Here's something I wrote a while ago:
The energies of a punch – Why you can never be 100% efficient
When you punch a target such as a focus mitt where does the energy come from and where does it go?
Chemical energy
The cells in your body convert the chemical energy derived from the food you eat into mechanical energy, via the muscles, into moving your punch to its target.
Mechanical energy
The punch is hurtled towards the target and energy is transferred to both the air surrounding the punch and the target that you hit. This energy is never fully transferred to the target. An insignificant portion is transferred to the air which is why you get a cool breeze when someone throws a punch at you. The key to a good strike is to transfer the maximum amount of the mechanical energy to the target. Some of the available energy is lost to the striking tool and the motion of the target itself. The arm is not a completely rigid structure and when punching the arm will stretch slightly, it has been determined, by whom I do not know, that the elbow joint can actually separate up to 2cm during a punch which is why it is advisable to keep the arm slightly bent when punching and tensioned at the end.
When hitting a solid target with the punch the skeletal structure will compress like a spring and so will the target. When the target is a focus mitt the fist will have a stiffer spring than the target but will be the other way around when the target is stack of boards (no spacers). This is why it hurts more to hit a stack of boards, especially when they don’t break, than it does to hit a soft target. It is the compressive forces on the target that is the key to hitting hard. Some of the mechanical energy is transferred to the target by making the target move. The more the target moves the less impact it receives, this is why boxers try to ‘roll with the punches’ when they get hit. Some of the mechanical energy is returned to the striking tool in the form of rebound, this is what occurs when you kick a solid target with a side kick and your leg is still bent. Any energy that does not get transferred to the target is wasted.
Sound energy
When the target is hit a thumping sound is often heard, this is energy that is wasted that is not transferred to the target.
Heat energy
When mechanical energy is utilized some energy is always lost as heat, when the hand is used to strike a focus mitt, especially when using knife hand strikes for several minutes you feel your hand getting warm. This because of 2 different mechanisms; some of the heat energy is produced by the body, as can be seen when your body temperature rises when you exercise. The other source of heat energy is from the conversion of mechanical energy in the focus mitt and the hand during the impact of the strike.
Summary
So in summary the mechanical energy of the punch is produced by the chemical energy produced by the body. The energy lost during the punch hitting the target is by the air resistance, the compressive forces on the hand and arm, the motion of the pad after it gets hit, the sound it makes and the heat produced. Therefore it is impossible to transfer 100% of the energy of the punch to the target..
There are ways however to maximize the compressive forces that can be delivered to the target. By striking fast you can minimize the movement of the target, this is how boards held in the fingertips can be broken without the board flying away intact. You can also support the target from behind, such as with the elbow strike in Yul-kuk hyung where the palm is placed behind the head while the elbow strikes the head from the front, because the head is not allowed to move maximum force is directed against it. Another way is to stiffen the spring of the hand by conditioning the arm and the fist. By conditioning the fist you are strengthening the bones of the hand because of Wolff’s law. By strengthening the muscles of the arm and tensioning at the right time you minimize the compression of your arm and maximize the compression of the target. So although you can never deliver 100% of the power to the target you can maximize it with proper training.
