Will a certain rep scheme build more muscle? Burn more fat? Create robotic-like endurance? Is a 3 rep program for no-necks, 10 reps for beefcakes, and 30 reps for human battery packs?
Years ago, in personal training school (which, years ago, could either be a deep study of bodybuilder magazines, or several years of college) we were convinced by the protein gods that sets of certain rep ranges directly correlated to very specific muscle trajectories. The complicated scientific protocol was always some version of this formula:
1-3 reps for strongs
8-12 reps for bigness
15-20 reps for less fat/more endurance.
(We dared not talk about 4, or 7, or 22 reps under penalty of humiliation for asking stupid questions. I mean really. Who does four reps of anything? Jeez.)
So athletes looking for Strength, whatever that was, would keep their reps low. Once a set broke the 3 rep range, forget about it. You're just building purposeless muscle with no ability. Meanwhile Bodybuilders (and basic fitness enthusiasts) should hone their reps to that middle range, where the prettiest, size-iest muscles are, strength be damned. Any more and you'd be going FULL CATABOLIC, each extra rep eating away all that hard-earned size you've been cultivating so eagerly in a tortured-but-shallow-artist way that could fill a therapist's notebook in just a session or two. And heck, if you ran marathons and stuff, 15-20 reps was some magical sweet spot that built mostly your heart, I guess. Also, the fat burning zone, or some shit.
And this was the way it was.
Then strength sport athletes ended up getting leaner, even though sets over 5 reps were practically considered cardio and mostly avoided. And on the other end, crossfitters were getting bigger, even with their rep ranges in the lung-searing hundreds.
And bodybuilders kinda just stayed bodybuilders. But we'll get back to that.
Let's start with muscle size. First, to create substantial muscle gain (size-wise), you'd either have to be genetically prone to it, eating a ton of calories and protein, taking the right drugs, or some combination thereof. If only it were as simple as just a rep scheme.
This also goes for fat loss... just flip that calorie formula from "a ton of" to "less."
What about Strength and Endurance, those classic keywords that would go on to become overused hashtags? I've written entire books about how we, as a culture, as individuals, as trainers, as a fitness industry, failed the very first task of using those words in the first place. We didn't define them.
We commonly agreed on some vague understanding of what those words would represent. That really helped in building naive formulas to use on personal trainer exams, and it created a lot of dialog amongst workout folks that sounded smart but meant nothing.
A fun example of the blurred-definition game that you could play at home would be to drop "cardio" into a regular conversation and step back to observe how everyone can have an idea of what it is, but no one has any idea of what it actually is.
Now try it with the word "christian." See? Words are fun.
So how many reps.? Why can't this part be simple?! I mean how cool would it be if muscles knew math?
Well they're even smarter than that. They know so many of the sciences and they respond to stimuli with a far greater intelligence than our yearn for child-like arithmetic. But rep schemes do have purpose, even the famous bodybuilder style, and that purpose is teaching the body how to turn on those muscles for different scenarios. But the numbers alone mean very little. It's the application of multiple factors during, and after, those reps. If we treat the muscle system like a city's power grid, then we can see how the body needs to access power in spectrums, turning on just enough to cover what's needed at the moment. If a neighborhood has a stadium, then certain times of the week, that power grid has to cultivate more juice for certain nights. If the city is lightly industrial, it has to produce a slightly higher than average power output for sustained periods of time.
That would be the difference between, say, a powerlifter (the stadium) and an endurance athlete (the industrial neighborhood). But here's where it gets tricky in the training, and it's what those particular athletes rarely want to admit.
Training a spectrum of strength, or a variety of those power output scenarios, helps specific strength. While training JUST the part of the spectrum you use the most can actually impede progress. For instance a marathon runner should sprint and a powerlifter should learn to count to 10 sometimes (like a bodybuilder!).
The body does not automatically understand the spectrum of strength when specifically trained for one part of it. In other words, chances are your name brand workout will not prepare you for much beyond the limitations of that label.
BUT understanding all of the spectrum to some degree will absolutely support the specificity in ways that I won't go into now. That's a lie. Yes I will. When the power grid is simply supplying low levels of output for long periods of time, it very well might not be ready for when a surprise high level is needed. It would fry the grid. Training the grid to live at the high level for small periods will teach it to recover quickly and not completely collapse. And vice versa. To be able to train those high levels bursts regularly, the system needs to have a higher baseline to recover quickly from those bursts. In other words, are low reps better than high reps? Yes. Just like high reps are better than low reps. In my extended programs, I cycle the rep scheme. Initially higher reps for some weeks, then every cycle we drop the reps and up the intensity in other ways. Because that's the goal, keep the intensity high, but through different variables, or manipulatable factors, as I call them. And there's a bunch of them... speed, volume, load, duration, movement selection, and plenty more. So whether your rep scheme is 3 or 30, the biggest factor is what I call the 3 Ps.
Be Precise.
Be Present.
Be Purposeful.
Then a 5 rep set and a 15 rep set might actually have very similar outcomes...
Ya get stronger. Especially when you take the time to define what strength really is. I wrote two books about that. Get reading! So what's the right rep scheme? All of them. At some point. As long as the three Ps are there. I realize this in no way answers your questions about reps, but it actually does. By creating a template to ask more questions. Sorry, and you're welcome.