I want to give a background first of something that's quite important to understanding the whole thing involved. We're discussing advanced lifters here, not beginners or so much intermediates.
Is the outcome of training gaining say 1% per week, as the usual outcome and how progress is made?
Well, how many of those who have been training, as adults, many years this year for example bench 300, the next year 450, the next year 675, the year after that 1012, etc?
How many such lifters overhead press this year 200, the next 300, the next 450, the next 675, etc?
This doesn't happen.
Yet most such lifters can in fact make good progress during these years. But the mode of it will not be gaining 1% per week as the ordinary outcome.
Nor, I think, gaining one part in 1000 per week, either, as some kind of exceedingly incremental slow progression.
No, important results that build upon each other come in jumps which may be 1% or greater. There are also fluctuations around a homeostasis, but these fluctuations don't build on each other.
Two general kinds of things can happen that are perceived as muscle growth:
1) The cells stay essentially the same -- same number of nuclei per cell, same number of mature muscle cells -- but the cells either "plump up" somewhat or not, there being a definite limit to how much they can do so in that state, so the "plumping up" does not build on itself incrementally once having already achieved near maximum.
2) The cells undergo what you might call a quantum or categorical change: one or more nuclei are added to a cell from satellite cells, or satellite cells differentiate to become mature muscle cells.
This latter category DOES build on itself and so to speak permanently raise the bar for how large that muscle can be.
It seems that so much of training effort is devoted to what is probably only optimizing the degree of "plumped up" the cells are. Without drugs achieving in contrast the quantum changes is going to be a relatively infrequent event, and will never occur to the full same extent that can be achieved with drugs, but should happen to some extent a number of times during the training year, for the advanced lifter.
Where drugs really come into play is stimulating, in combination with exercise and diet, the quantum changes.
Insulin plus AAS plus GH can trigger changes that don't occur, or trigger them to occur more rapidly, than either AAS alone or AAS plus GH.
That is why insulin is used, to better achieve these categorical changes in the muscle.