You'd have to use a foam that wouldn't actually harden, I think. Something that could compress and bleed off the energy, rather than transfer it. It would basically mean the difference between punching a wall where there IS a stud, or punching it where there ISN'T.
If there's a stud there, it's instant decel, and it freakin hurts. If there's no stud, the drywall gives some, and deadens the hit. Doesn't hurt.
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