The Physics of Karate Strikes (Journal of How Things Work).
It sounds like it's not a very long ebook (the product description calls it an "essay"), but it's only 99 cents, so maybe it's worth the price.
In recent years, the ancient eastern art of Karate-Do (a Japanese word, literally translated as “the way of the empty hand”) has become popular in the western world. Karateka—practitioners of Karate—often break boards, cinderblocks, and other solid materials in order to demonstrate the strength that their training develops. Much can be said of the history and culture associated with the expansion of martial training, but this essay—it is, after all, a physics paper—will examine the collision mechanics of a hand strike to a solid target like a board.
The Physics of Karate Strikes