The knee's anterior cruciate ligament (ACL) can be stretched or torn during activities that strain the knee. A torn ACL may require extensive therapy or surgery to repair it. Here are primary symptoms of a torn ACL, courtesy of the U.S. National Library of Medicine:

-- A loud popping noise as the knee it hit or twisted.

-- Immediate, severe pain in the knee.

-- Significant swelling of the knee in fewer than six hours after the injury.

-- Difficulty walking or standing, and general weakness of the knee.

(HealthDay News)