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exploiting the head loop in V1 skate
Everybody who's looked at a video of an elite World Cup racer doing V1 skate
(a.k.a. offset, paddle-dance) up a hill has noticed that in a front view the racer's head makes an obvious loop. Today I think I figured out a detailed understanding of how this loop enables them to go up hills easier and faster. And how I had gotten the timing of my head loop all wrong in my own V1 video on Sunday. The explanation is long and tricky . . . There's four parts to the loop, and they're not what I thought they would be. But getting this loop right is like "free gasoline" for my skating up hills. (Even if you don't believe the loop has any benefit, there's an valuable surprise in the fourth phase below.) Here's how I now understand the head loop: V1 skate has a "hang" side (where I push down and back on the poles) and an "off" side (where I recover my hips and poles up and forward). On each side I need to further break down the skate leg-push motion into two phases: the first phase where ski lands in the snow and glides mainly in line (with also a sideways push by the hip abductor muscle), and the second phase where it pushes mainly out to the side (and to the back some too), and also glides in-line. So the leg-pushes of the full V1 stroke cycle have four phases: Hang1, Hang2, Off1, Off2. Here's what I see happening in each one: Hang1 -- focus on pole-push down: Head and shoulders are quickly come to a stop, transferring their previous downward momentum to the arms and poles. Arms move down to the skier's waist -- both transmit the "inertial" force from the dropping head and shoulders to the poles, and add direct force of their own. Hips and knees are fairly quiet in this phase. Some push on the ski out to the side by the hip abductor muscles, which gets the ski more on edge. Hang2 -- focus on leg-push out to side (and back some too): Head and shoulders move strongly _sideways_ away from the direction of the leg-push, and already _upward_ lifted by the back muscles. The _reactive_ forces from this combined move go down and sideways in the direction of the leg-push, so the upper body _adds_ to the force of the leg-push (by Newton's Third Law). Knee joint extends and pushes the ski out to the side with the quadriceps and rear-gluteus muscles. Arms finish pushing the poles backward. Off1 -- focus on lifting the upper body: Head and shoulders move further upward, and rotate back the other way to face the skiers overall line of forward travel. Land the ski with on well-bent knee. Knee straightens to lift the hips and the weight of the entire upper body. The back muscles continue the extension they started in Hang2 and further lift the weight of the shoulders and head (this lifting adds to forward-motion power by building potential energy which is later released in the pole-push). Off2 -- forward fall and leg side-push: Head and shoulders move strongly sideways and then crunch strongly down onto the hang-side, _before_ the hang-side ski lands. Hips stay mostly level. Reactive force from the sideways shoulder move gets added to the skate-push out to the side by the "off" leg. Reactive force from recovering the arms forward adds some force to the backward component of the skate-push. But reactive force from the downward fall of the shoulders _subtracts_ force from the skate-push, so it's better to delay that move to the last part of this phase and then do it quickly. Conclusion: This loop of the shoulders and head enables the upper body to deliver an amazing combination of sideways reactive force, downward reactive force, inertial force, and gravitational potential energy -- to add to the obvious direct pushing forces thru the skis and poles (and minimizes extra load on the primary skating and poling muscles). Comments: (1) My big flaws in my video clips that Sharon took of me skating V1 on Sunday are that I make all my side-to-side shoulder moves one phase too _early_, and start all my up-down shoulder moves half a phase too _late_. (2) The timing of the downward "crunch" by the abdominal and chest muscles came as a big surprise to me. Instead of directly helping push the pole tips into the snow, the crunch force is used to generate momentum in _advance_ in the Off2 phase, which is then applied thru the pole tips during the next Hang1 phase as an _inertial_ force. (3) It's best make the sideways moves of the head and shoulders during the phases when the skis already edged (Hang2 and Off2). Because if the ski is gliding nearly flat (like in much of Hang1 and Off1), then it doesn't to a good job of transmitting the sideways reactive forces to the snow. Note: In this respect skis on snow are completely different from rollerskis or inline skates on dry pavement, which easily transmit reactive side-forces when rolling flat -- or even when rolling on the "wrong" outside edge. That's how I had gotten my timing of my side-to-side shoulder moves all wrong: I figured them out practicing on pavement. (4) Some coaches see the Off1 phase as a waste to be minimized, because it looks like a loss of _momentum_. But actually it's a very valuable phase for delivering forward-motion _power_, but in a clever indirect way. And perhaps it is physically the most _efficient_ phase, since simple direct lifting is an excellent way to apply the big power of the big power to the main problem of hill-climbing: namely moving the weight of the body uphill against the force of gravity. So many new moves to play with -- I can hardly wait to get back on snow. How could the forecast for Wednesday be for rain? Ken |
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