there are only 2 variables that affect muscular strain, namely load and speed of contraction. slower reps are counterproductive, because as you purposely slow your reps down you have to also lower the weight (load) to be able to continue. basically, muscles will either a)grow, or b)shrink. thats about it, unless you consider flexibility or nueromuscular efficiency. longer sets of slower reps do not give you "leaner" or longer muscles. they are just less efficient at building muscle, so as you lean out from diet/cardio you tend to notice your nice lean (small) muscles coming out.