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Resting long enough to maximize your training volume could be the key to success, i.e. strength and size gains.

If you have been following the various affords to ascribe differences in strength and, even more so, size-increases to a specific training variable, you will remember that the only promising parameters that appear to be supported by more than the literal "outlier study" are training load and volume.

Of these, the former is pretty much uncontested. The latter, however, is still questioned by a camp of inconvincible skep- tics, who simply ignore the fact that there's ample evidence that "[h]igher-volume, multiple-set protocols have consistent- ly proven superior over single set protocols with respect to increased muscle hypertrophy" (Schoenfeld. 2010).
What still isn't clear, though, is the role of other training parameters, such as the time you take to recover between multiple sets and exercises aka the "rest intervals". As Schoenfeld et al. point out in the introduction to their most recent study, "several studies have investigated the effects of varying rest interval length on muscular adaptations," (Schoenfeld. 2015) albeit with contradictory results: While Ahtiainen et al (2005) were unable to find a significant inter-group size or strength difference in well-trained subjects (6.6 +/- 2.8 years of continuous strength training) who rested 2 minutes compared to those who rested only 5 minute in response to their 21-week training intervention, Buresh et al (2005) reported more recently that significantly greater size increases of the arms and a trend for greater muscle hypertrophy in the legs in young, albeit untrained subjects who rested for 2.5 minutes instead of just one.

Figure 1: Previous studies found "conflicting" evidence. While Ahtiainen et al. found no effects of 2 vs. 5 minutes in trained, Buresh et al. found effects of 2 vs. 1 minutes rest in untrained subjects. With different subjects, different workouts and most importantly different rest times that were compared it is yet not exactly right to say that the studies contradict each other.

Now, obviously, the ostensible "contradiction" I alluded to in the previous paragraph does eventually not exist. With trained vs. untrained subjects, different workout protocols and most importantly different rest intervals (1 vs. 2 minutes and 2 vs. 5 minutes) the studies by Ahtiainen and Buresh cannot really contradict each other. The same must be said of an even more recent study by Villanueva et al. (2014) the surprising findings of which, i.e. "longer rest periods compromise the gains of older trainees", I've discussed last year, already.Eventually, however, this does not change that there is, as Schoenfeld et al. write that "a need for more research to provide greater clarity on the topic" (Schoenfeld. 2015). A "clarity" Schoenfeld et al. sought to find with a study that "used current rest interval recommendations for hypertrophy and strength of 1 versus 3 minutes, respectively, and employed validated measures to directly assess site-specific changes in muscle thickness" (ibid). In that, the researchers speculated that ...

"[c]onsistent with generally accepted guidelines on the topic (Willardson. 2006), we hypothesized that short rest intervals would produce greater increases in muscle growth and local muscle endurance while long rest intervals would result in superior strength increases" (Schoenfeld. 2015).

As you will know if you didn't miss the headline of this waredownloadsoft article, this hypothesis was only partly validated. The data in Figure 2 confirms that the subjects, "experienced lifters (defined as consistently lifting weights for a minimum of 6 months and a back squat / body weight ratio ≥ 1.0)" (Schoenfeld. 2015), gained significantly more strength, when they rested 3 versus just 1 minute between the 3 sets of their three weekly workouts (Figure 2 does also tell you that the strength endurance increases were identical in both groups).

Figure 2: Changes in markers of strength and strength endurance; * denotes significant pre- vs. post difference, # denotes significant inter-group difference (here in favor of long(er) rest periods | Schoenfeld. 2015).

What was Schoenfeld et al. did not find, however, were increased size gains in the short-rest period group whose 24 workouts that were performed on non-consecutive days over the course of the 8-week study period, were otherwise identical with those of the long-rest period group and comprised a total of 7 exercises for all major body parts, namely...

• three leg exercises, i.e. barbell back squats, plate-loaded leg presses, and plate-loaded leg extensions), 
• two exercises for the anterior torso muscles, i.e. flat barbell presses and seated barbell military presses, and 
• two exercises for the posterior torso muscles, i.e. wide-grip plate-loaded lateral pulldowns, and plate-loaded seated cable rows

This is a highly significant result even for you who is - according to an older waredownloadsoft Poll - probably training according to a split regimen, albeit most likely with very similar exercises. What may be different from the some, but obviously *smile* not your workout though, is that the supervision by members of the research team ensured that the subjects stuck to the prescribed cadence of 1 second for the concentric and "approximately 2 seconds" (ibid.) for the eccentric part of every the exercise. This as well as the imperative progression to higher weights, whence the prescribed number of 8-12 reps per set could be performed is unfortunately overlooked by many recreational trainees - with disappointing consequences in the form of inferior or even no size and strength gains, by the way... but I am digressing, let's rather take a look at the already mentioned, unexpectedly superior strength size gains in the long(er) rest interval group (Figure 3).

