Thursday, May 18, 2017

The Physics of the Front Handspring

I’ve been working on my front handspring for a while now. After repeated rounds of getting it, backsliding, improving, and backsliding again, something has finally clicked for me. I figured out the physics of the front handspring and that allowed me to identify what I was missing. I’ve looked online to see if there is a write-up of this anywhere, but I never found anything that really satisfied me. So here it is.

First, some basic concepts. In a front handspring your body is flipping over, so the physics of rotation is important here. There is something called “conservation of angular momentum”, which basically means a body cannot simply start rotating or stop rotating. If you’re floating in space, you cannot induce your body to rotate. You would have to push off of something, or something would have to hit you. Imagine you’re floating in space and you try to do a crunch. You cannot just move your upper body. If you crunch and tuck your chin to your chest, it will pull your feet up. Crunching puts a torque on your upper body, but it puts an equal and opposite torque on your lower body. The net torque on your body has to be zero, otherwise you'd start to rotate.

Let’s apply this to the handspring. You start by taking a hurdle-step and then kicking up with your back leg. The kick with the back leg is simultaneous with your reaching to the ground with your straight outstretched arms. Your arms and back leg should be in a straight line, so you look like one of those toy drinking  birds dipping down for a drink. Okay, now imagine you make the common mistake of reaching to the ground with your hands without kicking up your back leg. You’ve failed to build up proper rotational momentum by kicking up your leg, so you have to “catch up” by kicking your leg up very quickly. By kicking your leg up in a powerful arc very quickly, you are placing an equal and opposite torque on your upper torso and head. (Not quite “equal and opposite” because you’re not floating in space. You have the floor to push against. But your kicking leg is still working opposite of the rotation that you need for your handspring.) This is why you need to keep your arms and leg in a straight line. If you reach to the ground without kicking, or if your kicking leg lags a bit, it will put a torque on your body in the wrong direction when it tries to “catch up.” 

Let me demonstrate this with the help of my friend, fully-poseable Spider-Man.

Here, Spider-Man hasn't kept his leg and arms in a line. He'll have to bring his leg up in an arc to catch up, which will place a torque on his upper body in the wrong direction.

Here, it's even worse. Spider-Man simply reached to the ground. He'll have to swing his leg up even faster, which will put an even greater torque on his upper body in the wrong direction. This torque on his upper body is against the direction he needs to rotate to finish the technique.

When your back leg is kicking high and your hands touch the floor, your front leg needs to push hard. This accomplishes two things that allow you to complete the technique: it raises your center of gravity and it increases the speed of your rotation. When you go into your hurdle-step, your front leg should bend so that you can push with the full strength of your leg. Otherwise, if your leg is already straight, you will only be pushing by extending your foot, which will not generate enough power. See how the front leg is effectively pushing on a lever made by your outstretched arms and leg, generating more rotation.

In this image, Spider-Man is in proper position. His outstretched arms are connected to the floor, that point being the pivot point of a lever. The push with his front leg pushes against that lever, rotating it. The harder the push, the faster the rotation, and the easier it is to finish the technique. If you aren't rotating fast enough through the skill, you will land with your feet in front of you, possibly on your heels. This is a hard and unpleasant landing. Rotate faster (i.e. kick harder), and you will land more softly and on the balls of your feet.

The push off the front leg is probably the most important piece. I hate to say that, because botching any step could cause the technique to fail. But I'm tempted to say that a strong enough push off the front leg will give you enough lift and rotation that you can get away with some sloppiness in the other steps. This was the key failure point for me. Once I nailed this, I was landing my front handsprings consistently. Also the next step, the block off the floor, depends on getting this lunge off the front leg down correctly. Do a ton of hurdle-steps into a handstand position. Make sure that your front leg is bent, not straight, when your front leg lands from your hurdle step, so you actually can push with it. You can even place a yoga ball in front of you and fall forward onto it to get a sense of what coming down feels like.

