The question of “how many reps should you do?” is very common. This topic (like many others in the fitness industry) is somewhat contentious, but there seems to be some universal recommendations based on a variety of different factors. When choosing your reps or rep ranges, there are several things to consider:
What's Your Goal?
First, it’s important to identify a primary goal. Are you training to get stronger? Get bigger? Build endurance? Depending on your answer here, your desired rep ranges can change.
Strength: When it comes to strength, there should be a high priority on heavier loads. Specifically, spending most of your time in the 1-6 rep range will likely give the most benefit. Since strength in this context refers to how much weight you can move on any particular movement, getting practice lifting heavy and prioritizing “load on the bar” makes sense.
Hypertrophy (muscle growth): If you are instead looking to prioritize muscular hypertrophy (muscle growth) then the recommended rep range actually widens greatly. Anywhere from 6 to 30 reps have been shown to be fairly equivalent in terms of growth (with some caveats ill mention soon). This higher rep range means a focus on lighter to more moderate loads.
Muscular Endurance: If endurance is the goal, then you will definitely want to stick to higher loads (20+ in most cases) as the main goal isn’t to lift heavy, but to improve the work capacity and/or our ability to “flush” metabolites (lactic acid for example) from the particular muscle group.
It’s important to understand that these rep ranges don’t have hard boundaries that cannot be crossed. In other words, your hard set of 4 you did on bench press last week still likely stimulated some muscle growth. Likewise, sets of 8 on squat aren’t going to be useless for improving strength.
What Exercise Are You Performing?
Another thing to consider when deciding on a rep range is the exercise itself. Certain exercises may suit higher reps, others, lower reps. Let’s use back squat as an example. It’s quite rare to see someone program or recommend sets of 20-30 for back squat. Why is that? Well, if you’ve ever tried high rep sets of back squats before, you know the answer to this. It sucks. Even with lighter loads (which is needed in this scenario), back squats demand resources from different parts of the body that aren’t your quads and glutes- the main targets of the exercise. For many, the low back, lungs, or even shoulders/upper back will become the main limiting factor here.
Any time the limiting factor of an exercise is something other than the muscles you’re initially trying to target, then you may want to re-assess your approach. Another example could be bicep curls. Really heavy sets of bicep curls (less than 8 reps or so) can be pretty rough on the elbows for a lot of people. Thus using more moderate or lighter loads on some isolation exercises can be a more sustainable approach for you.
Again, it’s important to know that these are not rules. Can you do sets of 20 on back squat? Sure! Give it a try. You may be an exception and maybe it wont suck for you. Can you program heavy sets of 3-6 on bicep curl and get crazy arm gains? Maybe!
Conclusion
This leads me to my final recommendation, which is to vary your rep ranges over time. Putting yourself in a corner and only doing sets of 3-6 on the same exercise till the end of time usually isn’t a great idea. Repeated exposure to the same type of stimulus over time can cause overuse injuries, plateaus in your progression, and boredom.
Apart from switching exercises in and out occasionally (which I also suggest), sometimes simply varying your rep range, such as going from 10-15 reps on your lat pulldowns to 6-10 reps, can give you the second wind you’ve been looking for. Some amount of trial and error is very important in establishing preferences and determining what rep ranges work best for you, your joints, and your goals.