The Way You Train Shapes Your Muscle Size and Function - Study in Powerlifters, Bodybuilders & Controls Suggests Effects Go Beyond Hypertrophy & MHC Fiber Composition

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.