Gravity tens to be a harsh mistress, once the body is off balance and is not going to be able to recover we do only one thing, Fall.
So why am I teaching you about falling? well if we are not standing in place indefinitely we are all falling be it walking, running, sitting down, non controlled falling (tripping, lose balance) and then controlled falling (dropping to the floor on the knees, planned place to land).
Torso governs movement, the hips control the directional movement of legs and the shoulders control the direction and swing of the arms. The chest governs posture and lean and the different between the sheaths of muscle holding the stomach and the spinal cord and the rib cage which is flexible to a point but is rigid in that if you move, the rib cage holds its shape.
One of our senses is our sense of balance, how we stay up. When our seeing and hearing does not match up, you have problems with balance (people with vertigo have a sensation of the ground getting magnified but their ears telling them they are up higher than what they see, resulting in feeling a little bit dizzy). When our weight cannot stay on balance, we cannot counteract the pull of gravity and we have to fall.
There is a difference between controlled falling and uncontrolled; the brain knows that it is falling it doesn't come as a shock that you are falling. Walking, the body catches itself on the next step. Running the body encounters one foot hitting the ground in the pass through, the body has to stay on balance with one foot.When you need to fall, sitting or other ways (in stunts, martial arts and in acting) you are conscious of not trying to protect the body while falling. You fall the right way, you don't' throw your arms up to catch yourself or towards the ground to brace the impact through your arm. You are limp and relaxed not tense and like a plank.
Next time you are watching a scene in a movie, watch the stuntmen falling/rolling on the floor, also notice the drawn out dying scene and at least once in most of these movies someone sits down in a chair. Notice how people negotiate with their bodies the idea of falling with doing it on purpose. Or you can always notice in the usual funny videos on YouTube for non-controlled falling.
Until, next time happy animating!