from the article above
Big Versus Little- Muscles in Isolation
Let's say that we remove a biceps muscles from two different endurance trained rowers. One muscle is 50% bigger than the other (cross-sectional area). We hook up these muscles to an artificial machine and perform a test ( I know this is gross but it is just a hypothetical situation. We would never do this to well trained rowers!) Which muscle will be able to perform more work in a 6 minute all out test? Well, the bigger one will, of course. That is, assuming that both muscles are well adapted to repetitive work (lots of mitochondria) and both are receiving plenty of oxygen. So if all other things are equal, the big muscle outperforms the small muscle.
Big versus Little - Muscles as part of a Package
If we extend the above situation to say a big body builder and a skinny guy like me, does the bodybuilder win? Probably not ( I hope not at least). Why? Because now the rules have changed, or I should say the performance limitations have changed. In the isolated muscle above I said that the muscles were 1) equally endurance trained and 2) supplied with unlimited oxygen. When we put the muscles back inside a real body, neither of these conditions are true.
Mitochondrial Dilution
When a bodybuilder trains, the goal is to make each muscle fiber as big as possible. Muscle fibers have contractile protein, mitochondrial protein, and other components. Increasing the relative proportion of one component (like more contractile protein) means that you have relatively less of everything else in the same fiber (like mitochondria). From an endurance standpoint this is not a good adaptation. We even give it a name in sports physiology circles, mitochondrial dilution. The bodybuilder's muscles may actually become more easily fatigued as they get bigger, because their mitochondrial density is not increasing at the same rate. The bodybuilder accepts that because the name of the game is size, not endurance.
It is possible for the endurance athlete to gain some muscle size and maintain mitochondrial density, but it requires that the volume of endurance training be maintained. If you are a runner and you decide to get stronger in the weight room by really doing a lot of strength training 3 days a week for an hour, you will probably drop some of your running volume to fit it in. After 6 months you have gained 5-10 pounds of muscle, you look really good, and you are running 2 minutes slower for 10k! Why? Well besides having to carry around 5-10 more pounds of muscle that you can't use when you are running, you have probably lost endurance capacity in those bigger stronger quads. So, you have a lower lactate threshold due to the detraining of your leg muscles, plus you are less efficient due to the increased bodyweight (and decreased training volume). Oh well, at least you LOOK Fast
