Originally Posted by Diomedes
But you're forgetting about the delivery of the force. If the box is fairly soft, and partially crumples on impact, while it may still deliver the same amount of force in total it will do so at a slightly slower rate than if the box were completely solid with no "give" in its structure.
Now, it's an incredible over-simplification but it may be necessary to illustrate the point - two guys, same mass, throw a punch but one has a weak arm and one a strong arm. The boxer with the strong arm hits with a very solid, rigid arm that successfully transfers all of his power to his opponent, in an instant. The other guy hits, but instead of delivering all of the force at the end of a solid arm, his weaker arm "buckles", and force is absorbed by his elbow joint, shoulder joint etc.
Of course in reality a great deal more parts of your body than just your arm are important for delivering the force of your punch. There are many links along the chain that could be weak and reduce the impact.
You're right in a way, for all high speed movements the greater the isometric contraction at the distal ends of the link the more the force is transferred to the tendons, which then recoil at a higher speed with higher force (as long as the muscle-tendon unit is stiff enough).