Ripped & Buffed vs. Skinny and Sinewy: Training Velocity, not Load, Appears to be Sole Determinant of Exercise Induced Shifts from Slow- to Fast-Twitch Muscle Fibers.

Image 1: Who would you like to be?
And how do you train to achieve
his physique?

Sprinter or marathon runner? Ripped and buffed or skinny and sinewy? Although this is, after all, a question of muscle vs. fat, bone and tissue mass, it is upon closer examination as much a qualitative question, as it is a quantitative one - a question that may well be influenced by the way you train!

Unlike our adipose tissue which has almost unlimited capacity to grow, the size of our muscles appears to limited by a number of factors, among which the individual fiber-make-up, i.e. the ratio of slow-oxidative endurance-type fibers (type I) to fast-twitch type IIA (fast-oxidative glycolytic), and fast twitch IIX (fast glycolytic) seems to play an important role, when it comes to getting big and buffed or skinny and sinewy.

Figure 1: Slow- and fast-twitch faber composition in athletes and non-athletes (data based on Carrol. 1998; Widrick. 2002)

As the data in figure 1 goes to show, athletes, unlike untrained individuals, who have about the same amount of fast and slow-twitch fibers, exhibit discipline specific adaptations in muscle fiber composition, with sprinters having the lowest and middle distance runners the highest ratio of slow to fast twitch muscle fibers. According to data from Aagard and Andersen, Bergh et al. and Fry et al. (Berg. 1978; Aaagard. 1998; Fry. 2003), the range of slow to fast twitch fiber ratios extends from ultra-endurance runners with a 90:10 slow to fast twitch ratio down to weight lifters and sprinters with a minimum of 20:80 slow to fast twitch fiber ratio.

Muscle fiber type and weight loss: Contrary to what you may have guessed, or read elsewhere, obese patients with a higher amount of oxidative slow-twitch fibers have been shown to lose weight easier than their "heavier muscled" peers. In a 2002 study Tanner et al. report (Tanner. 2002):

With weight loss intervention, there was a positive relationship (r = 0.72,P < 0.005) between the percentage of excess weight loss and the percentage of type I fibers in morbidly obese patients. These findings indicate that there is a relationship between muscle fiber type and obesity.

Image 2: For someone who already got morbidly obese, a higher ratio of type II fibers may well be counter-productive if his/her overall goal is weight loss.

Another result of the same study, which could easily be misinterpreted as politically incorrect is the genetically determined higher raio of type II muscle fibers within the African American part of the female study population, which made it increasingly harder for these women to burn the fat. And just in case, you still wonder why a type I fiber, something obviously only skinny people have in excess would help with losing fat, just think about the term "oxidative muscle fiber" for a moment, then add to that the experimental observation that type I fibers have greater mitochondria volume densities than type II fibers (Sullivan. 1978) and you will realize that a highly oxidative muscle fiber is more valuable when it comes to burning fat than a glycolitic one, reagardless of whether or not the latter may "look" better ;-)

In a recent review of the literature Wilson et al. provide the following biological explanation for the differences that exist between endurance and strength athletes (Wilson. 2011):

[...], type I fibers have been observed to have both greater mitochondria volume densities as well as capillary-fiber contact length when compared to type II fibers.  In addition, mitochondria volume density was highly correlated (r = 0.99) with O2 diffusion coefficients across three different muscle groups (retractor, sartorius, soleus) suggesting greater aerobic capacity in type I fibers.

While type I muscle fibers will thus figuratively carry their owners in 80 days around the world, type IIX and IIA fibers exhibit a 10x and 6x greater peak power and a 4x and 3.3x greater contractile velocity than their oxygen-hungry slow twitch cousins.

