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Assumptions + Principles

0 - Set-down phase

1 - Underneath phase

2 - Central push phase

3 - Extension phase


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Assumptions + Principles

V02max capacity versus many-muscles approach

Q: But isn't the true limiting factor on skating speed and power "VO2max" -- the capacity of the athlete's cardio-vascular system to deliver oxygen?

A: Actually for many athletic performances, the work-level most of the time is not done at an intensity that reaches VO2max: e.g. skating a marathon. For performances longer than a few minutes, the ability for specific "peripheral" muscles to avoid lactic-acid-related fatigue is the key to not burning out early -- so spreading the power load to other muscles that are near their lactic-acid "threshold" limit can be a big help.

But even for short high-intensity situations, it's not simply an either/or, VO2max versus many-muscles, because V02max is not a fixed constant. So in training for several months, the more different muscles getting developed to make an effective contribution to propulsion implies greater muscle mass available to impose a greater demand for oxygen during high-intensity short-interval training workouts. Greater total demand for oxygen puts more stress on the central cardio-vascular system, which can be a key stimulus for it to grow higher VO2max capacity.

That's why elite cross-country ski-skate racers often have higher VO2max than elite bicycle racers: The ski-skate racers do a lot of pushing with their ski poles using their upper-body muscles, so they're using more muscle mass than the bicycle racers.

In some actual-race-performance situations (e.g. final sprint), a racer might choose to channel this increased V02max capacity into a fewer-muscle-moves technique, while in other actual-race situations the racer has sufficient V02max capacity to support a many-muscle-moves technique. That's a choice which training with many-muscle-moves technique offers.

0 - Set-down phase

Overlap of two legs.  Not!?

Q: How come some inline speedskaters almost have both skates in the air for an instant between strokes?

A: I don't know. My main theory is that some overlap of the phases of the two legs is good, like overlapping part of Phase 3 of one leg with Phase 1a (or Phase U1) of the next leg. And in the race videos I've analyzed, it seems like 10K and marathon skaters usually follow that theory.

But from looking at some of the 200m and 300m winners at the 2004 inline World Championships, looks like some of the short-distance short-track winners (e.g. Gregorio Duggento and Valentina Belloni) do not overlap their leg-pushes. They even show a little gap in between strokes, with neither skate fully on the ground. It's still a puzzle for me.

emphasize sideways "fall" helps propulsion?

Q: The skater's body tends to fall downward and sideways in phase 3 of the push. If set-down is delayed, the body has more time to fall further and faster. Does this help propulsion? Is it a "free lunch" from gravity?

A: Quick response: My analysis of the physics says: Delay in itself does not help propulsion. Falling directly downward faster doesn't help. It's not the "fall" part that helps, it's the sideways. If keep pushing strongly sideways in Phase 3 of other leg so that the sideways speed of the body increases while Set-down is delayed, that does help propulsion.

It's not a "free lunch": The skater really has to keep pushing with the obvious leg-extension muscles. If fall farther and faster before landing, must land with knee and ankle joints more flexed and absorbing more downward impact -- which puts more strain on the obvious knee-extension and hip-extension muscles.

Falling farther can be a result of pushing through a longer range-of-motion of leg-extension (either by starting from a more flexed configuration, or by finishing in a more extended configuration) -- in which case, provided that the skater pushed with the same average force through that range, propulsion will be increased. Or falling farther can be a result of landing the foot farther out to the side, in which case (other things being equal) it does not help propulsion.

Playing with the feeling of sideways fall and managing the balance of delaying the set-down move can be very helpful as mental image and control sensitivity. But overall gain in propulsion comes only if the imagery and balance result in the skater actively pushing more.

