A rep is one complete cycle of an exercise movement-a contraction of the muscle followed by an extension-that is, lifting a weight and lowering it again. A set is a group of these repetitions.
How many reps you include in a set depends a great deal on what kind of set you are doing.
For example, both research and experience have shown that bodybuilders get the most results using a weight in each exercise.
That represents about 70 to 75 percent of their one-rep maximum-that is.
The amount of weight they could use doing one full-out repetition of that particular exercise.
If you use this amount of weight you will generally find you can do sets of:
8 to 12 repetitions for upper-body muscles; 12 to 16 repetitions for the major leg muscles.
These figures are just approximations, but they work well as general guidelines.
Why can you do more repetitions for the legs than the upper body?
Simply because the fall off in strength over the course of a set is slower in the legs than in the upper body-upper-body muscles just don’t have the same kind of endurance as leg muscles.
But in both cases the amount of weight used represents the same 70 to 75 percent of the one-rep maximum ability of the muscles involved.
Occasionally, there are reasons for using less weight than this (and therefore doing more reps) and some very useful types of sets which involve heavier weight (and fewer reps).
Such as low-rep sets for maximum strength and power.
But these guidelines represent the majority of training bodybuilders do-and this is especially true for beginners.