When you are upside-down, you will “pop” off your shoulders, pushing your hands into the ground. If you had a good running start, this will also increase your rotation. (You can complete the handspring without a running start. But if you have forward momentum, the pop off the shoulders converts some of this to rotation.) Think of a running body tripping over a wire. The wire blocks the bottom of the body while the upper body is free to rotate, generating a face-plant. Or think of someone running face-first into a bar. Their head and upper-body are blocked, while their lower body is free to rotate. Their feet will fly up and they will land on their butt or back. Your block against the floor is accomplishing the same effect. Imagine a powerful wizard picks you up with his mind, holds you in a perfectly vertical upside-down position, and throws you upside-down and back-first across the room. But you manage to reach down to the floor and pop off your hands, generating rotation and landing a front handspring. It’s like being “tripped” or “running into a bar,” just from a strange orientation. The block off the floor converts some of your forward momentum into rotation. 

See Spider-Man below, being turned upside-down and flung toward a wall by a telekinetic super-villain:

He manages to catch the floor and "trip" himself, inducing a rotation. Notice the arrows, which represent his velocity. Initially they are all the same length, representing the same speed. His lower body will be blocked, reducing its velocity to zero. The middle of his body will be roughly unchanged, and his legs will be moving faster. Obviously this will result in his body rotating. Now if  he can stick the landing he can recover and face his foe. This is what the block off the floor is doing for you.

The push off the ground also gives you a few more inches of height, which will give you a few tenths of a second more to rotate through the technique. Imagine a powerful wizard (the same one who tried to fling you into a wall) suddenly bringing up the ground by a few inches when you're trying to do the front handspring. You will probably land awkwardly on your heels with your feet in front of you. The push off the floor gives you two things: more height and faster rotation.

A common mistake is for people to try to tuck forward to spot their landing. This is done after the pop off the hands, while you’re still in the air. At this point your body really is floating in space, so the physics of rotation are very important. Your body has as much angular momentum as it will have for the technique, and you can’t push off anything to generate more rotation. If you tuck your chin and try to “sit up” to spot your landing, it will pull your feet up. You will land hard, with your feet in front of you, and with your knees bent. This is a recipe for exploding your knees, so don’t do it. You actually want to arch your back and tilt your head back as much as possible, which will rotate your feet down toward the ground. This way you can land with your legs straight (well…straighter anyway), on the balls of your feet instead of your heels, and with your feet beneath you. When I do a good one, I feel myself “running forward” out of the technique.

So here is Spider-Man floating in space.

If he crunches his body to spot his feet (similar to trying to spot your landing on a front handspring), it will pull his feet up:

If instead he arches his back and looks back toward his hands, it will pull his feet down. He will land on the balls of his feet and with his feet under him, rather than landing hard on his heels with his feet in front of him. No explody knee-caps:

You can think of this in terms of "net torque has to be zero" or "net motion has to be zero". As in, rotating his upper body one way will rotate his lower body the other way ("net torque is zero"). Or: to push his hips up he must push his upper and lower body down (the net motion of his center of mass is zero). Once you've blocked off the floor, you are effectively floating in space for a few tenths of a second. So the physics of free-floating bodies becomes very important.

There is also a body-mechanics reason for not tucking your chin. It becomes almost impossible to arch your back if your chin is tucked. The arch in your back has to suddenly switch directions for your chin to tuck, so your spine is making an “S” shape. If you tilt your head back through the end of the technique, your back will be able to arch more. Your feet will land underneath you, or at least they won't land so far in front of you. 

I am still no expert at the front handspring, by any means. But I have recently figured out how to consistently land softly, and it's because I finally figured out the physics of the technique. That understanding has allowed me to spot some of my mistakes and fix them. My hope with this post was to achieve some sort of synthesis. I have a serviceable front handspring and a graduate degree in physics (probably a rare combination). Athletes who learn the front handspring hear all kinds of tips, do's, and don't's. Well, here are the why's for those do's and don't's.

"Don't just reach your hands to the ground. Kick up with your rear leg to drive your hands to the ground." Yes, because reaching to the ground and letting your leg "catch up" will fight against the rotation you need to finish the technique. "Don't leap off your front leg until your hands reach the floor, because you'll experience a loss of power." Yes, that's because you don't have a rotating lever until your hands are on the floor. "When your weight is on your hands, don't bend at the elbows." Yes, because bending at the elbows causes a loss of height and fails to convert your forward momentum into rotation. "Don't sit up to try to spot your landing." Yes, because doing so pulls up your feet and causes a heavy feet-in-front landing. Surely most people who learn the front handspring do so without learning the physics behind it. I hope this post will help someone who is stuck on one of the steps. 

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