If you fall on your back, your speed is too fast for your body to keep up, hold still a bit for your body to lean forward and continue running.
If you fall on your face then your speed is too slow for your body, edit around 100 frames before the fall and lean your body a little less
Running/Tricking/Parkouring (and other creative things in TB) is about failing, realize your mistakes and try to fix them, friends. Even if the physic is changed, you can still do it if you know what you're doing.