"We utilize movements most representative of those found in nature. They use universal motor recruitment patterns. They incorporate core-to-extremity movement patterns, and they move the body or other objects effectively and efficiently. They are also 'compound' (multi-joint) movements, which cannot be reduced. In other words, no amount of bicep curling will lead to a pull-up." (CrossFit Journal, 2004).
Watch "What Is CrossFit?" Part 3: Compound Movements