Here’s a thing you’ll hear a lot: kinetic energy is mass times velocity squared, so if you can get more mass into your strikes then you’ll have more power. By moving your body into your strike, you have more mass behind the strike. Seems self-evident, right? I actually used this one myself in a lesson just last week, and it’s certainly true, but it’s not the whole story.
Another perspective: when I teach my students how to punch, I normally explain it as establishing structure through your arm and then hitting the back of that structure with your body. I teach it that way in order to make it clear that you need to move your arm before your body, reach full extension at the moment of impact, and that (most of) the power in the strike is coming from your body and not your arm. This is also not the whole story.
If you were leaping into your strike with a rigid arm, the kinetic-energy explanation would be correct. At that point, you’re effectively a thrown brick, and you’re going to deliver precisely mv2 joules of energy. Things aren’t as simple as that in a real strike, though, because your muscles are continuing to generate force up to and even after the moment of impact.
To take a counterexample, consider a simple push. There’s no particular kinetic energy going on here; you’re moving slowly and your body is relatively stationary. You can still generate quite a lot of force, though, by pressing continuously against the target and balancing that force with a push into the ground.
There’s an intermediate stage as well: picture jumping straight up, and at the peak of your leap pushing horizontally against a target. In this case, you’re not pressing into the ground and your kinetic energy isn’t contributing to the push; it’s the inertia of your body plus the energy of your muscles that combine to create net force.
A real strike is a combination of all of these. Thinking about a simple straight punch, you generate force with your muscles into the ground, which moves your body forward. You generate more force into your arm. At the moment of impact, the energy delivered is your body’s kinetic energy plus the “push” through your arm, supported by your body’s inertia plus an additional push into the ground.
Losing any one of these components means that your strikes won’t have the power they need (see my previous post on the principles of power). If you don’t move forward, you lose out on the mass of your body in the kinetic-energy calculation. If you don’t generate power through your arm – i.e., hitting with a rigid arm – you’re losing out on that component. If you don’t simultaneously press into the ground, the power in your arm will go towards pushing your body away from your target rather than delivering that energy where you want it.
The tl;dr here (too late, I know) is “striking is complicated”. You start out by standing still and punching with your arm, and then you layer all these other factors on top over the course of your training. When you start considering strikes more complex than a linear punch – say, a sword cut – it gets even worse, since you’re having to generate power and body movement along lines that aren’t necessarily obvious or convenient. As Mushashi is so fond of saying, “you must think deeply on this.”
Martial Traveler 