This is from wiki: http://en.wikipedia.org/wiki/Muscle.
Maybe the pec differs from the dorsal in muscle type.

Skeletal muscle is further divided into several subtypes:

* Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
* Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:[2]
o Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
o Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.[3]
o Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.