[quote=Schradinger]
Couple things. Strength plays no part whatsoever in how hard a person can hit. The only role strength has is to generate the speed that actually does matter when you're hitting something. The stuff that matters is mass and acceleration. Since Mifune is airborne, all of his mass is going to be used to power the blow, and he has over 1,000 times the mass of a bullet and half the speed. Herriman is getting knocked on his ass (so to speak) no matter what happens to Mifune's hand.
[/quote]

I think you might have acceleration confused with velocity. Acceleration is defined as the rate of change in velocity; velocity is just the given speed of an object traveling in a set direction. 

Physical strength does play a major role in how much damage someone can do with a punch (greater muscle mass equates to an increase in a fighter's body mass and a consequential gain in punching velocity; greater mass and greater velocity results in heightened amounts of kinetic energy generated), but merely being burly isn't going to cut it. If UFC and boxing ring matches are anything to go by, technique is arguably the defining factor, since the individuals that compete in these two combat sports aren't 400 pound Olympic weight lifters, but men and women that hover around an optimal 105-200 pound weight margin.

I'm not a professional by any means, but I do know that a punch's power is deprived (at least) from the mass of the fighter delivering the blow, the velocity of the punch, proper rotation of the hips, and the fighter's footwork. 

If Mifune is airborne and not grounded, where is his maximum momentum coming from? Where's the power? There's no opportunity or even space for him to set himself for a proper hand strike; there's no room for adequate footwork or suitable torso and hip rotation. If Mifune can still fell Herriman to the ground using an attack like that (and if we go by legitimate physics), then Mifune is no longer a speed-based character, but a scaled down hulk that uses a sword for some obscured reason.