Figure 2: Relative peak power and contractile velocity of fast twitch fibers vs. slow twitch fibers (data based on Wilson. 2011)

The reason that the two guys from image 1 do not only perform but also look completely differently, lies yet in the greater capacity of type II fibers for exercise-induced hypertrophy (Schoenfeld. 2000). The relative number of type II to type I fibers is thus of paramount importance, if you want to look like a sprinter - not like a marathon runner and if you want to lift heavy weights instead of running cross-country. Fry et al., for example found strong correlations (r = 0.94; almost "causative") between the percentage of type IIA fibers and 1 repetition max snatch performance in national caliber Olympic athletes (Fry. 2003). Now the obvious question is: "How can I influence my individual fiber composition, or is this simply genetically determined?"

It stands to reason that genetics is a major determinant of fiber composition, but, hardgainer or not, with appropriate training and nutrition everyone can - at least to a certain degree - shift his muscle fiber make-up from a slow-twitch oxidative to a fast-twitch glycolytic type, even without the use of clenbuterol and other beta-2 agonists which hav been shown to trigger respective shifts from type I to type II muscle fibers in a rodent model (Zeeman. 1988).

Training for shifts in fiber composition

From Wilson et al.'s review of the literature it becomes quite obvious that standard exercise regimen, like jump squats at either 30% or 80% do not provide satisfactory results for someone looking to increase the number, not the size of his glycolytic muscle fibers (Wilson. 2011). In a study by Liu et al. (Liu. 2008), a 5x3RM bench press protocol, performed 3 times per week for 6 weeks, on the other hand, triggered a shift within the type II fibers. It increased the percentage of type IIA fibers from 44.9% to 66.7%, but decreased the type IIX fibers from 33.4% to 19.5%, thus leaving the percentage of slow twitch type I fibers unchanged. A second group from the same study who used a more versatile routine, with the same 5x3 regimen on Mondays, 10x concentric-repetition bench press throws at 30% of their 1RM on Wednesday and 10 stretch-shortening type push-ups on Friday for 5 sets, each, were able to increase the number of type IIA muscle fibers (from 47.7% to 62.7%) without decreases in the number of type IIX fibers, but a profound -50% reduction of slow-twitch oxidative fibers (from 18.2% to 9.2%). Wilson et al. go on and cite several other studies that were able to show the modulatory (increase in type II, decrease in type I) effects of high-velocity contractions on muscle fiber composition (Wilson. 2011) and corroborate that results with findings from other studies which corroborate these results with ...

[...] findings that the percentage of type I fibers may be increased with various types of aerobic training protocols such as endurance cycle training (+12% Type 1) and long distance running (+17% Type 1), [where, on the other hand] studies indicate that sprint training may facilitate the change of slow twitch fibers to fast twitch fibers.

Interestingly, Hortobagyi et al. were able to show that laziness taken to the extreme, i.e. 3 weeks of knee immobilization, also reduced the amount of type I fibers (-9%) and increased the number of type IIX fibers (+11%) in 48 recreationally active men and women (Hortobagyi. 2000). These results should yet be treated with appropriate caution and I would strongly advice against lying on the couch to increase your propensity for muscle growth by decreasing the number of slow twitch and increasing the number of fast twitch muscle fibers, because "recreational activity", for most people, consists of aerobic type of exercises, playing soccer, tennis or whatever - all sports that by and out of themselves would trigger shifts towards a more oxidative (predominant type I) muscle composition. It is thus not surprising that refraining from such activities for 3 weeks would reverse those changes.

So how should you train, then?

In view of the paramount importance of speed, not load in the few experiments which challenge the hitherto established paradigm that muscle fiber composition was largely determined by genetics and transformation was possible only within type II fibers, i.e. from type IIA to type IIX and vice versa, the incorporation of respective training techniques, e.g. concentric-repetition bench press throws at 30% of your1RM, as they were used in the study by Liu et al. (Liu. 2008), into a more versatile hypertrophy-specific routine which would

1. trigger a hypertrophy response, on "classic" strength training days (like 3x5 or 3x8-10), and
2. increase propensity for growth, on "speed-rep" days with exercises like plyometric push-ups, concentric-repetition bench press throws at 30% 1RM, etc.

would appear to be the most reasonable way to train for anyone out there, who does not belong to the "genetic elite" of born sprinters.