>>Dr. Ketchum: The goal of this video is to review the muscle fiber types. And so remember
that we have two primary or two dominant muscle fiber types, and those are slow oxidative
and fast glycolytic. Remember the intermediate form is fast oxidative. For each of the various
muscle fiber types we have different characteristics that you can think about and be able to answer
these questions without memorizing this table, okay? So if you think about the name of the
muscle fiber type and what the name means—then you’ll know what the characteristic is.
So let’s start with slow oxidative. Remember that “slow” is referring to how fast the
myosin ATPase is able to hydrolyze ATP. So the myosin ATPase activity is slow versus
in a fast glycolytic fiber, it’s fast. Versus is a fast oxidative fiber, right, it’s fast
as well. Now the primary metabolic pathway for ATP, this is an oxidative—in the slow
oxidative form—it’s oxidative. Oxidative means that you’re doing complete oxidative
phosphorylation; here we’re doing aerobic respiration, therefore. Fast glycolytic, the
glycolytic means that the only respiration pathway occurring is glycolysis, and therefore
that means that this is an anaerobic metabolic pathway. In the intermediate form—the fast
oxidative—this is mostly aerobic respiration. However, there is some anaerobic respiration
taking place. And that’s why when you look at the name of this fiber, they’ll say fast
oxidative and they may hyphenate it glycolytic.
So let’s return to the slow oxidative fiber. Because it’s slow and it’s doing aerobic
respiration, aerobic respiration requires oxygen, and myoglobin binds oxygen. So you
should have a high level of myoglobin then in that muscle fiber type. In anaerobic respiration
we’re not using oxygen, therefore there would be low myoglobin content. Now in fast
oxidative, where we’re doing mostly aerobic respiration, you’re going to have a high
myoglobin content as well, because you’re utilizing a lot of oxygen. Now in terms of
glycogen storage, remember that aerobic respiration—we’re not using glycogen storage directly—we’re
just using glucose. So there’s going to be a low glycogen storage; we’re going to
use the glucose, we’re not going to store it. In fast glycolytic, since we’re doing
anaerobic respiration, we’re relying on those glycogen storages. So we could take
glycogen, break it down to glucose, and then do glycolysis with that—with the fast glycolytic
muscle fiber type. And then the intermediate form, we have an intermediate amount of glycogen
storage, because remember, you are doing some anaerobic respiration. Remember that recruitment
order we’re not concerned about.
The rate of fatigue. Now if it’s an aerobic muscle fiber, it’s using oxygen and it’s
also slow, meaning that we’re doing cross-bridge cycling at a slower rate. So we’re not utilizing
ATP quite as fast; so the rate of fatigue, then, is slow. Another way to say this is
that slow oxidative fibers are fatigue resistant. Fast glycolytic fibers, on the other hand,
are doing cross-bridge cycling at a very fast rate, so they utilize ATP very quickly, and
so these muscle types are going to fatigue fast. And in the intermediate form, remember
that they are fast as well. They’re not as fast as fast glycolytic, so they’re intermediate
in terms of how fast that they fatigue. Now remember that the color of the muscle is due
to the myoglobin, and that myoglobin is what binds the oxygen. So if we have a lot of oxygen,
which we do in a slow oxidative fiber, it’s red—and it’s red because of the abundant
myoglobin. Lot’s of myoglobin, it’s red. Now fast glycolytic, we have low myoglobin.
So we’re not storing a lot of oxygen, so it’s going to be a pale or white color.
And then fast oxidative is an intermediate; so this would be more of a pink. Now fiber
diameter is going to be telling us about the ability to generate tension. So in a slow
oxidative fiber, we’re doing cross-bridge cycling at a slow rate. So because we’re
doing it at a slow rate, we’re not relying on generating a lot of force. So this is going
to have a small muscle diameter versus fast glycolytic, which does fast cross-bridge cycling,
meaning it does generate a lot of tension. So to generate a lot of tension, it’s going
to have to have a large fiber diameter. The greater the fiber diameter, the more sarcomeres
you have. And then again, fast oxidative is kind of in between; it’s an intermediate.
You don’t generate as much force as a fast glycolytic, but you generate more force than
you do as slow oxidative. Then we move to the mitochondria.
The mitochondria—that’s where we do oxidative phosphorylation. That’s where we take oxygen,
we do electron transport, chemiosmotic coupling and aerobic respiration is what’s carrying
out that process. So we’d have lots of mitochondria in a slow oxidative fiber. The same is true
for fast oxidative, because we’re doing mostly aerobic respiration. Because we are
only doing anaerobic respiration in fast glycolytic, we’d have very few mitochondria. Now in
terms of the capillaries—think about what the capillaries are doing. They’re bringing
a blood supply to the muscle fiber. So if you bring a blood supply, you’re also bringing oxygen.
So in a muscle fiber type that uses a lot of oxygen, it’s going to need a lot
of blood. Therefore, it’s going to have lots of capillaries. Versus fast glycolytic
where it’s not relying on oxygen, so you’re not relying on a blood supply, so it’s going
to have fewer capillaries. And then you’re also going to have lots of capillaries in
the fast glycolytic fiber, because it does mostly aerobic respiration. So then based
off of this, we can look at the muscle fiber types and what activities that they’re best
suited for. So we know fast glycolytic. Fast glycolytic—it does cross-bridge cycling
very quickly, does anaerobic respiration, so it’s going to fatigue fast, but generate
a lot of power. So the activities that generate a lot of power but would fatigue quickly include
powerlifting and hitting a baseball. Slow oxidative, on the other hand, slow—does
not generate a lot of power, oxidative—does not fatigue quickly. So those activities that
rely on slow oxidative fibers would be running a marathon and maintaining your posture.
And then we have an intermediate. So if you’re sprinting, then walking, sprinting, then walking.
For the sprinting part of it you’ve got the fast, right, cross-bridge cycling. So
you can generate a lot of force, but then you also have oxidative so that you don’t
fatigue as quickly. So that would be sprinting and walking—fast oxidative.