Additional information about muscles can be found at kidshealth.org/kid/body/muscles_noSW.html (KidsHealth for Kids) and www.bbc.co.uk/health/kids/muscles.shtml (BBC).

You can learn more about the Centers for Disease Control and Prevention study on stretching at www.acsm.org/publications/newsreleases2004/prep030304.htm (American College of Sports Medicine).

For information on childhood sports injuries and their prevention, see www.niams.nih.gov/hi/topics/childsports/child_sports.htm (National Institute of Arthritis and Musculoskeletal and Skin Diseases) and www.amerchiro.org/media/tips/kids_sports.shtml (American Chiropractic Association).

For an alternative viewpoint on the value of stretching, go to www.mayoclinic.com/invoke.cfm?id=HQ01447 (Mayo Clinic).

Books recommended by SearchIt!Science:

Health Science Projects About Sports Performance — Robert Gardner, Barbara Gardner Conklin Published by Enslow Publishers, 2002. What's the science behind sports performance? The first chapters explain proper stretching techniques and teach you how to do such things as measure your heart and breathing rates. The fourth chapter invites you to explore the effects of exercise, body position, and conditioning on your body, and the fifth chapter discusses proper nutrition and health. Science projects about muscles and how they work are suggested in the sixth chapter. Investigate your reaction time, learn about muscle pairs, and learn what causes muscle fatigue. The final chapters involve exercise and sports psychology.

To the Limit — Jeffrey Crelinsten Published by Harcourt, 1992. Your body carries enough fat to fuel a 67-hour run. So why do we get tired after only 10 minutes? It's because our blood can't carry enough fatty acids to keep our energy level high for that long. Athletes have trained their bodies so that they can endure high levels of exercise. This book looks at the ways skiers, dancers, climbers, and runners have super-tuned their body systems to reach peak performance. Drawings and photos are included to show how the heart, lungs, skin, muscles, and digestive system work during training and competition.

Peak Performance: Sports, Science, and the Body in Action — Emily Isberg Published by Simon & Schuster, 1989. Top gymnast Siri Larsen practices gymnastics more than 6 hours a day to keep in shape, but when it's time for competition, her mental attitude is just as important as the condition of her body. Before Larsen approaches the balance beam, she visualizes herself performing each part of her routine perfectly. For athletes like herself, peak performance depends on many aspects of their lives—the food they eat, the workouts they do, the way they think. This informative volume examines the routines of top athletes, exploring their training techniques, how they respond to injury, and how technology has affected the way that they stay in shape.

ligament A tough band of body tissue that connects two bones or holds an organ in place.

The ligaments of a human knee joint connect the bones of the upper and lower leg and help keep the joint stable. One of these ligaments is known as the anterior cruciate ligament, or ACL.

muscle A part of the body that can shorten or contract to produce movement. In humans and other vertebrates, muscles that are attached to the bones of the skeleton can be moved at will. The muscles of the heart and other organs contract automatically.

Copyright © 2002, 2003 Houghton-Mifflin Company. All rights reserved. Used with permission.