Let’s keep this extremely simple: once you are able to complete all sets of a specific exercise with a certain weight, if for some reason you decide to keep that weight constant without ever increasing it in your future workouts, then the target muscle will simply not grow any further. Muscle growth happens when said muscle needs to adapt to a bigger demand that what it can currently meet, hence when the “bigger demand that what it can currently meet” goes missing, muscle growth cannot happen. Simple as that. The muscle may become more defined, if proceed to lose fat by either reducing calories intake and/or burning more calories, but it doesn’t grow any further in terms of mass.
When we consider exercises in isolation, it’s really as simple as that. And for the first exercise of each of your sessions, this concept applies without the need for any additional considerations. Let’s look again at the Upper Body A session from the Muscle Building Workout Routine by Jay (Links page), where the first exercise is the Bench Press:
W = warm-up
S = sets
M = target movements (reps)
R = rest
Jay explains that when on your first set you manage to do the target number of reps, and then in the remaining sets your reps fall within the target interval, while always maintaining proper technique, then you are ready to increment the weight by the smallest possible increment for that specific exercise in your next workout: this is progressive overload. In the case of the Bench Press, the target number of reps is 8, and the target interval is between 6 and 8. If with a certain weight you manage to do all 8 reps in the first set, and then in the remaining two sets your reps are in the 6-8 interval, then you are good to go, you can increment weight next time.
Progressive overload is required for muscle growth. Let’s assume that when you joined the gym you started bench pressing with very low weight, and then you incremented it progressively up to your body weight: well done! Now you can do 8 full reps in all three sets with your body weight, maintaining proper technique at all times. Let’s further assume that you maintain the weight constant for two or three Upper Body A workouts in a row, since bench pressing your body weight was your initial objective when you joined the gym, and now you have reached it. That’s fine, but you need to be aware of the fact that, if don’t increase the weight any longer, your chest won’t get any bigger , since the muscle is already big enough to bench press your body weight, and the only way to grow bigger it by having the need to adapt to a bigger demand. Since you want your chest to keep growing, you proceed to add the smallest possible weight increment, which usually means adding one 1.25kg plate on each side of the barbell.
The first time you bench press with this higher weight, it’s possible that you do all 8 reps in the first set, 6 reps in the second, and 3 reps in the third: in this hypothetical scenario, you are not ready yet to increase the weight again. The second time you bench press with the higher weight, you may do all 8 reps in the first set, 7 reps in the second, and 6 reps in the third: assuming you always maintained proper technique, you are now ready to increment the weight again, and you proceed to add another 1.25kg on each side of the barbell. Rinse and repeat, rinse and repeat. Obviously, this process cannot continue forever, since otherwise you will be lifting tons eventually. At some point you may need to add even smaller increments than that, and eventually you will reach your genetic limit. Eventually. I’d say most guys are far away from that point though.
The smallest possible increment is fixed and depends on the equipment of your gym. Still, for exercises involving big muscles (e.g. incline leg press) you may want to increment more than the smallest possible increment, since an increment of 1.25kg is a bit small in the case. Dumbbells have the opposite problem though. Most gyms have dumbbells going up one kilo by one kilo up to 10kg, then two kilos by two kilos up to 30kg, etc. Fact is, the absolute increase it takes to go from a 10kg dumbbell to a 12kg dumbbell is the same it takes to go from a 28kg dumbbell to a 30kg dumbbell, it’s two kilos. But there is a hell of a lot of difference in terms of extra demand on the muscle in these two scenarios: the extra two kilos represent a 20% weight increase when you were previously using the 10kg dumbbell, but only 7% when you were previously using the 28kg dumbbell. A hell of a lot of difference indeed. So, what do you do if you want to increment the weight by less than 20% in one go? This is what I did:
I bought two half-kilo plates, and two one-kilo plates, and created eight sections of rope to tie them to one side of each dumbbell. Say I want to increment the 10kg dumbbell by less than two kilos, I can add 0.5kg to each dumbbell the first time, 1kg the second time, 1.5kg the third time, and then I can finally move to the 12kg dumbbell. The weight increment is much more manageable like that. The plates should not go anywhere since each one is tied to the dumbbell through four different ropes, but for extra safety, I always added the plate to the external side of the dumbbell, which was pretty much never on top of me during the sets (I’m thinking mainly of the dumbbell press, incline dumbbell press, dumbbell shoulder press). You can also decide to increment one kilo by one kilo, skipping the halves, up to you. And if you decide to go ahead with this workaround, just make sure you know how to tie knots which won’t untie themselves during the sets :)
Before we were talking about the ideal scenario of exercises in isolation, and the first exercise of each workout in an example of that. Then, for the exercises in the middle and at the end of the workout routine, it’s gets a little trickier, since you may have already increased the weight for one or multiple exercises beforehand. That’s when you listen to your body, and decide whether it’s appropriate to increase the weight there as well, or whether it’s better to just aim at maintaining the weight from the previous workout, which may be challenging already.
The beautiful thing of the beginner’s phase is that you keep increasing weight constantly, session after session, since you cleverly decided to start with very low weights and then increment them gently, in order not to injure yourself. When, after a few months of regular training, you move on to the intermediate level, you find yourself needing a couple of attempts or more before managing to increase the weight. And that’s fine. These days I like it to “confirm” the weight two or three times before increasing it, so as to make sure that I maintain proper technique at all times. Consider this: this time around you have increased the weight, and you barely managed to stay within the target reps interval in all sets. Actually, you cheated a little bit on the final two movements of the last set, just to have the movements considered as “done.” You consider the range achieved in all sets, and you make a note on your paper to increase weight next time. Next time you proceed to increase weight as per the plan, and it’s only natural for you to lose proper technique now, since in the previous workout you had to cheat a little to stay within the target reps interval, and now you are using an even heavier weight. This is why I now like to confirm the weight two or three times, before adding the smallest possible weight increment: like that I maintain proper technique at all times, and I won’t take the risk of running out of energy, of not being able to position the barbell back while doing the bench press, and of needing to ask for some help to get all that weight off my chest.
 In theory one could keep the weight constant in the initial exercises of the workout routine, and seek muscle growth by increasing the weight in the middle and final exercises, but that’s an inefficient way of doing things: the workout routine is designed like that for a reason, and for more info please head over to Jay’s “A Workout Routine” .com (Links page).