I know only the eyes & testicle is human body parts that cannot be strengthened. But The chi or Ki is the most powerful part of human (you can see it but you can feel it). May be you won't believe yourself can do unbelievable thing after you practise and strengthen your ki. for example: your body hard as rock and can't be chopped with sword, and like kyo or iori you hand not burnt, floating on the water, etc. In india there's a yoga i call it flying yoga. The yoga practitioner when they meditate themselves at the peak of meditation (like dhalsim in Street Fighter). Their body is flying for about 10cm or more, depend skill. Maybe only our ancestor can do that. People like us cant do that, because too many sins, polution in our environment, bad habit, lotta jobs we have to do, etc.. etc. Have you ever heard human can control the clouds, so rain can be controlled? So i believe the fighting techniques and martial arts that shown in fighting games such as King of Fighters is true.