Would you rather?

[MadMojoMonkey]

Make your own arms super lightweight, accelerate your fists to ludicrous speeds with your normal strength muscles. Just prior to or during impact, dramatically increase the mass of said, speedy fists.
KE = (1/2)mv^2

The energy in your moving fist scales linearly with its mass. So you speed it up by making it low mass, so it's easy to get high v^2 for your energy input. Then once it's going fast, cheat around the conservation of energy and make it massive. Getting hit by a truck hurts*, even if it's only moving "normal" fist speeds.

*allegedly

[boost]

Make your own arms super lightweight, accelerate your fists to ludicrous speeds with your normal strength muscles. Just prior to or during impact, dramatically increase the mass of said, speedy fists.
KE = (1/2)mv^2

The energy in your moving fist scales linearly with its mass. So you speed it up by making it low mass, so it's easy to get high v^2 for your energy input. Then once it's going fast, cheat around the conservation of energy and make it massive. Getting hit by a truck hurts*, even if it's only moving "normal" fist speeds.

*allegedly

haha, this is awesome.