Progressive overload is the key to muscle growth. This means that you gradually have to lift heavier weights in order to (continue to) grow. And funnily enough: that you can lift heavier weights is because you have grown. Still, you can’t use more weight every workout. How do you decide when you can add weight to the bar?
There are countless progression models and one is not necessarily better than the other.
Perhaps the simplest (but no less effective) method is to focus on your target RIR and reprange.
RIR stands for Reps In Reserve and is the number of reps that will keep you away from muscle failure. With compound exercises we usually use 2-3 RIR, with isolation exercises 1-2 RIR (so we don’t train everything blindly until muscle failure). The easiest is to use a fixed number of RIR, although you could make progress in that too.
The rep range is the number of reps you want to achieve with your sets, for example 10-12 reps. For muscle growth, we usually operate somewhere between 5 and 30 reps, depending on the type of exercise.
Assuming you do three sets in a row for an exercise, it might look like this:
1st set: 12 reps with 2 RIR*
2nd set: 11 reps with 2 RIR
3rd set: 10 reps with 2 RIR
* This means that after 14 reps you would reach muscle failure.
You add weight the moment you have achieved these targets.
Example
Week 1: Training weight 40 kg
1st set: 10 reps with 2 RIR
2nd set: 9 reps with 2 RIR
3rd set: 8 reps with 2 RIR
Week 2: Training weight 40 kg
1st set: 11 reps with 2 RIR
2nd set: 10 reps with 2 RIR
3rd set: 9 reps with 2 RIR
Week 3: Training weight 40 kg
1st set: 12 reps with 2 RIR
2nd set: 11 reps with 2 RIR
3rd set: 10 reps with 2 RIR
Week 4: Training weight 45 kg
1st set: 10 reps with 2 RIR
2nd set: 9 reps with 2 RIR
3rd set: 8 reps with 2 RIR
And so on.
Again, we assume a static RIR here. But you can also vary that, provided you do so strategically. For example 1-2 RIR two weeks before your deload week and 0-1 RIR (until muscle failure in the last set) in the week before your deload. After all, the shorter you train for muscle failure, the greater the fatigue.