In the late 19th century surgeon Julius Wolff proposed that bones adapt to the stresses to which they are exposed. Bones which experience continual stresses become increasingly dense. Bones with little or no stress become thinner and may become brittle. This concept is often referred to as body hardening. Parts of the body that receive consistent contact or stress will often adapt and change to better handle those stresses. Please note that this does not apply to physical stresses applied to your brain – you brain will be irreparably damaged from repeated physical trauma.