Adaptation through a introduction of a new variable (usually by volume or intensity OR by the modification of an exercise OR by the introduction of a new exercise) is the foundational principle of modern strength training. For the body to respond to a stress placed upon it, that stress cannot be habituated, i.e. it cannot be repeated over and over again. Do the same thing consistently and the body ceases to respond.
Again, this is the foundational principle of modern strength training. It's clearly stated in both eastern and western methodologies of training, though the approaches are different. It's found in virtually every legitimate strength training program for years and years. And yes, it could be crudely termed "muscle confusion."
So, what separates adaptation from "muscle confusion"? Well, the latter is the bastardized commercial version, and it gets a few things very wrong. First, the planned progression is haphazardly designed and pretty much incoherent, both in the training and in the diet. "Muscle confusion" operates like so:
A, then B, then C, then D, then E, etc.
In other words, do workout A, then do a different workout (B), then another different workout (C), and so on, and so on. It's a sort of organized chaos, and it's different from a strength training program like starting strength:
A, then A+, then A++, then A+++, then A++++, etc.
Which is to do workout A, then to do it a little better (A+), then to do it a little better than A+ (A++), with the intervals between these over time slowly closing as per the law of diminishing returns. It's not that dissimilar to 5/3/1:
A, then A+, then A++, then A-, then A1, then A1+, etc.
In which volume is downgraded relative to intensity over a three-week mesocycle, followed by a one-week deload (A-), and returning with an A1 week in which intensity is upgraded slightly over A but volume remains the same. Or, the westside template:
A, then B, then C, then D, then E, then B+, then D+, then A+, then E+, etc.
In which main exercises are cycled through at a regular basis (usually randomly) and adjusted slightly when they're returned to, because the lifter is slightly stronger than before.
The difference is that all of these abide by planned progression in some form. There is a starting point (always A), and an end-point, or goal, in mind, and that goal is measured by the change in performance between A and the end-point. "Muscle confusion" does not follow suit, save for the vague end-point of a "90-day better you," which in an untrained person is achievable pretty much regardless of the type of exercise or the type of programming, provided the intensity and volume are high enough so as to require significant effort.
Hope that clears it up, as this is a pretty important distinction to know.