[ more to be added ]

( ?? Is there a way to allow body to fall downward in phase 3 without helping propulsion)

1 - Underneath phase

inward-knee-roll move?  XC skiers versus Ice + Inline

Q: How come elite cross-country XC ski racers usually make a distinct inward-roll-knee move (combined with an ankle-flexion move) when they skate (i.e. knee goes inside the hip-heel-toe plane)?  While elite inline + ice speedskaters make at most a small inward-knee-roll move, none?  Also ice + inline technique coaching shows little interest in the ankle-flexion "knee drive" move.

Non-elite skaters are different:  Many non-elite XC ski racers often do not use the ankle-flexion "knee drive" move, and non-elite ice + inline speedskaters do make an inward-knee-roll move.

A: I'm not sure why. At first I thought it was because XC ski racing tends to require lots of high-force situations (hills and soft snow). But inline skate racers in short distances on a short oval track are also in high-force situations, and they do not use inward-knee-roll.

Here's some possible explanations I've encountered so far:

(a) Elite cross-country ski racers usually ski "classic striding" style as well as skating, and classic striding uses the ankle-flexion move effectively for propulsion, so that muscle set is well-trained for speed and endurance. The inward-knee-roll move enables them to use that well-developed muscle-move for propulsion in skating. While inline speedskaters do not tend to do much "striding" or "running" sports.

(b) Inline and ice speedskating uses the upper body mainly for reactive side-force, so it's important to achieve maximum transmission of side-motion of hips and upper body -- and inward-knee-roll move partly tends to absorb that side motion, or not to create as much of it. While XC ski racers use their upper body mainly for pole-push force, so a slight loss of side-motion transmission is less important for them.

(c) Race courses for elite cross-country ski racers have steep hills on them -- more and steeper than most inline race courses, and most snow is slower than most pavement on outdoor inline race courses. This requires that cross-country ski racers aim their skis out much wider in critical race situations. The inward-knee-roll move combined with ankle-flexion move is more effective for propulsion when the aim-angle of the ski is farther toward the side -- because the ankle-flexion muscles naturally direct their force backward (while the lateral-hip-adductor and inward-hip-leg-rotation muscles favored by inline racers are not as good for directing force more backward). Since the inward-knee-roll + ankle-flexion moves are more successful (and well-trained) for their most critical situations, cross-country skiers tend to rely on them also in other situations even where the ski aim-angle is less.

Perhaps some speedskaters in large aim-angle situations: like the start of a race, use some inward-knee-roll: see Valentina Belloni in her start in winning the 200-meter time trial in the Inline World Championships 2004.

(d) Perhaps the inward-knee-roll move it has something to do with coming down from having the body up high to set up for double-poling in cross-country skiing. In V1 skate, the inward-knee-roll move seems more noticeable on the poling side than on the recovery side. But sometimes racers also do it when they're not poling (perhaps out of habit?).

Is roll-over propulsive?

Q: Ice and inline skaters when doing normal-push stroking usually land with their foot tilted toward the outside, then "roll it over" during Phase 1 so that the foot is then tilted toward the inside. Is this "roll over" propulsive? Is it a pre-requisite "trigger" for propulsion?

A: Quick response:

[ ski: ] On skis, roll-over is important in the physics because the ski cannot transmit propulsive force effectively while its running flat, not tilted toward one side of the other, and perhaps there are frictional costs or control risks if try to do the roll-over while also trying to apply significant propulsive push through the ski. So expert ski-skaters tend to just avoid the roll-over by landing the ski alreadly tilted slightly toward the inside (or perhaps land it flat).

[ inline: ] For inline speedskaters, whether the foot or boot or wheel-frame is tilted slightly to one side or to the other or standing straight vertical has no significance in the physics of propulsion or control. So for inlines "roll-over" is important only as a mental image.

What matters for propulsion during roll-over is the four muscle moves described in Phase 1. If the mental image of "roll-over" helps the skater engage and develop those muscle moves, then it's positive. If it gets the skater not to do any effective pushing until after the foot is tilted toward the inside, then the mental image of roll-over is negative for propulsion.

