Physics, Push-Pull with the longsword, bad cuts…. and you!

I’ve always admired how much cutting mechanics seem to stand apart from other aspects of HEMA instruction. Take a dozen instructors and ask them how to do a Krumphau, and you get a dozen different versions; and add a couple more versions to the total if you ask them again a week later. But every cutter who performs at a high level has arrived at the same conclusions on mechanics! This really shows you something about the consistency and reliability of our knowledge about cutting mechanics*.

You can still find the following concept floating around: you should “push forward with the top hand and pull back with the bottom hand” to generate a longsword swing. This (or anything where you use a leveraging action between two hands) is what is known as a Push-Pull Cut.

Such an action is in direct violation of one of the big, fundamental tenets of good longsword cutting mechanics: to create a solid structure with the body. This means that both hands should be ‘locked’ into position while delivering the cut — not moving independently of each other. If you are–like me–the child who asks “why” a dozen times, you might appreciate a little more of an in-depth mechanical explanation as to why this is.

*It also says something about the reliability of our interpretive processes, but that’s a whole other issue….

Follow Through

I don’t have anything really ‘sciencey’ to say here. When you push-pull, you cannot follow through the target properly. Try it yourself, and you will see that you either have to change the alignment of your elbow or release the grip on one of the hands.

Centers of Rotation

One of the first explanations, and one many instructors use, is that you want to use a bigger center of rotation — one centered on the body.

Start by looking at a simple push-pull cut.

If you bought a sword that wasn’t exactly 1m long, then you probably aren’t a true physicist.

Which more or less means nothing, until I compare it with the same rotation, just moved back into the center of the body.

Correcting your arms to be exactly 1m long is true commitment.

You can see that by moving the center of rotation away from the hilt, we made the tip move faster. You can also think of it as moving the tip further from the center of rotation.

This is also the point where people with formal physics training also become a little bit skeptical, because of a little principle called compound rotation.

Wow! Amazing! We just cut even faster!

This seems like a good idea, but only if tip speed is the only factor in the cut*. There are other reasons that this becomes a very bad idea.

* If you actually try to put this into practice you will also find out how difficult it is to combine the timing. Even if we didn’t have the other failures of this approach, I don’t know that I would recommend it.

Body Structure

There are a few things at play when the sword impacts the target:

  1. The energy/momentum of the sword. (Center of Rotation)
  2. The force from the cutter’s body transmitted into the sword. (Body Structure)
  3. The force from the target slowing down your sword. (Edge Alignment)

We’ve already covered the first one, now let’s look into the importance of body structure for the cutter.

When you impact a target, there is going to be some resistance on your blade. This will do one of two things:

  1. Slow the blade down
  2. Twist the blade

Both are bad, but the second is much, much worse. Once the blade is twisted it will:

  1. Slow the blade down at a faster rate
  2. Be prone to twist even more

From this, we can see why a strong structure is important. And there are two main things that compromise your structure:

  1. Having the joints out of alignment
  2. Having the muscles in a weaker contraction

(I’m done chaining unnecessary lists now, I swear.)

Joints out of Alignment

If you are doing a push-pull motion, the joints in your arms are in constant movement. By definition, they are only going to be in the correct position for one moment in time — and in reality it is possible that they don’t even pass through that correct position to begin with.

You might be curious about whether this principle opposes the ‘pull with your pinkies right before impact’ approach. No. When you contract the muscles in your hand, they have reached a static position before you impact with the target.

Muscle Structure

When a muscle is contracting it is weaker than if it is standing still.

Source: Simon Fraser University course slides.

Some terms from the diagram you might not have already known:

  1. Concentric – A normal muscle contraction. Imagine lifting a weight over your head.
  2. Eccentric – When a muscle is lengthened by an external force.  Imagine struggling to keep a weight above your head which is too heavy, and is slowly inching its way down.

(I lied about the lists.)

So if you are moving your hands/forearms/arms when the sword hits the target, your structure is automatically significantly weakened.

Edge Control

The last — and most important — failing of the push-pull cut is edge alignment. You may look at the previous section and scoff. “Supporting a lot of force in a cut isn’t necessary! You don’t need a lot of force to cut.” To counter this, I would say:

  1. You have not read the last section carefully and appreciated how small deviations compound. It is very important to keep the sword as stable as possible.
  2. Your edge alignment will be exceedingly difficult to control.

(Not sorry for the list.)

To quote a historical master* on the subject of moving the arm into position:

“This would not be the case if it had descended by its own movement; for it is impossible to raise and lower it at the same time.”

This is telling us that the arm can only try and do one thing at a time. If you are trying to move the arm as fast as you can to Point B, you can’t simultaneously have the arm tracking to a specific target in the vicinity of Point B. Instead, move the body so that the arm moves towards point B, and leaving the arm free to perform the precise movements necessary.

If the arms and hands are engaged in a levering action of the sword, there is something very important they are not doing:

Edge Alignment

If you have ever cut, or heard anyone talk about cutting, you know just how important this little detail is. And if you are utilizing a push-pull motion, you have not generated a stable platform for the small muscles in your hand** to make the necessary fine corrections to the edge.

* Guess which one!

**Your grip muscles aren’t really in the hand, they are all located in the forearm.

Summary

So in summary:

  1. Push-Pull doesn’t allow you to follow through. You either slow your sword down on contact or wreck your structure trying to keep the blade rotating.
  2. Push-Pull produces a slower tip speed than rotating the sword about the center of the body.
  3. Push-Pull significantly weakens the body structure behind the sword.
  4. Push-Pull makes it almost impossible to have good control of the edge alignment.
  5. Listen to the advice of people who have figured out how to cut well.

(Ok, that was really the last one.)

About Sean Franklin 35 Articles
Sean has a Bachelor's Degree in Mechatronic Systems Engineering, and is currently employed as a Controls Engineer. He is passionate about developing more analytical ways to view sword fighting, wishing to develop evidence based standards for protective gear and rule sets informed by tournament statistics. His martial arts history includes competitive success, medaling in international competitions for Longsword, Messer, Grappling, and Cutting. In addition to competition Sean has been invited to instruct at a number of events across North America.