When I got certified as an instructor, we used a manikin w/a strip that printed out the side. They checked that all your compressions, and breaths were correct. After a few tries, I passed. Then we had 2 person. My partner could NOT get it right. I couldn't pass till she passed. I was determined to not come back another day. So we went for who knows how long. Finally, one of the instructors had mercy on me and partnered w/me to pass. I went home. The next day, I was sore from my neck, wrists, arms, abs, to my knees. And, don't forget my glutes and shoulders. Oh, ouch. It was the next week before I was normal.