In my horror story, there's a scene where a serial killer, a man in his early 20s with an average build, is chasing down a young girl around the age of 10. The girl is supposed to escape from the serial killer in this scene.
Now realistically, an adult male should be able to easily catch up to a girl this young. In order to make the escape more believable, I wanted to give the serial killer a leg injury earlier in the story. This leg injury would prevent him from catching her easily, while still making it possible for him to capture her.
This leg injury shouldn't be severe enough where he can't properly run for long periods of time, but it should slow him down enough where a little girl could escape, even with some difficulty. He's unable to get this injury treated - both the serial killer and the girl are isolated from the outside world.
What kind of leg injury should I give this serial killer?