Science concepts: impulse, torque, friction
Taking people down to the ground is an essential part of most martial arts. To train those techniques properly, our training partners need to know the safest way to fall. Falling well is particularly important in grappling heavy arts like Judo. Some Judo schools will spend upwards of 30 minutes per class working on this skill.
Over my years of training, I’ve been told and/or led to believe things about proper falling technique that just aren’t true or are half true. I want to address the science behind the principles of breaking your fall.
Here I take the term break fall to mean any movement meant to reduce the painful impact with the ground. A fall could happen as a result of a misstep, slip, trip, push, takedown, or throw. I take this to include falling in place as well as rolling out (ukemi in Japanese martial art vernacular).
Getting hit hard is not fun. When it’s the ground that’s doing the hitting, there’s not much you can do to counter it (gravity being a physical law and all that). What you can do is break the impact up into smaller pieces so that no one piece actually inflicts injury. This is largely a matter of impulse, which I’ve described in a previous post on dealing damage with strikes. The same physical principles that apply to dampening impact from fists apply to the ground as well.
Another principle of break falls is to make contact with the ground using parts of the body that are less likely to be injured. This generally means that you want to use soft, muscled/fatty (depending on your body composition…I’m not judging) parts of the body rather than bony parts. This, more or less, is also because of impulse. The more time that any given impact takes to change your momentum, the less painful it will feel. In practice, this tends to provide several guidelines like “don’t let your head touch the ground” and “don’t roll along your spine” and “don’t ram your elbow into the ground”.
Let’s talk specifically about rolls since they’re more complicated than the “regular” breakfalls. For now, I’ll assume that you have some amount of forward momentum (i.e. you’re not falling straight down). This could be as a result of some kind of throw like a Tomoe-nage (otherwise known as the Captain Kirk throw). If your body makes a single impact with the ground and then stops, then you take all of the organ-shocking impulse in one shot…not a good thing. If you could cut that in half and take two impacts it would be better. Keep cutting the impulse down with more impacts and you might just save yourself from some serious injury.
The ideal we’re shooting for is a wheel. A wheel has a (virtually) infinite number of impact points with the ground as it moves forward. The human body doesn’t really resemble a wheel (at least most people’s bodies don’t), but we can try to approximate it. This is traditionally done by using the following body parts, in succession, as impact points: hand, forearm, upper arm/shoulder, upper back, middle back, lower back, glutes, foot, foot (the feet go in succession as well). The back should be relatively round compared to the arms. So, you get 8-10 contact points with the ground. Now we’re talking. Ten 10% strength impacts are definitely preferable to one 100% impact.
Realistically speaking though, not every point of impact is going to take exactly 10% as an equal distribution. Nor would we want it to. The feet are much better at taking impact than the hands are. The glutes, being a large muscle, are good for taking impact, but probably not as good as the feet. What we want is for each point of impact to take a small enough impact to avoid pain while dissipating the entirety of the impact. That might mean putting 5% on each of the hand, forearm, and shoulder, then 15% on each of the upper, middle, and lower back parts, then maybe a 20% on the glutes, and 10% on each foot. (These numbers are just off the top of my head…not a goal to shoot for.)
Assuming you know what your impact distribution should look like for your body (maybe your hands are super-awesome at taking impact compared to the average person), we can break the fall down into the appropriate pieces.
Like most of moves in martial arts, it will be helpful to break things up into vector components to see the effects. There are a number of different scenarios to examine. First, let’s look at what we don’t want to happen.
Though the motion of your center of mass is taking a parabolic trajectory through the air (by nature of the constant acceleration due to Earth’s gravitational pull), your instantaneous direction of motion is what to be concerned with.
|The thin black line represents the ground.|
The thick black line represents the trajectory of your center of mass.
The gray arrows represent your instantaneous tangential velocity/momentum vectors.
|The blue line represents your forward most support.|
The red and green are the components of the momentum relative to the support.
