How reps should be done and how many of them should be performed has already been clarified on other pages. What exactly one should achieve during the course of a set -again - shouldn’t be a mystery to anyone. If it is to you, go to “reps” and “training to fatigue” to read up on it.
Now it’s time to go one step further and look into sets. How many of them are needed and generally, what an exercise should be built up of.
It is a generally accepted truth that if one manages to hit exactly the right amount of weight (again, around 70% of one’s one rep max) then four sets are enough to fully stimulate the target muscle-group. Doing more than four sets can be beneficial - specially for larger muscle-groups that need a more detailed approach- or you may risk to overtrain - in the case of smaller groups that have a slower recuperation rate.
There is an obvious difference in how many sets small and big muscle-groups can and should take.
Let’s look at the leg. The thigh is made up quite a few muscles grouped together. In this case one needs a few sets for every major muscle involved like the quadriceps muscles and the adductors. For a complete workout one has to hit all specific areas with power sets. In the case of the biceps, four sets are just about enough as it’s basically a small muscle.
Despite this fact, the biceps possess the fastest regeneration rate of all the muscles, so for best results one should hit it with a few more sets as it can well and truly take them.
The calf-muscles are again a small muscle-group, however, designed by mother-nature to do endless sets and reps all day, they’re hard to work out efficiently unless using supplementary sets again.
The rear deltoid is again a very small muscle-group compared - let’s say - to the back, and it doesn’t have a particularly efficient regeneration rate either.
Hitting this area with 4 sets should be well enough. Generally speaking, the bodyparts that we use all day - like the legs - are less responsive to stimulation thus needing a greater number of sets to shock into growth. Large muscle-groups need more sets because of their complexity. Small muscle-groups that aren’t a subject to everyday wear and tear respond a lot better to training thus needing fewer sets to stimulate.