In the book American Shaolin: Flying Kicks, Buddhist Monks, and the Legend of Iron Crotch: An Odyssey in the New China, the author describes his exposure to, and training in, some of the so-called "iron techniques" of martial arts. The techniques, according to the author, involve subjecting specific areas of the body to repeated and consistent abuse, while treating them with "medicinal herbal mixtures". The result is that the area of the body can supposedly sustain tremendous stresses, and becomes much more durable and resilient than untrained body parts.
The author trained in "iron forearm", because he feared the disfigurement of joints, and general swelling, that apparently accompanied training in some of the other variants. At one point I believe he mentioned that you could easily spot practitioners of "iron fist" because one of their hands would be significantly larger than the other, and marred by swollen and grotesque fingers resulting from them being repeatedly broken and reset during the course of training.
Other styles include "iron stomach", "iron skull", "iron legs" and even "iron penis". "Iron stomach" demonstrations frequently involve placing the point of a sharp spear against the abdomen, and leaning into it until the spear bends almost in half, without piercing the skin.
These styles also have a history of appearing in various martial arts entertainment movies.
Have their been any studies comparing the reputed strength and durability of limbs trained with these techniques versus the limbs of athletes employing more traditional training and conditioning practices?
Is the reputed resistance to piercing and cutting attributed to skin subject to this training significantly different than the properties of "untrained skin"?
Edit:Here is a video of Shaolin Monks demonstrating one of the more extreme versions of the technique. Is this merely a trick exploiting the natural elasticity of human skin (which is often underestimated), or is this really the result of special conditioning to toughen the skin far beyond norm?
Here is another demonstration that shows spears supporting a man's weight, with a demonstration of the supposed "iron crotch" (although the video looks like the stick may actually miss the monk's testicles, there are numerous references to demonstrations that were too graphic to be easily faked in the book mentioned earlier).
Here is a monk putting the point of a spear against his throat and pushing a van with the haft of the spear.
Here is a demonstration where blunt force is applied to the stomach with no apparent effect.