I played offensive line in college football and tore my ACL during a non-contact rep during fall camp. Felt a pop when it happened, but was able to finish practice and go about the rest of the day. Next day when I woke up it swelled up and my leg would 'give out' every tenth step or so... I didn't know for sure until the next day something was wrong (though the 'pop' should have been an indication). One of my teammates (also an Olinemen) tore his ACL in high school but played on it the rest of the year as his team was on a state title run. Point of all of this is that (in my experience) is that it's not necessarily a painful injury... just makes the leg very unstable.