I use to do that. One of my mishaps left a mark on the heater duct in the shop in Germany. That was in 1971 and I was there for a visit and to show my wife where I use to work and heater duct still is dented in where my knees hit. This heater duct is around 10 foot off the ground. We were tring to get a bolt out on an entrenching machine and I was on the fender on my knees and the other guy was on the other side with a breaker and bar and cheater pipe. I was pulling and he was pushing well the dam 3/4 inch bolt broke. He caaught himself and I did a somesault off the fender into the heater duct and fell 8 foot to the floor. Another guy happened to walking thru and he caught me and broke my fall which probably saved me from breaking my back. As it was I screwed up my knees and spent 3 day in Landstul hospital. The good part is I got put on bed rest for a month so I got to sit on my balcony at my apartment drink beer and talk to the German ladies walking by