When you approach an instructor and ask, “How many reps should there be in a set if I want to increase muscle strength?” The answer may vary: 6-8 / 10-12 or something else. Where are these numbers taken from?

The muscles do not count the number of repetitions. The relevant parameter is the duration of the set.

How long should the series be?

The answer to this question depends on what your goal is. Let’s take a look at the most common target: micro-injuries (according to the theory of muscle damage, the destruction of old fibers to obtain stronger new ones).

One of the most important results of the model used is: micro-injuries to muscle tissues are created when creatine phosphate stores are depleted and the rate of ATP recreation (using lactic and lactic anaerobic energy systems) is less than its speed of consumption.
Therefore, we must look for time limits that satisfy the aforementioned condition.
You probably know that there are two main types of muscle fibers: red (slow twitch) and white (fast twitch). We will look for the conditions according to the type of muscle:

o Within the fast twitch fibers (white) after 7-10 seconds of peak intensity work, the creatine level drops to half its original level. After 30-40 seconds, the anaerobic lactic energy system is in its high production, in addition, the acidic products of metabolism accumulate within the muscle. Therefore, our limits are 7 to 40 seconds. Probably the optimal duration of the series is somewhere in the middle of the mentioned interval and its 20-30 seconds.

o After 60-120 seconds of setting duration, the alactic anaerobic energy system (slow twitch fibers) reaches its high production. Note that when the stress is above 30% of the maximum, the blood circulation almost stops (it hinders the transport of oxygen, necessary for the alactic energy system). Therefore, the set duration must be between 60 and 90 seconds.

A simple rule of thumb that is correct for many people: upper body muscles consist mainly of white fibers and lower body muscles consist of red fibers.
There are also other tests to determine the ratio of white / red fibers in your muscles.
General principle that you must take into account: if you want a maximum advance you must train both types of fibers.

The exercise range is also something to consider. The higher the range, the longer it will take you to do each rep (unless you maintain a constant rep length, for example 24). In 20-30 seconds you will probably do 6-8 squats, 8-10 bench press reps, and 10-15 calf raises.

As a conclusion to this section I would like to bring the following example: in training a you have completed 7 repetitions in 30 seconds. In the next workout, you have finished 8 reps in 28 seconds (with the same weight). Some will say that you have advanced. Instead of 7 reps, 8 were completed, but in fact the duration of the set decreased by 2 seconds (intensity decreased) and that’s what matters.