This is a very very important question and will impact your workouts and development on a great scale. As I mentioned in this* blog, your muscles grow when you have worked a muscle and exhausted it more than it used to be, and then let it rest to recover. If you give your muscles an easy job, it has no purpose or reason to get stronger(bigger).
Let’s take an example;
To get to your new job and your office, you’ll have to walk 15 stairs. The first day will get your muscles a little bit exhausted and if you are not used to workout or walk in stairs, there is a chance that you will get sore. Now, you do this everyday and after time those stairs will be nothing for you. You have become stronger. And your muscles have grown a little little bit. What happened? Well, you forced your body to adapt to your new resistance. And it forces your muscles to get stronger.
Let’s take a new example. This is the first time you do squats and you use 30 kg on your back. You can barely do 12 reps, so in set 2 you only do 10, and the 3rd you do 8 reps. The next day, since you are a beginner, you will get sore, since your body is not used to this heavy resistance. When it comes to weightlifting, you often follow a structure, or you should do. So, after 3 weeks, you can do 20 reps, 8 more than when you started. As in the “stair example” you have become stronger. Your body adapted to the 30 kg. Here is the key: If you don’t adapt the resistance(weight) all the time, your body and strength will remain the same. Since you don’t give it a reason to grow. If you stay on the same weight for months, your muscles will remain the same.
So, how do you pick the correct weight?
Well, it’s very easy and after a couple of weeks you will be able to pick a weight very easily by experience. Let’s say my workout schedule tells you that you should do 12 reps. Pick a weight, let’s say 50 kg. And you do 16 reps. This is too easy for you right, since your goal is to do 12 reps. So, now you have to increase the weight a bit. You add 10%. Let’s try 55 kg. You came to 9 reps and weren’t able to do 12. This means that this weight is too heavy for you. So, you try 52,5 kg and now you can do your 12 reps. This is how you do, and in the beginning it might sound tricky but in a couple of weeks you will not need to do these steps since you have learned which resistance is perfect for you.
I choose to mix my rep-range between 5 up to 20 reps. Why? The reason above. Your muscles are going to get used to a specific resistance. Think about it. The resistance is very different between walking on stairs and doing lunges, but it’s the same movement. If you want a muscle to get bigger, you have to make it “sweat”. Never, never, do the same rep-range over and over again for more than a couple of weeks at maximum.