This anatomical, mechanical study demonstrated the following: 1. The only selective function attributable to the vastus medialis is patellar alignment. 2. The extensor lag accompanying knee lesions is a function of great loss in mechanical advantage of the quadriceps during the last 15 degrees of the extensor range, a 60 per cert increase in force being needed to complete extension. 3. The clinical prominence of the vastus medialis is related to the marked obliquity of the distal fibers, the lowness of the insertion, and the thinness of the fascial covering of this portion of the quadriceps. 4. Early atrophy of the vastus medialis prominence and loss of terminal extension after injury are indicative of general quadriceps weakness.