How does endurance exercise training increase the capacity to transport glucose into skeletal muscle fibers?

• A. By increasing the ability of insulin to promote transport of glucose into the muscle
• B. By increasing the number of GLUT4 glucose transporters
• C. Both A and B
• D. Neither A nor B

Answer: (C)

Endurance training boosts how muscle fibers take up glucose by two complementary changes: more GLUT4 transporters and greater insulin effectiveness. Regular endurance activity increases the amount of GLUT4 protein in muscle, so there are more carriers available to bring glucose into the cell when signals arrive. At the same time, the muscle becomes more sensitive to insulin, so insulin more effectively triggers the signaling that moves GLUT4 to the cell membrane and promotes glucose entry. Training also enhances contraction-related pathways that push GLUT4 to the surface, even without insulin, and these effects compound with the increased transporter pool. Together, these adaptations raise the capacity to transport glucose into skeletal muscle, making both increasing GLUT4 numbers and improving insulin-mediated transport key contributors.