Type IIa fibers are the in-between team. They squeeze fast and strong, but they can also keep going for a while.
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Your muscles are made of long cells called muscle fibers. There are slow ones, very fast ones, and Type IIa fibers in the middle.
Type IIa fibers are pinkish red. They have a good number of mitochondria, the cell's power makers, plus a store of sugar fuel called glycogen (GLY-ko-jen).
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Type IIa fibers can use oxygen and stored sugar to make power. So they are faster than slow-twitch fibers, but they do not tire out as fast as the speediest fibers.
They help in things like a soccer game, where you run hard, rest a bit, and run hard again.
Exercise can change your fibers a little! With training, some of your very fastest fibers can start acting more like Type IIa fibers, so they get tired less easily.