I get asked a lot, "how many reps should I be doing per set?", which is a question that can't be answered without me asking a question back.
"What is your training goal?"
The answer to that question is what determines your ideal rep range.
A person training for athletic power will do a certain amount reps which will be different compared to the guy training for muscle gain which is again different from the woman training for strength.
"So what is your training goal?"
Let's go over some common ones.
Muscle size: 6-15 reps
Oh yeahhh! Probably the most common goal in da gym. Getting bigger muscles has been shown to be optimally achieved through quite a large rep range.
6-15! Yeah some good choice here.
Two types of muscle gain can occur in this rep range.
The lower end more likely produces functional muscle gain, which is an increase of the actual muscle fibres inside the muscle.
The higher end causes sarcoplasmic size increase, which is an increase in the fluid inside the muscle. This still makes the muscle look bigger but won't really assist in strength levels.
Increasing the muscle fibres is harder to do then increasing the fluid, so use the full rep range and make gains from both!
Athleticism and power: 1-3 reps
Are you an athlete who needs to develop power? Or a lifter who just like to lift heavy stuff?
Your rep range is going to be 1-3.
You will not be lifting maximal weights here as the goal is explosiveness. You want to accelerate that weight fast. Anything over 3 and you will probably have fatigue from the other reps and slow down.
An argument can be made that whenever your rep speed decreases you are no longer trainer power. Each rep must be EXPLOSIVE and fast.
Endurance: 15+ reps
For those people who enjoy those long punishing runs or spending hours in the pool. Nothing against it, sometimes I get out there and have a go myself.
You want to do 15+ reps per set.
This is going to focus on the endurance aspect of the muscle. It becomes less about powerful muscle contractions and more about how long it can keep contracting before giving up. This includes increases in mitochondria and your muscles buffering capacity - two things you really want for endurance!
It will burn...
Strength: 3-8 reps
Probably the second most popular goal after muscle size, and there is definitely some overlap.
When training for strength you want to be lifting heavy weight with perfect form.
This means you will be sticking within the 3-8 rep range.
Anything more and you probably aren't lifting heavy enough. Anything less would be more recommended to advanced trainees.
Resting at least 2 minutes between sets will ensure you are fresh for each set and are ready to lift some heavy weight. Any fatigue from the last set will impair your ability to move the load and nobody like getting trapped under the bar...
Fat loss: 10-15 reps
More then just your rep range comes into play when developing a program for fat loss. Still, we want to cause a metabolic disturbance in the body so the rep range of 10-15 will be good for that purpose.
Keep rest short and heart rate up!
You also don't need to stick to one rep range. Mix and match goals that are similar to each other. Want more power? Increasing your maximal strength will help with that. Just don't mix completely different goals like endurance and power. Use separate training sessions for that.
BONUS: Optional reading if you're interested in why those rep ranges are best for each category.
Now we understand the basics of rep ranges we can take it to the next level. From the curious people out there, I also get asked,
"WHY am I doing that many reps?"
Lower rep ranges (1-8) are strongly correlated to the neuromuscular system. This includes things like the motor unit innervation of a muscle, the amount of neural drive to the muscle and the firing rate of this neural drive, as well as general coordination of muscle firing patterns (aka form).
Higher rep ranges (8+) become less about how strong your muscles can contract and more about how long they can keep contracting before giving up. Here, we are attempting to cause damage to the muscle, pushing it out of homeostasis and forcing an adaptation (that being through myofibrillar or sarcoplasmic hypertrophy).
When you pass about 15 reps you begin training other aspects of the muscle. This includes increasing the mitochondria count and the muscles buffering capacity (lactic acid clearing) as well as just the muscles general ability to work under continuous contractions.
Note: there is no immediate switch from one to the next (like as soon as you hit 15 reps it is no longer muscle gain, it is now endurance training). Think of it as a continuum. When you hit 15, it's just a little less muscle gain and a little more endurance.