Distance runner Paula Radcliffe has won dozens of long-distance races and held the women's world record for the marathon since 2003. Scientists, trainers, and athletes alike have wondered about the extent to which muscle structure and function contribute to success in athletes such as Radcliffe. What makes elite distance runners so good? Are their muscles somehow different from those of less successful athletes and non-athletes? Predict who would likely have a greater proportion of fast glycolytic fibers in their gastrocnemius (calf) muscle—an elite distance runner or an elite sprinter. Explain.

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1. Understand the types of muscle fibers: There are primarily two types of muscle fibers in humans - slow-twitch (Type I) fibers and fast-twitch (Type II) fibers. Slow-twitch fibers are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. Fast-twitch fibers are better at generating short bursts of strength or speed than slow-twitch fibers but fatigue more quickly.
2. Identify the muscle fiber types predominant in different athletes: Elite distance runners, like Paula Radcliffe, predominantly use their slow-twitch muscle fibers because these fibers are more efficient at using oxygen to generate the ATP needed for endurance activities. On the other hand, elite sprinters rely more on fast-twitch fibers, particularly the fast glycolytic fibers, which provide quick bursts of speed and power but fatigue quickly.
3. Compare the muscle fiber distribution in distance runners and sprinters: Given that distance running requires endurance and efficient energy usage over time, distance runners have a higher proportion of slow-twitch fibers. In contrast, sprinting demands rapid, intense bursts of power and speed, which are characteristics of fast-twitch fibers, especially the fast glycolytic type.
4. Predict the muscle fiber composition in the gastrocnemius muscle: Based on the demands of their respective sports, an elite sprinter would likely have a greater proportion of fast glycolytic fibers in their gastrocnemius muscle compared to an elite distance runner. The sprinter's muscle composition is optimized for speed and power, whereas the distance runner's muscle composition is optimized for endurance and efficiency.
5. Conclude the muscle fiber type distribution: Therefore, in comparing the gastrocnemius muscle of an elite distance runner and an elite sprinter, the sprinter would have a higher proportion of fast glycolytic fibers, which are crucial for the short, explosive movements required in sprinting.