However, Oyama did have thick calluses on his knuckles. Callused or keratinized skin is caused by repeated striking or any form of repetitive application of force to an area of the skin. This may be seen by inspecting the area where your pen rests against your fingers when you write. If you write frequently, you will notice a small callus of keratinized skin. The body naturally protects itself if it detects that a part of the body needs extra padding.
What about the supposed bony development of the metacarpal phalange joint of the knuckles? As discussed above, the bone is not malformed but is actually a combination of the thickening of the skin of the knuckles and a thickening of the metacarpal extensor tendon. This tendon covers the metacarpal phalange joints and when repeatedly struck, it becomes thicker. Over time, this tendon may become quite enlarged and it is this, combined with the callused skin, that causes the appearance of enlarged knuckles. When left alone, callused skin will tend to return to its original form.
A traditional karate forging tool is the Japanese makiwara (rolled straw) punching pad. The primary purpose of the forging post is to develop a strong punch or strike and to condition the hand and wrist to absorb some impact so the punch may be delivered correctly and effectively. Working the forging post has the added benefit of ensuring that correct breathing and form are used as poor technique will cause pain. The knife-edge of your hand and the first two knuckles of your fore fist must be conditioned before striking against hard objects. This does not have to be a rigorous process. Just lightly strike these areas against hard objects a few time a day.