The Myth of the Mat Slap
“In my Aikido class, sensei requires us to slap the mat with our hand when executing a forward roll. He says this helps dissipate the force of the roll. Do you do this?”
Chongdo students are expected to execute fast rolls onto a variety of surfaces without the use of hands, and roll back to their feet. If their hands are free, they serve only to guide the torso into position, but this is not required except for beginning students.
“Sensei cautions us that we will be hurt if we don’t slap the mats, because we will land harder on our sides or back.”
Many Japanese-influenced systems promote this, such as Aikido, Judo, and even Hapkido by extension. However, when the Aikido student is thrown while his hands are tied up by the thrower, he does not slap the mat; yet, he is not injured. How then does slapping the mat prevent injury in one version of the technique, but not in all?
The reality is that slapping the mat with one hand during a breakfall or diveroll serves no practical function, but only—as we’ll see later—more of a psychological one.
Many systems do not slap the mats (or more importantly hard concrete) when executing these techniques, further testifying to the mythology of this practice. So what’s the reason? If you don’t care for math, don’t worry: despite the formulas that follow, we are taking shortcuts through the math.
A diveroll, or a tumbling breakfall, is designed to prevent the injury caused by a body (yours) from slamming into the ground. “Slamming” means that energy is being released, and it’s this energy that does damage to your bones and internal organs. When an opponent flips or throws you, he is trying to injure you by dropping your mass from a height. According to the laws of conservation of energy, that formula is well-known as mgh = ½mv²... which is “your mass, times gravity, times the height you’re dropped” is the same as “half of your mass times your speed times your speed.” This is the same formula you would use to study how an egg, dropped from a height, splatters on the floor.
It’s a great idea to throw you, because there’s nothing you can do about your mass, gravity, the height he drops you from, or the speed at which you hit. There’s no way you can change those variables. Or is there?
Here’s why a diveroll works. As he flips you, you roll across the ground and (in theory, but not always) come back to your feet. Why? Because that circular motion supplies a variable you can control.
The energy of a mass rolling across the floor is described by KEr = ½Iω². This is means that “the energy of your roll is equal to half of your moment of inertia times your angular velocity times your angular velocity.” Here, angular means “rolling”.
That’s the formula, but it still does not quite explain why rolling out of a flip or throw changes anything. Instead, we need to look at what goes into that piece of it. For that, we will dissect the concept of angular momentum. This is what helps drive that new equation.
Ordinarily, momentum is simply mass times velocity (p = mv). But in a rolling object, you have to factor in the rolling motion. Angular (that is, rolling) momentum has a different formula: L = mr²ω, or “your angular momentum is equal to your mass times the radius of the roll, squared, times your angular velocity.”
This is the crucial difference. As before, you can’t change your mass. You also can’t change your velocity, because he is controlling that when he throws you. But you can change the radius of your roll!
That’s all a rolling technique is: he he throws you, he places his body against yours and wheels you over. There, the radius of that throw is very small: either zero (in a hip throw) or about the length of a slightly out-stretched arm (in an Aikido style throw). But by increasing your radius by rolling (your circular-shaped body is bigger than zero or an out-stretched arm by far), you change the variable.
Consider an ice-skater spinning on her skates. Crouched into a ball, she rotates rapidly. But when she extends her arms and stands up, her spin slows down dramatically. Identical principle: by increasing her radius by stretching out, she slows down the spin. Similarly, by relaxing your body and rolling out of it, you counter the velocity of his throw.
And by slowing your velocity, you dramatically lower the overall energy with which you hit the ground. That is how the technique works.
“So what does slapping the hand do?”
Nothing. It makes a louder noise on the mat, but it does not dissipate the energy: your roll does that. One explanation often given is that cupping the hand, in particular, provides a cushion of air that helps absorb the impact. You are welcome to try that technique on solid cement; however, you will likely find that any cushioning effect will be an insignificant percentage of your total mass. The amount of force generated by a body hitting the ground is about 2000 Newtons. The force generated by a short, cupping slap to the ground is tiny. Plus, there is another problem with that theory.
The hand slap is a separate event in terms of physics. In other words, the opponent flipping you onto the ground is one set of equations: the hand slap is a separate set unrelated to the first. The net effect is no different than you laying on the ground, raising your hand a foot or so off the ground, and slapping the mat. It’s basically your elbow and wrist working together to strike the mat. It has nothing to do with the rest of you.
For an analogy, the effect of slapping a mat on a throw is similar to you knocking on a door while running past it. Just as the knock won’t slow you down, neither will the hand slap. Again, this is evidenced by the large number of styles and systems which do not slap the mats and the practitioners suffer no injury.
“I tried this a few times at class, and it seems that it hurts less when I slap the mat. If your math is right, why am I feeling the impact more?”
This is the psychological effect of the slap. This is not to dismiss the effect as imaginary. Indeed, the effect is likely very real. However, what’s happening is that you are making a conscious effort not to slap the mat, which means you’re keeping your hand tucked closer to your body. By doing that, you’re tightening the radius of your roll—which means you are hitting the ground harder. In other words, slapping the mat has become a habit for you, in that it reinforces time and again the need to widen your radius.
Instead of fighting the urge to slap the mat, simply relax it. Instead of a vigorous slap, try a gentle pat or even a brushing motion with your hand. You’ll eventually replace the mat slap with no special motion at all, and retain the same skill at countering throws as you always have.