When you started to carve your Thanksgiving or Christmas turkey, did you ever wonder why some of the meat is white and some is dark? The explanation for the color difference is pretty 
simple. The dark meat that was on your holiday feast is ‘red’ or slow-twitch muscle; the white meat is ‘fast-twitch’ muscle. And most animals, including us humans, have some combination of these two fiber types.
Some of our muscles, like those in your back used for maintaining an upright posture, have a higher percentage of slow-twitch fibers than fast-twitch ones. The calf muscle for example is made up of around 90% slow-twitch muscle fibers. The tiny muscles controlling eye movements are at the other extreme, with only fast-twitch fibers.
The exact proportions of each type of fiber is partly genetic, but for most people each type of muscle fiber makes up about 50% of the whole system, but varies considerably from muscle to muscle according to its function.
The reason that they have different colors is that the red muscle fibers are more efficient at using oxygen to produce the muscle fuel ATP because they have more mitochondria than white fibers and mitochondria bind with hemoglobin (blood cells) which creates the darker color.
Red muscle is better at maintained sustained effort over a period of time due to this more efficient energy source. White muscle dominates in muscles used for rapid movement and can generate large amounts of effort over a short period of time. Their energy source is the sugars that are formed into ATP, and this energy source is powerful but runs down rapidly.
Our bodies can adapt and modify the types of fibers in each muscle according to use. So a 100 meter sprinter may have as much as 80% fast-twitch (white) fibers, while a marathon runner may have 80% red slow-twitch fibers. Power-lifters need fast-twitch fibers for quick bursts of strength, and distance swimmers need slow-twitch fibers for the stamina to cover a large distance.
So the next time your bodybuilder buddy refuses to help you do some yard work by saying that his body is decorative and not useful, he may not be too far from the truth!