The particular part of this that is important is the point on the ground that is directly in front of your instantaneous motion vector. In relation to the picture above, this would be the point that the gray lines hit the ground, should they extend far enough. Why is this point important? Because if our contact point with the ground is ever beyond that point, then our motion will provide a downward torque rather than a forward torque. This is bad. Have you ever attempted a breakfall roll and ended up jamming your shoulder really badly or have your body crumple into the ground rather than roll nicely? It’s probably because your contact point got out ahead of the motion vector-ground intersection point. By putting that support point so far out, it causes a portion of your motion vector to torque down towards the ground. Sometimes, if the points aren’t too far apart then your momentum will overcome this downward torque. It still won’t feel comfortable on whatever the next joint up your body is. [I opted to use drawings for this rather than get a picture of me doing it because the potential injury just isn’t worth having the real picture…just take my word for it or try it out yourself.]
Now, let’s consider the situation in which you put your support point somewhere between the motion-vector-ground intersection and the spot on the ground directly underneath your center of mass.
This is going to have a different torque than the previous situation and certain effects on your vertical and horizontal motion. Let’s break down the motion vector into components using the support point as our rotation point.
|Red and green are the component vectors, just like above.|
This shows us that we will torque forward rather than down. This is good, though the portion of the vector that goes towards torque isn’t huge here. Much of the impact is being taken on this support point. We could do better.
What if you put the support point behind your center of mass? You’d end up with something like this:
This has a much bigger portion of the motion going towards torque, which is good…sort of. The problem now is that the torque vector is angling you down at the ground again! This support point does some of its job, breaking down the fall. Where the support point that was too far forward took too much force at once, this point takes too little. There’s a big portion of the impact still left over for the next support point. Experience tells me that it’s either the shoulder or lower back that will usually suffer in these situations. We could do better.
We’ve tried reaching way out. We’ve tried reaching out in front of our center of mass. We’ve tried reaching back behind our center of mass. How about right under it (there's a Goldilocks joke in here somewhere...)? We end up with a picture like this:
|This article is brought to you by the backwards number 4|
This actually looks pretty good. The torque is taking us parallel to the ground (i.e. the non-ouchy direction) and a fair portion of the motion vector that isn’t too big is handled by the support arm. This is the best of both worlds.
There’s another benefit in this situation. The aspect that I’ve glossed over until now is friction. You’ve probably assumed that your support point would actually stay put once placed on the ground. Well, on a slippery surface that might not happen. Imagine you’re doing the roll on wet ice. If you reach out in front of yourself, then your hand might slip forward into the bad range and cause you to fall flat (I’m pretty sure you can see this in action at any mall ice skating rink if you watch long enough). If you reach back, your hand could slip even further back, negating the benefits of the using the support point at all. However, if you put the support point directly under your center of mass, then friction doesn’t even come into play. Well, at the instant of contact it doesn’t come into play. Once the angle changes it will start to be a factor, but it’ll be a very small one. Friction is the force that fights against two materials from sliding on one another. But it’s purely a reactive force. If you don’t try to slide (i.e. you don’t have a horizontal force component), then friction doesn’t show up and isn’t relied upon.
Not to belabor the point too much, but what if my initial assumption wasn’t true? What if you were indeed falling straight down? Putting your support point directly under your center of mass might mean taking the full impact on whatever hits the ground first. In that case, you actually do want to place your support point off to one direction or the other, depending on which way you can more easily make yourself approximate a wheel.
Rather than come up with video of my own to demonstrate all of the myriad ways this can happen, I found a good video on YouTube of a guy demonstrating various falls.
Back when I was in Taekwondo and Judo, we did a lot of breakfalls. When we did them (rolls or otherwise), each fall was accompanied with a loud hand slap on the ground. The louder, the better. There was a direct correlation between the loudness of the slap on the ground and the perceived quality of the breakfall. We trained on fairly soft mats. In the beginning, I questioned the slap, asking if it would work on hard ground. I was assured by brown and black belts that it would. They had years of experience and I was just a kid. So, I believed them and didn’t really think much of it afterwards. I allowed the doctrine to influence my technique.
