I have used them for a good period. I would say anywhere from 90# for speed and 270# for strength. If it's a prowler, grab the handles on the bottom of the other side and push low on the way back. Otherwise, attach a harness to it to pull it backwards facing the sled. You can also attach a fastrope to it and do rows or a Y-strap and do chest presses. The best part of a sled is the lack of eccentric loading so you can hit it hard pretty often and not beat yourself up.