Sets and Reps

Why is 3 sets of 8-12 reps usually recommended for lifting routines?

What if I stick to a particular weight, and instead, do 3 sets of 20 reps at that weight? Is that bad?

It depends on what you are trying to accomplish with your workout.

Sets and reps are just ways of organizing the volume and intensity of
your workout to meet your goals.

Three sets of 8-12 reps are almost always recommended for beginners
trying to improve body comp and strength. It's pretty good advice for a

3 sets of 20 is more of an endurance protocol. You are less likely to
make size/strength gains, but if you a beginner it is still possible.

This is really not an 'either, or' situation, to make progress you will have
to plan variations in the loading and volume of work. You can't just use
the same weights and reps all the time.

toddseney and HarryLime are correct. But to add a little bit of elaboration:

As a rule of thumb you can think in these terms:

1-7 reps - focus on strength gain throug neuromuscular adaption, possibly some hypertrophy (gain in muscle mass), little effect on endurance

8-12 reps - focus on mass gain through hypertrophy, will gain strength as well, some limited benfit for muscular endurance

12+ reps - focus on muscular endurance, limited mass or strength effect.

Of course in each case, the assumption is that the weight you're doing is the % of your 1RM that is appropriate for that rep range. Ie, doing 1-2 reps with a weight that you could do for 30 reps, isn't a strength protocol, just because you only did 1-2 reps.

So why is something like 3 sets of 8-12 recommended so often? Like HarryLime said, it's a good general purpose routine, works well for beginners, it's simple, easy to remember, etc. And it's not just for beginners... 8-12 reps is still the (rule of thumb) "mass range" even if you've been lifting a long time... but you have to keep the Progressive Overload principle working and keep adding weight as you get stronger.

Dont forget the rests between sets. Doing 8-12 reps for hypertrophy won't help you if you're resting 2-5 minutes between sets. 90-60 seconds between sets has given me great hypertrophy gains.

However, I've found that switching the regimen every 2 weeks from a 5x8-12 to a 8x3 (120 seconds between sets) scheme has been extremely beneficial in terms of strength and mass gains.

Total amout of reps per exercise/bodypart are a lot more important than reps per set.

Doing 10 sets of 3 reps allows you to use much more weight compared to 3 set of 10 reps.

toolin-- by "occluding" do you mean isolating a muscle, or constricting it or something? I've only seen that word in reference to constricted veins/arteries/lung passages.

Found a link, and I'm starting a new thread for occlusion training.