Knockback - why ST not HP?

[Gollum]

Originally Posted by mr beer

Really it should probably be based on mass rather than either ST or HP. But I can see how that would get fiddly.

Authors maybe chose ST because someone stronger is more firmly grounded.

Martial arts teach how to be more firmly grounded (to avoid being pushed back or thrown down) and it requires muscle activity (feet, legs, and abs).

It's a question of density rather than mere mass. And stronger people are more dense.