Force= Mass x Acceleration

In order to increase your force you have to have either size or snap speed or both. Punching with weights will increase your speed and amount of force you will put on your opposition. Cardio wont improve your punches strength, cardio will improve the amount of punches you can throw before you get tired.

I remember reading somewhere about people who could break cinder blocks and 1 layered brick walls with their hands. Apparently theres this chemical fluid or layer of something that your body has around some bones, and anyway its above the knuckles, and these people keep practicing and training their knuckles but continually punching these objects, later the body rebuilds a layer or 2 over it, and over time your punches are extremely strong due to the build up of layers of this tissue. Kind of like the process of building muscle tissue, if u know what i mean. May seem like bullshit but it does make sense if you think about it.