The key pre-requisite "trigger" for propulsion in Phase 2 is getting the foot out toward the side far enough away from hip, so that there's large enough vertical leg-slant angle to that extending the leg also has a substantial non-vertical component. As long as the foot is near to underneath the hip, most of the force of any push by the big obvious hip-extension and knee-extension muscles will just go straight into the ground, not into pushing the skater forward (or even sideways). The skate or ski can be tilted toward the inside after roll-over as long as anybody desires and can manage the balance, but Phase 2 cannot begin effectively until the foot gets further out to the side. Roll-over is not sufficient.

[ ice: ] The ice skate blade is not as effective for transmitting propulsive force while running flat on the ice, and perhaps there is some control risk during the moment of roll-over. But the ice skate blade can transmit substantial propulsive force through the outside edge of the blade, so there is no benefit to wait until after roll-over to start applying force through it (perhaps think of "pulling" on the blade toward the outside).

So on ice it could be valuable to get through the "running flat" configuration more quickly -- one trick is to use an ankle-pronation move. And of course learn to feel how to best apply how much force through the blade in each configuration before, during, and after the roll-over point. Otherwise the physics is much like for inlines.

[ double-push: ] The physics of the Aim-switch in double-push is a bit different from roll-over in normal-push. See analysis of the Aim-switch phase A.

2 - Central push phase

[ no questions yet ]


3 - Extension phase

No maximum leg extension?

Q: How come elite racers don't make all the moves available for maximum extension of the leg?

Especially glaring is that they often end their normal leg-push with the ankle in a pronated position (bent downward and inward from the hip-heel-toe plane). If they made an ankle-supination move in Phase 3, they could properly straighten their leg, so it would push their heel farther out from their hip, and also push their foot out along the surface of the ground.

Another possibility is that they could get more difference in hip-to-heel distance if they made a combined inward-hip-leg-rotation + ankle-flex move in Phase 1, and then a out-ward-hip-leg-rotation move in Phase 3.

A: There's more to propulsion than going through a "range of motion". I also have to push effectively through the motion. (physical Work = Force * Distance). Adding to my range-of-motion in some direction is only helpful if I have some muscle in a configuration to apply a significant force through that extra range.

If my ankle is pronated after Phases 1 and 2, then an ankle-supination move in Phase 3 will move my heel further from my hip. Unfortunately the force through my foot to the ground from that ankle-supination move is inward and downward. But the propulsive force direction for the Main-push in normal skating is outward, not inward. (Same problem with the outward-hip-leg-rotation move).

How about if we combine the ankle-supination move with a hip-abduction move -- since a hip abduction move in a Phase 3 configuration has an outward and upward component. But hip-abduction has little effective power out that far, and the bad upward component keeps getting worse the further it goes out: it reduces the downforce needed to make the ski or skate grip against the ground, instead of skidding out to the side.

falling helps propulsion?

Q: The skater's body naturally tends to fall downward in phase 3 of the push. Does this help propulsion? Is it a "free lunch" from gravity?

A: Quick response: Actually it's slowing the fall and stopping it which can help propulsion -- and starting the (eventually-required) rise upward again. But starting the fall (and stopping the rise upward) actually reduces the effectiveness of other pushing and exactly cancels the benefit of slowing the "natural" fall, for an overall net zero impact on propulsion.

Any benefit from this "natural" fall is not a "free lunch", since slowing and stopping the fall takes real muscular work, mostly down with the obvious big leg muscles.

It's also possible to have an "un-natural" fall (and eventually-required rise) of mass of only the upper torso and head, by using the back-extension muscles to handle the required rise. This still require accurate timing coordination to achieve any overall non-self-cancelling propulsion benefit.

Playing with the feeling of sideways fall and managing the balance of delaying the set-down move can be very helpful as mental image and control sensitivity. But overall gain in propulsion comes only if the imagery and balance result in the skater actively pushing more.

[ more to be added ]


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