The science of "blocking" roundhouse kicks: Part 1

Here are a couple of questions that were recently posed on the Traditional Fighting Arts Forums by "Kframe".  I thought it would be useful to post them here along with my answers:

"Ok, now here is a great question with regards to the round kick and movement.  We all know that moving up the circle past the apex will cause the round kick to lose a lot of power. How far does that movement have to be fore it starts losing power? How much power does it lose in the first few inches past the apex?"

A roundhouse kick loses force exponentially the moment it passes the apex.  How much is lost in a few inches?  It isn't easy to say definitively, but I'll give it a go in a minute (I'd need my brother to do some more precise calculations).

Let's just say that even a few inches and I think you'll have lost more than the kick was worth.  That's exactly why the Muay Thai defences below involve body shifting sideways (ie. "taisabaki" or "tenshin" in karate): while the movement sideways only takes a few inches off, it is enough to make it possible to absorb the kick without injury.

So the short answer to: "How much force is lost in a few inches?" is this:
"Enough."

The long answer is this:

I realise that in this picture the "50 cm" distance is
probably inaccurate - but you get the idea!

I'll assume for a moment that the kick loses 100% of its force after 50 cm (which is, I think, not an unreasonable assumption - based on my own experience).

Note that I'm talking here about useful force.  I know that your leg would still contact, but at 50 cm or so after the apex where you meant to hit with full force, I think it is a safe bet that your kick has become functionally useless.

This means that for each centimetre after the apex, it loses about 2% of its force.

One inch = 2.54 cm.  So 3 inches = 7.62 cm.

Which means that:

3 inches after your apex, the kick has lost approximately 15% of its force.

The body movement in the Muay Thai block equates to approximately 30 cm.  Which means that by the time you've moved into position to absorb the kick, the kick has lost as much as 60% of its force (on my rough calculation anyway).  No wonder the "block" works!

And no wonder that arts like karate stress so heavily taisabaki/tenshin (body movement/evasion) of this "upright" kind (cf. bobbing, weaving, ducking etc.).  Civilian defence arts try to take the "sting" out of circular attacks - which are the most common civilian attacks used by humans in any era and any culture.

"Same question in reverse, say we step in to the kick, how much power does the kick NOT have at a given distance before the apex?  I watched my friend do his round kick defense again, and he stepped left, into a front stance. That movement alone covered nearly a foot of distance from his starting position. That distance was covered going into the arc of the kick. Which means he would have intercepted the kick with his "universal block"(what its called in kenpo, I have no idea what that gator mouth block is called in karate) nearly a foot before the kick reaches the apex. (a credit to his great execution of the front stance, imho.)    So basically how much power does the kick lose if I move in to it a given distance?"

How much "power" does the kick lose when you move in?  Not nearly as much as it loses when you move away.  This is a fairly simple equation:

As an example, if your kick travels 1 m, and you move in to the kick by about 20 cm before the apex, the kick will lose only 20% of its force.

On the other hand, at 20 cm after the apex the kick would have lost approximately 40% of its force, based on my previous calculations - ie. twice as much.

It's a question of how much room the kick has to accelerate.  Moving in reduces that acceleration room, reducing the impact speed - but all via a rather linear equation.

So, by way of direct comparision, this means that:

3 inches before your apex, the kick has lost only about 7.62% of its force.

So if you move into a roundhouse, you face a kick that is potentially twice as powerful as the one you would face moving away.

On the other hand, if you move in early, you might jam the thigh and negate as much as 95-100% of the useful force - where if you move away early, your attacker might have time to alter their target point (the apex) making you face 95% of the force after all!

It's all in the tactics and what is best at a given time!  In Part 2, I will address some of these tactics from traditional martial arts - tactics that rely upon the science to which I refer above.

In the meantime, I note that Chuck Norris would always impact his roundhouse at the apex!

Copyright © 2013 Dejan Djurdjevic