ACL stands for Anterior Cruciate Ligament. It stops the upper and lower parts of your leg from sliding past each other.
A running back is a football player that is deceptively hard to tackle. They are good at "making people miss." They get a defender to think that they are running in one direction, then rapidly shift their speed and trajectory to go in a slightly different direction.
https://m.youtube.com/shorts/0wkXMXbuy-4
This doesn't work without an ACL.
ACL - Another Career Lost
NFL - Not For Long
3/N