No it's not based on empirical evidence but rather on scientific evidense regarding physiology, specifically response to training load and energy systems.
First I'll state that the rep zones assume a controlled tempo (not slow, but not fast either... each rep lasting around 4 seconds, 3 seconds eccentric and 1 second concentric).
Based on this tempo the duration of the different zones become:
1-3 rep range (relative strength) = 4-12 sec.
3-5 rep range (limit strength) = 12-20 sec.
6-8 rep range (functional hypertrophy) = 24-32 sec.
8-12 rep range (hypertrophy) = 32-48 sec.
12-15 rep range (strength-endurance) = 48-60 sec.
15+ reps = + 60 sec.
NOW... The relative strength zone focused only on the phosphagen energy system (ATP-CP) which has a capacity of 9-12 seconds.
The limit strength and functional hypertrophy zones correspond to the early portion of the anaerobic glycolysis pathway and thus still represent intense work.
The hypertrophy zone correspond to the portion of the anaerobic glycolysis pathway where lactate accumulation is the greatest.
The other zones, not as relevent to us, indicate an gradual increase in the utilisation of the aerobic pathway.
THEN ... we must consider the work of Zatsiorsky that explained the hypertrophy stimulation process. In his book (Science and Practice of Strength Training) Dr. Zatsiorsky explains that protein degradation is the trigger to stimulate muscle growth. Protein degradation is a factor of the rate at which protein is degraded during a set, and of the length of the period spent degrading the protein.
The rate of protein degradation is a direct result of the amount of intramuscular tension produced by the muscle (how hard a muscle has to contract to fight a resistance). So the more FORCE one has to produce = the higher the tension and the higher is the rate of degradation. Force = Mass x Acceleration, so in the case of lifting weights for hypertrophy purposes, increasing the amount of mass moved (weight used) is what will increase the rate of degradation the most.
So in simpler terms: big weights = high rate of degradation, small weights = low rate of degradation.
The second factor is obviously related to the duration of the set. The longer the set, the higher is the degradation time. For example, if a set lasts 10 seconds you will not be degrading proteins for as long as if the set lasted 30 sec.
To maximize hypertrophy (muscle growth) you must find the zone where total protein degradation is the highest per set...
HEAVY WEIGHTS = high rate/low duration
LIGHT WEIGHTS = low rate/high duration
Zatsiorsky (and several german sport scientist quoted in the early work of Gilles Cometti) established that the best tradeoff; the training zone where the result of rate x duration occurs with reps of 8-12, with a peak at 10 reps.
This is also backed up by other evidence, namely Dr. Jurgen Weineck who mentionned in one of his book that the beginner only needs 60% of his maximum to stimulate good gains but once passed the beginner stage, 70-80% is required. 70-80% (for most peoples) is, not surprisingly, the 8-12 reps zone.
Then we can also quote some empirical evidence. But empirical evidence based on the work of coaches who worked with hundreds of athletes. I like to base my work on Soviet sport science because their coaches:
a) were also scientists and able to perform proper data collection and interpretation
b) recorded every single detail of the training of their athletes. Reports that were sent to a central comittee for data metanalysis
With Zatsiorsky's work we established that the optimal zone to gain size was the 8-12 reps range. We can now turn to the analysis of top Soviet lifter's training to find the effect of the earlier zones. NOTE that Soviet analysed EVERYTHING... every training session for each athlete was recorded and sent to the misnistery of sport. At the end of the year an analysis was published for each sport (by qualification level) which stated the average training volume (tonnage), sets, reps, reps per set, exercise selection, etc. for each class of athletes.
One thing that can be seen in any of the work by old Soviet coaches (Roman, Medvedyev, Oleshko, Laputin, etc.) which is available in translated form at www.dynamic-eleiko.com is that:
1) lighter lifters (those in the lower weight classes) performed more sets above 90% and a much lower number of reps per set (1-3)
2) heavy lifters, especially superheavy lifters who are not restricted by a certain class limit, performed less set above 90% but an higher average number of reps per set: 3-5.
The reason is that ligther lifters cannot afford to gain excessive weight, even if it's from muscle bulk. The Soviet coaches wanted their lighter weight class athletes to be as strong as possible, without moving up a class. So they trained using a lower number of reps with more relative weight. This gave the lifter an adequate strength gain but only a slight increase in bodyweight. Thus it increased relative strength to a higher degree (this is why I call it the relative strength zone).
Heavier lifters did not have the problem of making weight. So they could afford to gain a lot of muscle. However they needed muscle that is as strong as it looked. Working in the Zatsiorsky hypertrophy zone (8-12) provided for more size than strength gains and such work is metabolically costly and thus limit the amount of work that can be done in a week. So they used an intermediate zone of 3-5 reps (assistance exercises were sometimes performed for 6-8 reps) which provided more strength gains than any zone because it did so via 2 factors:
1) Increase in muscle size, which increases strength potential (see it as increasing the number of employees in your factory)
2) Increase in neural efficiency which increases size realization (see it as motivating your employees to work harder)
This is why I called this zone "limit strength".
To make a short reap...
1-3 reps = very high neural adaptations, low muscle gain, very low metabolic adaptations
3-5 reps = high neural adaptations, moderate muscle gain, low metabolic adaptations
6-8 reps = moderate neural adaptations, high muscle gain, moderate metabolic adaptation
8-12 reps = low neural adaptations, very high muscle gains, moderate metabolic adaptation
NOW, for some more empirical evidence. Chris Aceto is widely recognized as one of the the top 3 bodybuilding coaches in the world, having worked with several top ranked pro. In his books he states that the number of reps that stimulate maximum muscle growth is 6-12 per set. Below that you work more on strength and over that you work more on resistance and endurance.
I honestly could go on and on ... even citing some studies by Dr. Kraemer indicating the ideal hypertrophy zones, but I don't whant to turn this into a novel.
Most peoples know my view of training to failure. To recap: train as close to failure as possible: work to the point where you perform as many "COMPLETE" reps as possible. In other words work until a point where you know you would fail on the next rep. This is done for all sets but the last.
The last set is performed to failure, going to the point where you cannot budge the weight an inch on your own during that last rep.
Actually the target bodyparts become more responsive to training after a specialization block. Can I explain it? Probably not entirely, but an educated get would be either IIB fibers overshoot (which has been shown to happend when there is a period of greatly reduced training after a period of excessive training) or better neural efficiency: your neuromuscular system being more effective at recruiting the target muscles because you asked so much from them and stimulated them often. However from what I've seen with all who used this system, I can say that the target bodyparts DO become more responsive afterwards.