Sitting with the Steadicam On
Sitting down with the rig on is definitely possible (and I love doing it when I can), but there are dangers to avoid. The two things to really avoid are sitting down with a too long vest, pinching your pelvis, and sitting down on a vehicle without a lot of prep and safety work.
Pinching yourself at the hip is the real danger. A vest basically becomes a big metal knife that cuts into your pelvis and thighs when you sit down if the front of the vest is long (which it often is for some people). So generally, you have to shorten the vest until the front side of the vest doesn’t cut into you as you sit down. This generally makes the vest less comfortable on the lower back, but the danger is definitely not worth the comfort. A lot of very important blood vessels and nerve pathways flow through your hip joints, and pinching them off can have very serious consequences.
I find when sitting it’s also helpful to be sitting with your leg-to-chest angle as an obtuse angle, never an acute angle. The lower you sit, the shorter the vest must be to not cut in, and the less comfortable the rest of the shot will be (and potentially the more damage you could do to your back). Again, it’s about eliminating pinching and the weight of the rig bearing down on a point on your hip joints or thighs.
The other big danger is sitting in a vehicle - soft mounting. I heard a story first hand of an operator, young and enthusiastic, who soft mounted on the back of a pickup truck on a rough drive. They caused enough damage to their body with the bottom of the vest digging in that they had to be taken to a hospital, and if I’m remembering correctly, they couldn’t walk for a while. Truly scary stuff. Yet, sitting on a vehicle is possible if done safely, and may need to be done for stepping off of a vehicle mid shot. But doing that safely is far more complex than I’d ever want to get into in an article on a website.