The Simple Fist
The most well-known form is the fist - the fingers are all curled in as tightly as possible, and the thumb is folded down over the second row of phalanges.
The striking surface is the major knuckles: the ends of the metacarpal bones of the index and middle fingers (arrows).
It is important to keep the wrist aligned with the two major metacarpals so that the force goes directly through them without any torque or side force being applied to the wrist bones. Generally, when a punch is delivered, the wrist is straight. The simple fist may be delivered two ways: with the wrist vertical (a standing, or vertical fist), or with the wrist horizontal (a flat, or overturned fist). The general consensus is that the vertical fist is slightly stronger biomechanically. However, if the fist is "screwed" into the horizontal position at the moment of impact it can create a vortex or twisting effect through the body structure and fluids resulting in a more penetrating strike.
It is important to make sure that the force of the punch is expressed through the two major metacarpals. If the ring and little finger metacarpals are brought into contact, they may be broken due to their much smaller size ("boxer's fracture").
A major modification is the Motobu Fist.