I recently had a whole battery of anorectal function tests.
No prep was needed. I lay on my left hand side with my knees drawn up, wearing a gown and still my top half clothing.
The doctor first did a DRE, and asked me to 'squeeze' and 'push' her finger, which I'd then repeat on the manometry catheter.
Then she inserted the manometry catheter, and taped it in place. She taped two very small electrodes to either side of my hole, which took quite a while to get right with her spending quite a bit of time repositioning them, all while I had the tube coming out of me. Then on command I had to squeeze, push or cough, and the graph showed the pressure along the length of the catheter, against time.
Then a balloon on the end of the manometry catheter was slowly inflated and I had to say when I could feel it. The balloon then got deflated and the catheter and anal electrodes removed.
A different wire was inserted that gave out small electric shocks, and I had to say where I could feel them.
Then I had an ultrasound, the probe was covered with a condom and only inserted in and out a short distance.
Then I had another catheter inserted, which inflated on both sides of my hole, and I had some more coughing and squeezing.
Then a different catheter with a scratchy balloon inserted, and the doctor also put in her finger to manipulate it into the right place. The balloon then got inflated until it felt really big and uncomfortable - astonishingly like I really, really needed to pass a BM - but that feeling instantly vanished the second it got deflated.
Finally I had a small catheter with a balloon attached inserted, the balloon inflated, and then had to walk through to the bathroom and expell it, while the doctor timed.
So so many different tubes up my ass hah... Only my backside was ever exposed