Let me start by saying that your muscles don’t know how to count! What they do know is time and tension. A rep range, allows the muscles to work for approximately 40-60 seconds. This is the time the muscle needs to be under tension to promote change. 40-60 seconds at about 3-5 seconds a rep is equal to 8-12 reps. That’s where these rep range numbers come from. It just makes it easier (and safer) than watching the clock while you lift. So the rep range doesn’t have to be exact. It’s simply a number to use as a lifting guideline.
What is more important is performing quality reps. Take your time and complete reps in a full range of motion (all the way and all the way down) instead of blasting through them just to reach the numbers you are aiming for. Similarly, lifters will stop at 10 reps hoping for strength gains, even though they are not fatigued and can still do 10 more. In this scenario, finish your set until you can’t do anymore and then raise the weight for the next set. There is no point wasting a set because the weight was too light.
The general rule is that you want to pick a weight that will fatigue you before 15 reps. More than 20-25 reps is considered cardio. Lifting a light weight many times over a long period of time will increase your risk of developing a repetitive strain injury.
Typical rep ranges are as follows:
Endurance and tone 15-20 reps
Strength and size 6-12 reps
Power 1-5 reps
There are some exceptions:
Body Weight Exercises
Your lower body for example, is comprised of very large muscles. They are also mostly used for endurance to get your body around all day long. It’s not unheard of to perform well over 20 body weight squats or lunges to hit fatigue. Once you start adding weight I would stick with the 8-15 range. More reps in lower body exercises raises your heart rate. This keeps you warmed up and also promotes fat burn while lifting.
An exercise like push ups should be preformed until fatigue. Some people can do more than 50 push ups and this is absolutely okay. Aside from adding weight on to the back, more reps is push up progression.
Abs should be treated like every other muscle. Would you do 100 reps for your bench press? If you are performing crunches correctly you should not be able to do more than 25. If they are weighted crunches you should not get passed 12-15. Abs also need breaks in between sets, anywhere from 10 seconds – 1 minute depending on the difficultly level.
I always say it but I’ll say it again, weight lifting is most effective when reps, weight and exercises are regularly changed. Change promotes results. You never want to let you body get used to a routine. The human body adapts very quickly and once it knows what comes next it doesn’t have a challenge.