There are different type of muscles in our body. Both biceps and quadriceps are voluntary muscles. However, biceps has a muscular fiber called red fibre although quadriceps has a white one.
Okay, that could seems cool. Nevertheless, how can we differentiate every type of muscular fibres? The answer of this question is molecular biology and histology.
In the animation above we can see a highly detailed scheme of the different types of voluntary muscles and their characteristics.
Type I muscles are those we use in quotidian, regular and repeated actions. Typically, we see a major quantity of these fibers in postural muscles such as quadriceps, rectus abdominis (six-pack), trapezius… Now, one can understand why type I muscles cannot be fatigued. Imagine that you are walking and, spontaneously, your right quadriceps stop working for a while; it cannot be possible!
Type IIB muscles , on the other hand, are those we use in a precise moment for a short time. Ocular muscles and hand ones are usually used to exemplify white muscles.
Everything I wrote before can be explained with molecular biology and histology and now we are going to see it:
Type I , slow twitch, or “red” muscle, is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red colour (there is the reason we called it “red” muscle). Increased density in capillaries gives more oxygen and nutrients , giving a regular sustain for its activity. So that, this muscle can maintain the Krebs Cycle for longs periods of time. Slow twitch fibers contract for long periods of time but with little force because Krebs Cycle is really efficient but it costs so much time. Despite the fact that muscular force is low, it is sufficient to held the body in an upright (“orthostatic”) position.
Type IIb is anaerobic, glycolytic, “white” muscle that is least dense in mitochondria and myoglobin . In small animals (e.g., rodents) this is the major fast muscle type, explaining the pale color of their flesh. Because of decreased density of capillaries and mitochondria, these muscle fibers cannot use Krebs Cycle and start an anaerobic activity. Anaerobic activity is less efficient but you can obtain a great amount of energy in seconds.