I am in no way endorsing this study, as I have yet to read the actual article, but it looks ok at the level of the abstract.
If you research Professor Chaplin at Penn State, he has an interesting article on the subject. He teachers biological anthropology and has apparently been in the MA for along time.
The theory is, as someone earlier stated, that thicker bones and conditioning results in harder blows and less likelihood of injury to the striker. The anecdotal evidence provided by pro-conditioning folks states that they notice their hands don't break as easy when hitting people, and are more effective. This may be true, but it's hardly a substitute for a real study. I have read the main reason for broken hands is because of the hook punch. An ER doctor noticed a specific type of injury from the hook created by the oblique angle of force hook punches create. I would image proper punching technique decreases the chance of injury more than bone-hardening would, and it may be difficult for the experienced fighter to know the difference.
Just trying to think what kind of study it would take to actually prove any of this.
33-100 kick boxers (train mainly with gloves, no makiwara training) enter Kyokushin tournaments. Then we see if there is a higher occurence of breaks in the kb'ers.
Do breaks happen more frequent amongst KK white belts (less time on the makiwara) than on black belts? (I'm only using KK as an example b/c they don't use gloves)
The few serious boxers that I know have knuckles as large as any karate guy I know.
As an aside, bone conditioning sucks. the stupid 'toe tip kick' has my doctor recommending surgery to remove the calcification on my big toes (I saw it on the x-ray). She says that if I keep it up, my toe will be completely rigid in 20 years or less.
I can't tell you what's happening to bones in this sort of thing.
I would think that muscles and bones would work similarly. When muscles heal, they heal through fibrosis, and the muscles become stronger. Bones heal through calcification, by laying down more bone. Perhaps they become stronger?
But, what I do know is, Iron Palm is not a bone conditioning program. It's all about the chi baby! WooHoo!
So even though I'm with you, and don't believe in conditioning the bones by hitting trees or any BS like that, when it comes down to actually working to try to prove something does or doesn't work, the score reads: TREEHITTERS - 1, YOU - 0.