By training volume we mean the number of sets per muscle group per week. What is the minimum number of sets for muscle growth?
EFFECTIVE REPS
Answer: that depends on the number of effective repetitions you do per set. Effective (or stimulating) reps are the reps closest to the point of muscle failure, usually about the last five reps of a set. The closer the reps get to failure, the more effective these reps become in stimulating hypertrophy.
If you train a set to failure, it will contain more effective repetitions than if you keep a few reps in the tank (Reps In Reserve, RIR). If you stop two reps before failure, that set will be half as effective as a set trained to complete muscle failure. If you train to muscle failure, you actually need relatively few sets to initiate growth.
THE NUMBERS
If, like an average bodybuilder, you follow a training program with a high relative intensity (training a lot or all of it to muscle failure), you can already build muscle with 4-6 sets per muscle group per week. That corresponds to roughly 10 sets that do not lead to muscle failure, but are trained, for example, with 2 RIR (that is, if you stop at the point where you could still do two reps). So those 4-6 (with muscle failure) or 10 (with ~2 RIR) are your Minimum Effective Volume. From this perspective, you don’t need to do as many sets as you might have thought, although the volume requirement does increase as you become more advanced.
Do you train for muscle maintenance (Maintenance Volume)? Then you already have enough with about a third of your Maximum Adaptive Volume, the volume for maximum muscle growth. If you do 20 sets per week with 2 RIR, about 7 sets per week will be enough.
FAILURE VERSUS RIR
Finally: why train with RIR and not everything completely to muscle failure? Well, training to failure requires a relatively large amount of fatigue and recovery capacity. If you train with RIR there’s less fatigue and so you can do more sets. On balance, this results in more effective repetitions and therefore more muscle growth. If you aim for maximum rapid muscle growth, you train with RIR. If you settle for slightly slower muscle growth, but also less work, you will train to muscle failure, such as with High Intensity Training.
IN SUMMARY
You can achieve muscle growth with a relatively small number of sets: 4-6 sets per muscle group per week if you train to failure, around 10 sets if you train with 2 RIR.