imo you don't need to do both, this could pre exhaust your muscles which will hinder working sets. Unless preexhaustion is part of your routine, just do a warmup set with 50 percent weight, focus on the negative to get a good stretch and you should be fine. It's also easy for beginners to overstretch which ends up tightening the muscles rather than elasticating them. I used to do it all the time before I learned correctly and it led to serious DOMs as well as a strained abductor which prevented me from running for weeks. Not good. [Edited on 01.03.2013 1:58 PM PST]