Researchers at the University of Kansas conducted a study in which subjects jumped in the air and a weight connected to their torso was dropped while in the air. The subjects landed on one leg and force, weight on the knee and other measures were taken. The results showed pulling an athletes' jersey from behind resulted in the most pressure on the knee. The findings are useful as contact sports have a much higher incidence of ACL injuries than others and that most such injuries occur when there is no direct contact to the knee made. They also illustrate the importance of building core strength in athletes to help prevent such injuries.