Uprighting is what human beings are doing as part of all sitting and standing activities. Only when we are fully supported in lying down or reclining are we not uprighting.
When we capture the force of our falling, we activate what I call our innate uprighting system. This makes sitting, standing and moving easy. Here, in a nutshell, is how it works.
When we command ourselves to fall straight down, we end up tipping slightly forward and inwardly (on each foot and each sit bone).
This tipping happens naturally, without the use of muscles. We tip forwards simply because we have a bit more weight in front of our center line than behind it. We tip inwardly because of the shape of the heels and sit bones. Being sensitive to the change in pressure that our falling and tipping creates, we do not tip very far. We stop our tipping near-instantly -- using optimal leverage points where we capture the force of our falling which activates our deepest musculature, beginning with the plantar muscles 
on the bottom of the feet. This kinetic chain of muscles, spanning the full length of the body, lift our bones optimally, segment by segment. This all happens in a flash. With the lifting of the head, we reach full extension. We then fall and tip again.
Innate uprighting is a repetitive cycle of flexing and extending, falling and lifting, of rest and activity. Our very own weight, when well directed, is the source of the energy that lifts us. It is an always available source of energy.