I wanted to mention something like this as a stage trick. I was not sure if it is done globally. An aikidoka friend of mine has formed an O with his thumb and index finger and asked me to pull it apart. He did not seem to be pressing them together hard at all, yet I could not even though I an fairly strong. He explained he was visualizing his fingers turning into an iron ring. I still don't understand how this can work. My best guess is 1) with lots of visualization practice he convinced himself 100% 2) due to his confident body language I doubted myself and my strength was sapped.

EDIT: wait, I found it online! http://bodymindandmodem.com/CoolKi/Finger.html

More tricks: http://bodymindandmodem.com/CoolKi/CoolKi.html

Thanks for the idea, this is what I meant by cool stage tricks.

EDIT2: possible explanation: http://aikidoforbeginners.blogspot.com/2007/06/unbendable-arm.html