I can and have drilled people though phone books with my PE fist, I used it as a Kubotan !!
Years ago the JKA did a study about muscular strenght and effort in strikes ( It can be found in Nakayamas book, Dynamic Karate) and they found that the first 3 strikes of combination were the most powerful, the 2nd being the most, but after that there was a CONSIDERABLE drop in force and speed.