Coloration of fast twitch muscles

Answer 1

Have you ever sat down for Thanksgiving dinner and found yourself wondering why turkeys have some dark meat and some white meat? Well, you were not the first. A scientist named Ranvier reported differences in muscle color within and among animal species back in 1873. The explanation for the color differences is pretty simple and has a basis in physiology. The dark meat of the turkey, or chicken, is "red" or slow-twitch muscle. The white meat is "white" or fast-twitch muscle. Most animals have some combination of these two fiber types, though the destinctions may be less obvious. Why are they differently colored? The slow muscles have more mitochondria (full of red pigmented cyctochrome complexes), and more myoglobin packed within the muscle cells. This gives them a darker, reddish color. Humans also have dark and white meat. Some of our muscles, like the soleus in the lower leg are almost all slow twitch fibers. Others such as those controlling eye movements are made up of only fast twitch fibers. Function dictates form in these highly specialized muscles. The majority of human muscles contain a mixture of both slow and fast fiber types. From an evolutionary standpoint this makes sense. Our not so very distant ancestors' daily survival sometimes dictated a long walk or jog in search of food. Other times, a fast sprint or jump may have kept one out of harm's way. The exact composition of each muscle is genetically determined. On average, we have about 50% slow and 50% fast fibers in most locomotory muscles, with substantial intra-individual (and muscle to muscle) variations. This variation helps make sports interesting!

Ref:http://home.hia.no/~stephens/fibtype.htm