Some info for you
I am in medical school and had this conversation with my physiology professor who has forgotten more human physiology than most of us will ever know. I can't say for sure that he is correct but I also read it in a few journals so here is what he told me if anyone is interested.
I asked him if I did slower reps would I build muscle faster. This is what he told me. It the short and sweet version.
He said your muscles build up when you are lifting weights because you are actually tearing them when you lift weights, then when you rest and eat properly they rebuild and get bigger. (duh) but here is what I was interested in hearing. He went on to say that you aren't really tearing the muscle when you are contracting it, almost all of the tearing goes on when you are extending the muscle ( the negative part of the rep) so extending the muscle slowly with a heavy weight will give you the best results if you are just looking to tear the muscle so that you can rebuild it for size gains.
So I went to the gym and used a little less weight than normal (becuse I had to) and did counted to six or eight during the negative portion of the rep and my muscles were burning like hell and I actually got a good pump from it. But the next day and the day after I was sore as fuck. Now maybe it was from changing the exercise or maybe I tore some more muscle, either way I felt like I was getting a better workout. I also gained 5 pounds that month.
Anyway just my experience thought I might share it. Anyone else have input about this?