Wow, you guys have dug up an old thread here! The "backpedaling" motion shown in this animation has been much discussed here at EpicSki, both before and since this thread came around (six years ago!). But the concept remains one of my favorites. I built a new version of the animation, with twice as many frames for smoother motion (the little bobble of the "wheel" is an unintentional error in the animation):
And CGeib took the idea further with brilliant animations showing what happens when your boots hold your shins too upright or too forward (I'll post links when I find them). Here's the photosequence that seems to have vanished from the initial post for some reason, showing the same movements in real life, with a World Cup bump competitor:
The importance of ankle flexion in this movement pattern has been a point of much contention in discussions past. The animation intentionally shows absolutely none--no dorsiflexion, no plantar flexion--as if the skier's ankles were completely fused. The reason is not
that I don't think ankle flex is important. Quite the contrary--there's no ankle motion here precisely because
I think that the ankle's ability to flex and extend is critically important!
Ankle motion is the key to fine-tuned, precise, and continuous fore-aft balance adjustment. Modern high-performance boots limit the range of ankle motion considerably, but the little bit of motion I have is precious to me. I do not want to use it up unless I have to. So I want to develop disciplined movements of my knees, hips, spine, and arms--shown in the animation--that keep my ankles free at all times.
While it is popular to advise dorsiflexing the ankle (pulling the toes up toward the shins) when coming into a bump to pull the ski tips up, and plantar-flexing the ankles on top to push the tips back down, these are the last
things I want to do. Literally! Once I'm up against the tongues of my stiff boots (Nordica Aggressor 150), that's the end of my ankle motion. They're locked up! Same thing if I'm pressing back on the boot cuffs. The "backpedaling" motion of the animation allows me to keep my legs neutral in my cuffs, maintaining my free ankle motion for subtle fore-aft adjustment at all times, in the troughs, on the front sides, the tops, and the back sides of the bumps.
So I allow my feet to move forward beneath my hips as I rise up the bump, just as you would when ascending the wall of a half pipe. Indeed, the "U-shaped" trough from one bump to another is quite similar to a half pipe, albeit in a much smaller scale. Last spring, I think, this topic came up in a thread about whether you could be in "balance" if your hips were behind your feet. I envisioned two half pipes right next to each other, and drew the following animation to suggest how you'd have to move to go from one to another, in balance, as an exaggerated version of the move in bumps:
I was delighted when Keystone put a feature in our A51 terrain park that required exactly this move this season. It's basically a quarter pipe with a horizontal rail across its top and a steep, rounded backside--just like the crux move in the animation above. I asked one of Keystone's freestyle trainers, Ben Atkinson, to ski over it for my video camera. Here's the result:
Note how Ben pushes his feet dramatically ahead off him as he skis up the front side. But once his feet get to the top, he holds them there, as he allows his body's momentum to carry him over the bar. It is the relative position of his body over his feet and his patience to let his body pass over his feet--not
dorsiflexion or plantar flexion of his ankles--that tips his ski tips up, and then down the other side. Only when he's in front of his feet again does he let them dive down the back. Look at the similarity of Ben's movements and those of the skier in the bump photosequence above.
So the body (center of mass) moves more-or-less continuously down the hill in bumps, while the feet move quickly forward beneath it through the troughs, then they "wait" for the body to catch up and pass them at the tops of the bumps. That's the "backpedal"!
Any way you look at it, it's clear that fore-aft movements are at least as important as up-down movements when absorbing bumps. The common image of "knees moving up and down like pistons" really doesn't describe the motion. Anyone who tries to keep "hips over feet" (an unfortunately common bit of misadvice) may not give himself to permission to make these critically important movements in bumps!
And while these movements are most obvious in bumps, the very same principles apply in all high-performance turns, in what Ron LeMaster has nicely and accurately dubbed the "virtual bump" effect.
Contrary to popular opinion, allowing the feet to move forward and back beneath the hips, and allowing the ankles to both flex (dorsiflex) and extend (plantarflex) as the situation demands, are critical in skiing!
These are controversial points, but when you look at these images and think about them a bit, I don't think you can deny the conclusions!