ACL Tear Symptoms: Signs, Treatment And When Surgery Is Needed A sudden knee twist while playing sports, running, or walking can cause a serious injury. You hear a "pop." Your knee swells and feels weak. It is easy to overlook these symptoms and expect the knee to recover naturally. https://www.tumblr.com/maheshnadgir/831971012245585920/acl-tear-symptoms-when-do-you-need-surgery?source=share