A few thoughts here.
To increase volume during the heavy training of my training system, I have folks leave 1-2 reps in the tank (this was before the RIR term was around) on compound movements during all but the last set, whereas isolation movements can be taken to muscular failure. The last set of a compound movement would be taken to failure safely.
(An advantage of an all out set to failure is that there's no guesswork as to effort in gauging performance - it's all out - so this is a good tool when using a log book to gauge progression.)
As a few guys have mentioned, there's a hell of a difference between stopping 3 reps and 1 rep shy of failure, as well as actually doing to true failure (and even something else to say if the last failure rep is one that someone fights for as long as possible). The main issue is really, I think, how this taxes the nervous system, which is the limiting factor very often for recovery.
So the notion that you can just lump the last 5 reps shy of failure together as equally "effective reps" is overly simplistic.
--- Recovering from leaving 2 reps in the tank vs. 0 RIR can be quite different. And 0 RIR (just not doing a failure rep b/c one's experience suggests that the next rep just isn't going to happen) can similarly be worlds apart from pushing into an "iffy" rep that ends slowly grinding to a halt at a sticking point against which you battle for several seconds during a max effort partial negative. That last rep could be 5 seconds of all out everything you've got effort in some cases...
--- The number of effective reps someone can get from a given set changes per the law of diminishing returns, I suspect. In other words, the more advanced someone is, the less of an effective stimulus it is for growth when stopping, let's say, 3 reps shy of failure. (That stopping point, if all sets were done that way, may not be effective at all, and I think this might have played a role when it came to Jordan's experiences. Sets at 4 RIR just were to easy and really not much of a stimulus at all for him perhaps... ) On the other hand, the more advanced guy can push harder, using heavier loads that take a greater toll on recovery. This explains why many advanced guys (Dorian Yates, being a prime example) find that the stronger and bigger they get, the less (volume) they can recover from. The paradox here is that they have be sure to train really hard (or even harder) to ensure that each set contributes "effective reps" to the training stimulus, which means that volume must be carefully monitored with a big emphasis on recovery.
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As far as "junk volume," my understanding of this idea is that it refers to sets that really don't contribute to the training stimulus, e.g., that no "effective reps" were performed b/c
---Effort was low,
---As well as sets that are of high quality (would otherwise contribute effective reps) but are part of a session where the maximal anabolic response has been set in motion by the previous sets (e.g., the last 5 sets of a 15 set session where 10 sets maxes out the stimulus), and/or these junk volume sets are sets that otherwise don't contribute to the stimulus of the session (e.g., the 4th set of an each exercise where doing only 3 sets of each exercise maxes out the stimulus).
Junk volume can also just be those that don't add effective reps b/c the session had dragged on so long and fatigue made it impossible to generate the effort, tension, etc. needed to add to the stimulus. There is something to say for the old school "bro" notion of stopping a workout when one loses the pump: You've probably slowed down (CNS or psychological fatigue, are getting low in glycogen (which can impair performance) and/or simply aren't training hard enough to create the metabolic stress that comes with a good pump, so it's time to call it quits.
In this paper, they refer junk volume as "wasted sets" in the context of training frequency and myofibrillar protein synthesis
1. Dankel SJ, Mattocks KT, Jessee MB, Buckner SL, Mouser JG, Counts BR, Laurentino GC, and Loenneke JP. Frequency: The Overlooked Resistance Training Variable for Inducing Muscle Hypertrophy? Sports Med 47: 799-805, 2017.
"Performing more sets per session while using a lower training frequency may reduce the time spent in a positive net protein balance because the large number of sets performed within a given session may exceed the ‘anabolic limit’, resulting in wasted sets. Additionally, performing more sets within a given session requires greater recovery time, causing muscle protein synthesis to return to basal levels until re-stimulated again during another training session."
-S