Infinity Ward Dev Writes: One of the most important pieces of the Call of Duty: Modern Warfare game-creation puzzle; an aspect of the game that bridges the new engine, art, narrative, design, and even the performance capture, is the believable animations the team at Infinity Ward aim to achieve. How the various characters in the game move and interact is of paramount importance to creating both an entertaining and immersive game.
Source: N4G PC Call of Duty: Modern Warfare Detailing Advancements in Animation and Authenticity