Figure 3: Changes in muscle thickness and corresponding effect sizes; * denotes significant pre- vs. post-changes, # denotes significant inter-group differences; overall it is obvious that there's a long(er) rest advantage (Schoenfeld. 2015).

As the single "#" in Figure 3 tells you, the inter-group differences and thus the advantage of the long(er) rest intervals was statistically significant only for the quads, though. If we also take into account the lack of statistically significant effects on the sleeve sizes (biceps and triceps) in the short rest interval group, as well as the obvious differences in effect sizes (Figure 3, right), there's yet little doubt that the hypothesis that shorter rest intervals yield greater size increases must be considered falsified - at least under the given experimental conditions (trained subjects, three full-body workouts per week, standard hypertrophy set and rep-ranges, etc.).
References:

• Ahtiainen, Juha P., et al. "Short vs. long rest period between the sets in hypertrophic resistance training: influence on muscle strength, size, and hormonal adaptations in trained men." The Journal of Strength & Conditioning Research 19.3 (2005): 572-582.
• Buresh, Robert, Kris Berg, and Jeffrey French. "The effect of resistive exercise rest interval on hormonal response, strength, and hypertrophy with training." The Journal of Strength & Conditioning Research 23.1 (2009): 62-71.
• Henselmans, Menno, and Brad J. Schoenfeld. "The Effect of Inter-Set Rest Intervals on Resistance Exercise-Induced Muscle Hypertrophy." Sports Medicine 44.12 (2014): 1635-1643.
• Schoenfeld, Brad J. "The mechanisms of muscle hypertrophy and their application to resistance training." The Journal of Strength & Conditioning Research 24.10 (2010): 2857-2872.
• Schoenfeld, et al. "Longer inter-set rest periods enhance muscle strength and hypertrophy in resistance trained men." Journal of Strength and Conditioning Research (2015): Publish Ahead of Print.
• Villanueva, Matthew G., Christianne Joy Lane, and E. Todd Schroeder. "Short rest interval lengths between sets optimally enhance body composition and performance with 8 weeks of strength resistance training in older men." European journal of applied physiology (2014): 1-14.
• Willardson, Jeffrey M. "A brief review: factors affecting the length of the rest interval between resistance exercise sets." The Journal of Strength & Conditioning Research 20.4 (2006): 978-984.

Don't worry, this exercise is not going to reverse all the effects of your training. Still, in view of the intriguing results of the study at hand, it would be interesting if the fiber-type unspecific effects in powerlifters are reversed when you stop powerlifting / exercise altogether and/or start lifting with higher volumes and lower intensities (bodybuilding style volume training).

There are visible and invisible differences between bodybuilders, powerlifters and normal men. The former ones are so obvious that it wouldn't be very interesting to address them in a study. The latter, on the other hand, are, other than you'd expect, hardly researched, but - as the results of a recent study from the Manchester Metropolitan University, and a bunch of other European Universities and research centers shows - more pronounced and significantly more fundamental than some of you may have thought.

As Meijer et al. point out in the introduction to their soon-to-be-published paper in the peer-reviewed scientific journal Experimental Physiology, the performance of a power athlete is largely determined by two traits: the maximal force and power generating capacity of the recruited muscles, and the ability to maintain force and power for a prolonged period of high intensity efforts.
Needless to say that the peak muscle power and force will to some extent dependent on muscle volume and physiological cross-sectional area of the muscle, respectively. In accordance with the above definition of what one's power will depend on, though, the mitochondrial density in the recruited muscle fibres does yet figure as well. I mean, you don't want to run out of steam on your 3rd rep of squats, do you? That's what I thought. Thus, Meijer et al. are right to point out that "[i]deally, an  athlete seeks to maximize both muscle power and endurance" (Meijer. 2015).

Myostatin Limits Muscle Hypertrophy in Everyone! Even Normal Gymrats May Benefit from Blockers, if any of them Actually Worked  Learn more!

