Let us start by subdividing muscle fibers into two types: Type I (Slow twitch, Oxidative) and Type II (Fast twitch, Glycolytic).
2. Their metabolism is aerobic.
3. They maintain relatively sustained contractions since they have low amount of Myosin ATPase.
4. Their energy utilization is low.
5. They have slow contraction rates and
6. They can work for prolonged duration.
Type II muscle fibers:
1. Also called White fibers because they lack myoglobin and have very few mitochondria.
2. They derive their energy from anaerobic glycolysis.
Remaining points are the reverse of what is mentioned about Type I muscle fibers above.
Now lets compare Usain Bolt ( a gold medalist Sprinter) and Eliud Kipchoge ( a gold medalist Marathoner).
Eliud has increased number of Type I muscle fibers in his leg muscles whereas Bolt has increased number of Type II muscle fibers in his leg muscles.
So we can say that the proportion of these two types of muscle fibers varies among the muscles of the body, depending on the function and also Training.
If Usain Bolt does a 100 m sprint, for the first 4-5 seconds the major source of energy(ATP) is creatine phosphate and then it is anaerobic glycolysis, using muscle glycogen as the source of glucose.
In contrast if Eliud Kipchoge ran a 42.2 km marathon, Aerobic Metabolism is the principal source of ATP and the major fuels are blood glucose and free fatty acids.
The sources of these fuels alongwith their respective time duration for which they can supply muscles with energy sufficiently are given as follows:
1. Blood Glucose- 4 min
2. Liver Glycogen- 18 min
3. Muscle Glycogen- 70 min
4. Triacylglycerol in adipose tissue- 4000 min
That'll be all!