[mralabama]

To do it correctly you are really supposed to do the exercise to "failure". I know for myself, when I was actively lifting doing machines, I did 2 sets of 15 reps. Once I could do all of the reps in both sets maintaining proper form and timing (i.e. using a slow count like 1-2-3 then hold and back down - something I still have problems with some times ) twice, I moved to the next weight level. You don't have to feel much if any discomfort and probably won't unless you are really straining. It might be that for some of those exercises you are right at the borderline of moving to the next weight level. I noticed that only when I moved to a new weight level where I couldn't do all of my reps did I notice discomfort. Otherwise I just felt muscle fatigue after the workout.