Testicle,Substance P,Wolff's law and Pain

From twitter, i knew these videos.
It is funny and let me think more about pain.

What is the pain? Where is the pain?...it is easy to question,but difficult to answer.

At last, pain is the gift.

Sports Science: "World Record Kick to the Groin" Part 1

Sports Science: "World Record Kick to the Groin" Part 2