Between 8 and 12 months of age, babies are in constant motion. They arch their necks to investigate their surroundings when lying on their stomachs and grab their feet when lying on their backs. All this activity strengthens muscles and prepares babies for crawling, a skill usually mastered between 7 and 10 months of age. While learning to crawl, some babies rock on their hands and knees, and others push themselves backward instead of forward (because their arm muscles are better developed than their legs). Some children never crawl, instead opting to scoot on their bottoms or slither on their stomachs. Whether they crawl, scoot, or slither to explore their world, babies are gaining skills and preparing their bodies for walking.
Shelov, S. P. (Editor-in-Chief). (2004.) Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.