How to stimulate muscle growth: use proper training volume
Many strategies have been tried and recommended as being the best way to strength train. And some methods may be better than others depending on your goals, such as endurance, strength, or power.
But when it comes to achieving muscle growth, there is one training parameter that stands out as being perhaps the only variable that matters. That variable is the number of stimulating reps.
But what is a 'stimulating rep'?
It turns out, that in a given set, performed to failure, the only reps that contribute to muscle growth are the last 5. For instance, if I perform 10 pullups, attempt the 11th, and fail, reps 6 through 10 are the only reps that contribute to muscle growth. Reps 1 through 5 have no effect on muscle growth other than that they allowed reps 6 through 10 to bring my muscles to fatigue, and thus stimulate muscle growth.
So, in order to stimulate maximal muscle growth for a given workout, we need to maximize the number of reps that we perform within five reps to failure. If I can perform 10 pullups, 5 sets of 5 will not stimulate muscle growth. It would be much more valuable for me to do two sets of 10. Even though, in the latter case, my total training volume is smaller (20 reps vs 25 reps), I would have performed 10 stimulating reps vs 0 stimulating reps in the 5x5 routine.
But there is one more factor to keep in mind: timing.
Some coaches advocate taking short rest intervals between sets, and if you are training specifically for endurance, this is valid, but if you are primarily looking to improve your strength, longer rest intervals (usually 4 or 5+ minutes) is better. To understand why this is, we have to understand something about fatigue, and about what triggers muscle growth.
There are two types of fatigue: fatigue of the muscles (aka 'peripheral' fatigue), and fatigue of the Central Nervous System (aka CNS or 'central' fatigue). As you near the end of a an exercise set, your muscles start to fatigue. At this point, your CSN will trigger your muscles to recruit additional muscle fibers than were required to lift the initial reps. It is the recruitment of these additional muscle fibers that contributes to muscle growth! This usually happens in the last 5 reps of each set to failure, and THAT is why the last 5 reps are the ones that trigger muscle growth.
But we still haven't explained why long rests (4 or 5+ minutes) are important.
After muscle fatigue begins to set in, and the CSN is called upon to recruit those additional muscle fibers, THAT is when CSN fatigue begins. If the CSN is fatigued when the muscles start to fatigue, then the CSN is not able to recruit the additional muscle fibers that is was able to recruit during the end of the first set. And remember: the recruitment of those additional muscle fibers is what stimulates muscle growth, so if that doesn't happen, then muscle growth doesn't occur.
As it turns out, the CNS usually requires a longer (4 to 5+ minute) rest period to fully recover from fatigue after a set to failure. So if you wait 4 or 5 minutes between sets, then your CNS should be able to recover, and you should be able to recruit those additional muscle fibers, and stimulate maximum muscle growth.
So, how do you know if you are not waiting long enough? The easiest way to tell is if your second and third sets of significantly smaller than your first. If, on your first set, you were able to do 10 reps before failure, you should be able to do 9 or 10 reps on your second and third sets as well. If you are able to keep your sets consistent, then you are probably waiting long enough. But if, on your second and third sets, you are only getting 6 or 7 reps, then your CNS is probably still fatigued, and you are likely only getting 1 or two stimulating reps, instead of the 5 that you want for maximum gains.