A concept known as RIR (Repetitions in Reserve) or RPE (Rate of Perceived Exertion) has become popular in the fitness industry in recent years and could prove useful for your own training. Both mean essentially the same thing and can be used for any type of training. However, in the context of our study, this concept is particularly relevant to strength training.
Suppose you're doing squats and your program asks you to lift 50 kg for as many reps as possible, but you leave 3 reps in reserve. This simply means that you stop your set of squats once you feel you could do 3 more reps.
It's not an exact science, far from it. It can be difficult to know what it feels like to keep a certain number of repetitions in reserve, and you may not always be precise. All training must be stimulating for the system. If you challenge your muscular system sufficiently, it will adapt accordingly. That's how you get stronger. However, if you don't apply enough stress, or if you apply too much, you won't get stronger. This is where reserve reps can be very useful.
A good rule of thumb for all strength training is to keep 1 to 3 reps in reserve for most sets. If you stay within this range, your bodybuilding work will be stimulating and help you progress. If you're constantly missing reps, or if the last rep is slow and unappealing, you're leaving too few reps in reserve. This puts too much strain on the system and makes it difficult to progress.
On the other hand, if all your sets are super easy and you leave 7 or 8 reps in reserve, that's not challenging either. It needs to be a little more challenging. Let's add here that you should record your workouts, especially your bodybuilding work. If you train regularly with 1 to 3 reps in reserve and slowly increase the weights, you'll be on your way to excellent gains in strength and fitness.