Tuesday, December 13, 2011


"Don't jerk your foot like that." Dance Coach has the frowny face as he watches me do the Evil Step Behind from Rhythm Blues. If I'm not paying attention, when I  bring the front foot up, I do it too quickly (just to get the thing over with) and my upper body will rock slightly in response.

Ah, yes. Move. Smooth. It's more than just style points. There's actual physics involved.

Since I'm a beginner, up to this point, most of my skating problems have been soluble by thinking about my center of gravity.  It's not until I got to the Evil Step Behind that I run into more complicated concepts.

For those of you pursuing Freestyle, I'd better explain The Evil Step Behind. Think of it as a back cross roll, without the back, or the roll. You skate forward, bring the free foot in front, then gracefully (this is ice dance) move it so it crosses behind the skating foot, to the opposite side, and switch feet so the free foot is now the skating foot. You don't 'roll' so there's not any margin of error of skate placement. This is usually the make or break point for adult ice dancers. And it's in a beginner dance. Fortunately, Coach Amazing helped me past this point with her advice to Point.That.Toe. Now, I have to achieve consistency and 'pretty skating'.

What happens when you too quickly move your foot up ahead of you, either in the first step or subsequent steps? Well, there's a disturbance in the Force. And that disturbance is called jerk. I do not make these names up.

What happens when that front foot snaps up too quickly is that your foot accelerates up, then suddenly stops. The rate of change of acceleration abruptly goes from something to nothing. You get a physical jerk, hence the name.

When you stop that foot too quickly the rest of your body has to respond to the resulting release of force. A beginner skater like me wobbles in the upper body to keep from falling. This doesn't happen when you're standing on the ground, but when you're in skates, you're on the rocker. Your stability is lessened and the upper body is sensitive to sudden changes in acceleration. Even small changes in acceleration such as from a badly done step behind can cause a wobble in the upper body. (More experienced skaters don't have this problem, as they have better body control and years of practice. This is a beginner problem. )

The solution to this is to Move.Smooth. The motion of raising the foot from the ice and bringing it to a stop has to be done so that there's a slow change in acceleration. It should be like stopping a car when your elderly great grandmother is sitting beside you. Don't just 'step on the brake' to stop. Plan that stop so it feels like a soft pillow gently being put on a sofa. Same thing with the step behind. Raise it (increase the acceleration from zero to something), then bring it to a stop in a planned manner. Smoothly.

Interestingly, in the early rocket programs of the 50's and 60's jerk caused the destruction of many rockets. Here's some motivational video of rockets exploding.


  1. I love these physics-y explanations. Even though I'm a words person (lit prof) rather than a mechanics person in most things, I find the technical details of skating to be so helpful. This is why I like slow-motion video examples so much.

  2. I'm glad you like them. I've got a few more coming. I'm working on some 3D figure software to make them cool.