Final answer:
Fast Glycolytic (FG) fibers are the primary muscle fibers active during bench pressing, which rely on anaerobic glycolysis for energy and are designed for quick, powerful movements but fatigue rapidly.
Step-by-step explanation:
The fibers that are primarily active when bench pressing are Fast Glycolytic (FG) fibers. These muscle fibers are equipped to use anaerobic glycolysis for their energy production, which is the process of breaking down glycogen to generate ATP quickly. FG fibers are characterized by their large diameter and lack of substantial mitochondria and myoglobin, which gives them a white appearance. They are designed for rapid, forceful contractions, making them suitable for the high-intensity demands of resistance exercises such as the bench press. These fibers are responsible for producing the powerful movements needed in such exercises and they fatigue quickly, being prevalent in muscles used for power activities. Additionally, resistance exercise like bench pressing can lead to hypertrophy, the enlargement of these FG fibers, which contributes to the increased muscular size commonly seen in athletes and bodybuilders.