Striking Bone is an optional rule, and part of Harsh Realism for Unarmed Fighters.
The DR 0 of shins and DR 2 of skulls are tougher than their DR would suggest. When striking unarmed against these surfaces, Hurting Yourself applies. Some fighters avoid this by not striking the skull deliberately. To avoid knocking shins however, get Striker!
References[]
- Martial Arts, p.124