Being a lifelong athlete can take its toll on your body. By now, you can probably recognize when an injury just needs to be walked off, or if it's something more serious. For competitive athletes, knee injuries are some of the worst there are. It can be difficult to tell if what you felt on the field is worthy of a hospital visit. Sometimes wishful thinking can keep you from going to the doctor to find out what's really wrong. To prevent you from missing what could be serious, look for these telltale signs that your knee injury could be serious.