to understand where the weight is, where the muscular effort is. But why? My answer: so that you can build an action correctly, so you know what to squash and what to stretch, and how to time that action. Hallelujah, brothers and sisters, squash and stretch yourselves!!!