Karate
Karate is a Japanese martial arts created in the early 20th
century. This style emphasize on punches, kicks, knees and
elbows for defense and counter attacking. Their style are more
known for their punches. Karate Punch
https://www.youtube.com/watch?v=0nJsti-ZzXI
What makes a punch strong depends on the force they can generate. The expression for this is Force = (mass X acceleration)
The way karate martial artist utilize this expression is by pivot their torso and shift their weight from one leg to the other to maximize their mass of the punch while maintaining their stance. They also rotate their fist 180 degrees as they swing their punch as shown in the picture above to increase the acceleration of the punch, resulting in a higher force.
The key to breaking a cardboard or concert is the force applied. When analyzing Newton's third law every force has an equal and opposite reaction we can then tell that when a karate master hits a concert brick it will exert the same force back at him. The question is why doesn't his hand break and only the brick?
The answer to that is when he breaks the brick the force that was plus to be redirect to his hand from the brick according to Newton's third law changed. As soon as the karate master breaks the brick with the force he applied his momentum traveled threw and so the force that the brick did to his hand was only a little because it shatter before it could apply the equal and opposite reaction. Momentum is always conserved. If however, the karate master hit the brick and it didn't break then the brick will redirect the full force back and potentially breaking the the bone depending on his bone structure.
Like most martial arts a common practice is to break cardboard and concert. What this does over the years with any type of martial art is harden the bone by a process called Cortical remodeling or wolff's law. This process happens when someone hits a hard object and does damage to the bone causing a micro fracture. Micro fracture are small cracks in the bone which in time will heal up and become stronger. This is a way to say that training pays off because it reduces the pain you feel when hitting or breaking a object with your fist.