With tests to determine the mobility of lower and upper extremities, 139 professional football players were classified as loose or tight, and subsequently checked for the incidence of major knee ligament rupture. Thirty-seven players sustained knee ligament rupture requiring surgery. Thirty-nine players had at least three indices of looseness. Twenty-eight (72%) of these ruptured their knee ligaments. Of the remaining 100 players with two or less indices, nine (9%) ruptured their ligaments. Only two (4%) players in the tight category (no indices of looseness) ruptured their ligaments. Increased likelihood of knee ligament rupture was found with increased looseness. Specific exercises should be designed to increase the strength of loose players and the mobility of tight players. By proper screening, it may be possible to reduce potential disabling knee ligament rupture in loose players in contact sports and in high velocity athletics.