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? To discover the relationship between muscle-fiber types and performance, researchers obtained tiny biopsies of the gastrocnemius of 14 elite distance runners, 18 trained but non-elite distance runners, and 19 untrained subjects. They categorized the fiber types as slow or fast. (At the time of the study, intermediate fibers had not been identified as a third type.) Some of their data are shown here (* means). What conclusions can you draw from these data?

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1. Examine the bar graph provided, which shows the percentage of slow muscle fibers in three groups: elite runners, non-elite runners, and untrained subjects.
2. Note the y-axis, which represents the percentage of slow fibers, and the x-axis, which categorizes the subjects into three groups.
3. Observe that elite runners have the highest percentage of slow fibers, followed by non-elite runners, and then untrained subjects.
4. Identify the asterisks (*) above the bars, which indicate statistically significant differences between the groups.
5. Conclude that a higher percentage of slow muscle fibers is associated with elite distance running performance, suggesting that muscle fiber composition contributes to athletic success in long-distance running.