Muscle Fiber
- This nice table is from Institude of Human Anatomy
| Characteristic | Slow Oxidative (SO) or Type I Fibers | Fast Oxidative-Glycolytic (FOG) or Type IIa Fibers | Fast Glycolytic (FG) or Type IIx Fibers |
|---|---|---|---|
| STRUCTURAL CHARACTERISTICS | |||
| Myoglobin content | Large amount | Intermediate amount | Small amount |
| Mitochondria | Many | Many | Few |
| Capillaries | Many | Many | Few |
| Color | Red | Pink | White (pale) |
| FUNCTIONAL CHARACTERISTICS | |||
| Capacity for generating ATP and method used | High capacity, by aerobic cellular respiration | Intermediate capacity, by both aerobic cellular respiration and anaerobic respiration (glycolysis) | Low capacity, by anaerobic cellular respiration (glycolysis) |
| Rate of ATP use | Slow | Fast | Fast |
| Contraction velocity | Slow | Intermediate | Fast |
Muscle Fibers for Endurance Runners
Obviously, the most important muscle fiber type for endurance runners is Slow switch fiber (type I, or Slow oxidative). It is fatigue resistant and very energy efficient.
Adaptation
The FOG (intermediate) muscle fiber can be converted to slow or fast switch muscle fiber depending on the needs and adaptation.
In fact, adaptation can even slowly convert between all three types of fibers (maybe in years).