Fast forward a few years to my Hapkido training. I was at a seminar that was taught by a Korean grandmaster. The guy didn’t speak a lick of English. So, you know he was good (he was actually quite amazing. I can only hope to be that agile at that age.). We were doing seated backward breakfalls to warm up. Now, the dojo also doubled as a dance studio. So, the floors were hardwood, which they had placed some folding mats on for the seminar. I was sitting in the back, on the last bit of mat. I was doing my stereotypical, macho martial arts thing and slapping the mat really hard when I rolled back. A few reps into it, my right hand had drifted slightly out to the right and found the edge of the mat. When my fingers hit that wood floor, the pain was excruciating. The bones hurt. The joints hurt. The skin hurt. I honestly think that having my fingers cut off would have been less painful.
Of course, I couldn’t let any of this pain show. That would have been contrary to the martial arts machismo. However, I never forgot that pain. All of the questions I had asked as a kid had now resurfaced in my, now post-college aged, mind. After the seminar, I asked some of the Hapkido black belts about the issue of the breakfall slap. They explained to me how to properly use my hands in a breakfall (though they admitted that some of the “traditionalists” want to hear a loud slap).
Now, I’ve heard many arguments for slapping hard and loud. One that almost sounds convincing is the one that claims that the loudness of the slap is the dissipated energy from the fall. With that logic, certainly louder is better. It sort of makes sense, but if you look at the people who make this claim you’ll notice that they slap the ground after their torso hits the ground. As a physics enthusiast, this tells me that they’re not dissipating energy from the fall at all. Their adding energy by swinging their arms, which is where the loudness comes from. And besides, anything they do after their torso has hit the ground isn’t really doing anything to protect them from the fall. The fall has already happened at that point! No, if you want your hands to be of any help at all, they need to touch the ground before your body does. This makes sense from the physics perspective. It also makes sense logically.
Friends don’t let friends do the macho breakfall slap. (The video above doesn’t have sound, unfortunately, but I am virtually positive that the guy is falling very quietly. Take a tip from the art of parkour: quieter impact is easier on your body.)
So, here’s the summary of the tips we get from the science plus a few “technique” tips I think will help. First, try to make your body as round as possible when falling in order to maximize the number of contact points and therefore the number of pieces the impact is broken into. Second, when rolling, try to keep your forward-most support point under your center of gravity. For many people, this is approximately the stomach/chest area. Third, to avoid hitting your head tuck your chin down to your chest and put your cheek to the shoulder of the arm that you aren’t rolling over. Fourth, keep your feet tucked in close to your body as long as possible. Many people open up too soon and end up having their knees and/or ankles slam against the ground, which stops all motion. I’d much rather you converted the remaining motion into the horizontal direction so you can walk it off…literally.
Common Mistakes and Fixes
- When rolling, if you go straight from your hand to your shoulder, then it means that your fingers need to point more backward.
- If your elbow hits hard to the point that you feel pain in your shoulder, then you are reaching too far forward.
- If your lower back is sore, that means you need to arch your back more. Make sure to tuck your head really far forward to get an arch in your spine.
- If your feet are hitting the ground hard, stopping you from standing up out of the roll then you need to keep your knees bent and your feet close to your body for longer. Wait until your feet touch before starting to extend your body.
If you’re like me, then you plan on training martial arts for a long time. If you want to prevent unnecessary injuries (as opposed to the necessary ones…) then you need to learn how to fall properly. Our training partners need to be able to practice the full motions of takedowns and throws just as much as we do, and we also need to know how to minimize damage to ourselves when someone successfully executes a move that would put us on the ground. There are only a few scientific principles we need in order to do that. Learn them. Generalize and apply!