Final answer:
Fast Glycolytic (FG) fibers are used for quick bursts of energy due to their ability to rapidly generate ATP through anaerobic glycolysis, ideal for very short, intense activities like sprinting.
Step-by-step explanation:
The fiber used for a quick burst of energy in muscles is the Fast Glycolytic (FG) fiber. FG fibers are designed to create Adenosine Triphosphate (ATP) quickly through the process of anaerobic glycolysis, enabling them to produce rapid, powerful movements. They have a large diameter and contain high amounts of glycogen for quick ATP generation, but due to their reliance on anaerobic processes, they have low endurance and fatigue rapidly. These fibers are white because they do not contain significant levels of myoglobin and have fewer mitochondria in comparison to oxidative fibers. FG fibers are ideal for activities like sprinting or weightlifting, where a short but intense effort is required.
For slightly longer activities, like a 200m run or quick swimming event lasting approximately 90 seconds, the body uses glycogen to make ATP without oxygen (anaerobic energy system), producing energy quickly though not as fast as the creatine phosphate system. In contrast, Fast Oxidative (FO) fibers can also provide quick energy and are more fatigue-resistant than FG fibers, but they do not produce the same tension as FG fibers and are less efficient for very short explosive activities.