The problem with this maxim, however, is that you cannot maximize both, the fibre cross-sectional area (FCSA) and its mitochondrial density. In fact, studies suggest that the FCSA at a given mitochondrial density is limited by the maximal extracellular oxygen tension (Van der Laarse et al., 1997; Wessel et al., 2010) or the myonuclear domain sizes (learn more).

It is thus only logical that studies on myostatin negative animals show that limit the amount of hypertrophy beyond which it becomes disadvantageous for sustainable power (learn more). Weak big, vs. small(er), yet strong muscles. That sounds like powerlifting vs. bodybuilding, right?
Well, that's obviously what Meijer et al. thought, as well, when they phrased the following hypotheses about the way the muscle fiber specific tension (F0) of 12 male bodybuilders (BB | BB: 29.8 ± 4.8y; 177.8 ± 4.1 cm; 91.7 ± 13.4 kg), 6 male powerlifters / power athlete (PA | 23.4 ± 3.9 y; 185.0 ± 4.3 cm; 103.0 ± 7.3 kg) and 14 non-competitive / weight lifting controls (C | 24.0 ± 3.5 y; 180.9 ± 5.3 cm; 77.9 ± 6.3 kg) differ in the introduction to their latest paper:

"PAs is characterized by high-intensity low-volume resistance training with supplemental aerobic exercise. Since BBs train for bulk and PAs for function we hypothesize that fibres from PAs will have a higher specific power and F0 than those from BBs. We expect an increase in specific power and F0 in PAs and BBs (for BBs especially in type II fibres) compared to C." (Meijer. 2015). 

To test whether this hypothesis is correct, the researchers compared muscle fibre contractile properties of biopsies taken from m. vastus lateralis of 12 bodybuilders (BB; low- to moderate-intensity high-volume resistance training), 6 power athletes (PA; high-intensity low-volume combined with aerobic training) and 14 controls (C). To do that you have to take samples from the muscle and test the maximal isotonic contractions on a single muscle fibre in vitro.

Figure 1: Typical examples of three IIA muscle fibres. Circles represent the measured force-velocity data. The continuous line shows the fitted force-velocity curve and the dashed line shows force-power curve (top). Specific tension of skinned m. vastus lateralis fibres is inversely related to muscle fibre cross-sectional area (bottom) - For both rows of data: Non-resistance trained controls (left), bodybuilders (middle), power athletes (right | Meijer. 2015).

As the data in Figure 1 shows, the scientists' analysis of their subjects' muscle fibers produced unsurprising and surprising results:

• BBS have larger muscle fibers (unsurprising) - The fibre cross-sectional area (FCSA) was 67% and 88% (P<0.01) larger in BB than in PA and C, respectively.
• There's no difference in fiber size between C and PA (surprising?) - Unlike the difference between bodybuilders, power athletes and control, the existing difference in fiber size between PA and C did not reach statistical significance. 
• BB and PA fibres are stronger (unsurprsing) - BB and PA fibers developed a higher maximal isometric tensions (32%, 50%, P < 0.01) than those of C. 
• BB & C fibers are significantly weaker than PA fibers (unsurprising) - The specific tension (F0) of BB fibres was 62% and 41% lower than that of PA and C fibres (P < 0.05), respectively. 
• The increased peak power of PA fibres was not related to fibre type (surprising) - Irrespective of fibre type, peak power (P) of PA fibres was 58% higher than that of BB fibres (P < 0.05), while BB fibres –despite considerable hypertrophy- had similar PP as C fibres. 

With the latter result standing in contrast to the long-held believe that training induced changes in fiber composition like an increase in the proportion of power-specific MHC IIa,b fibers in power athletes and an increase in "enduring" type II fibers (MHC IIx) in bodybuilders were responsible for the strength and power differences, the study at hand provides initial evidence that that the effects of the way you train go way beyond selective type IIa,b vs. type IIx hypertrophy.
References:

• Jürimäe, Jaak, et al. "Differences in muscle contractile characteristics among bodybuilders, endurance trainers and control subjects." European journal of applied physiology and occupational physiology 75.4 (1997): 357-362.
• Meijer et al. "Single muscle fibre contractile properties differ between bodybuilders, power athletes and controls." Experimental Physiology (2015): Accepted article.
• Van Der Laarse, W. J., et al. "Size principle of striated muscle cells." Netherlands journal of zoology 48.3 (1997): 213-223.
• Van Wessel, T., et al. "The muscle fiber type–fiber size paradox: hypertrophy or oxidative metabolism?." European journal of applied physiology 110.4 (2010): 665-694.