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Questions

 


Questions

see also Leg motion Questions

Why does V2 run out of gas going up steep hills?

Seems like V2 is the most powerful technique for elite racers on gentle terrain.  Isn't power just what is needed to go up steep hills?  So why should V2 not be best for steep hills also?

What matters for getting up a steep hill is delivering enough power to maintain enough forward speed to keep on skating.  Seems like the more times I get more help from more muscles, the more power I should get.  So two pole-pushes ought to be more powerful than one.  Or at least, if two pole pushes are more powerful on the flat, they also ought to be more powerful up a hill.

The obvious counter to this is that the "dead spot" of passive-glide time needed to prepare for each pole-push on each side slows the skier down more on a hill -- that it's the dead spot that makes the skier work harder to get speed back up again, puts more stress on arm muscles.  The slow-down during the dead spot up a hill is a larger percentage of the skier's average speed than on the flat.  And when failure comes, it comes at the dead spot. 

But in the physics the dead spot is not so important:  

  • The speed to switch from V2 to V1 offset is much higher than the failure point, so avoiding the failure point is not the real problem.
     
  • Each dead spot is less than 0.3 seconds, so for an athletic skier the loss of speed is not much more than 10%.
     
  • The skier can carry extra kinetic energy from the other phases to get through the dead spot.

The more important reasons in the physics are: 

  • Muscle-configuration "gearing":  The poling motion simply delivers less power at lower speeds.  Each motion has a Speed-Power curve.  In Classic skiing, poling is thought of as a "high-gear" motion, best for higher speeds.   The force must go through the shoulder joint, which is a long way from the surface of the snow -- so its leverage is good for converting shorter muscle movements into much longer snow-surface movements.  But this same leverage ratio makes it difficult to deliver high forces at slow speeds.  So their lower power output drags down the skier's average power at lower speeds. [ see more on this ] 
     
  • Pole-angle "gearing":  I can fight this gearing problem to some extent by focusing my pole-push on the "low-gear" sub-range of its motion -- the first part where the pole is angles back more.  But eliminating the first part of the pole-push cuts the range of motion, so I get less benefit from the cost of setting up for each pole-push.  Unfortunately the speed is slowest just at the start of the pole-push, and the start-position is the worst angle for delivering high force at low speed. [ see more on this ] 
     
  • Inefficiency of using leg muscles to drive poling up hills:  In pure double-poling in Classic skiing, using the leg muscles is a major source of the power of poling.  This is done by dropping the hips and upper body down and back to drive the poles, then using the big leg muscles to lift the body-mass above the hips up high in recovery to build "potential" energy to be released in the next pole-push.

One the flats, using the capacity of these "hip-lifting" muscles like this makes sense -- applying vertical work to drive forward motion -- the hallmark of "Big V2".  

But when climbing up a substantial hill, over 80% of the skier's power is applied to fighting vertically against gravity -- not just moving forward.  The skier lifts the hips and large body mass above them, only to drop it down again to drive the poles, then lift it all that again to really finally move that mass up the hill.  Lifting it once is required, lifting twice it through the same vertical range has the smell of inefficiency and wasted energy.  Moving one part of the body down to move another part forward and up sounds strange and like wasted energy.

Better just to lift the body mass above my hips once, and "lock in" that gain. 

Yet another reason why each poling stroke is not delivering as much benefit as on the flats.

  • There's no isometric bent-knee problem for leg muscle lift to solve up a hill.

On the flats the use of leg muscles to raise the upper body also solves a different problem:  the isometric strain of gliding on a flexed knee joint.  At high speeds on the flats, it is difficult to start pushing immediately with the leg on the new ski -- difficult to set the ski down on its inside edge with reliable control and balance. 

So I need to passively glide a little ways on the new ski.  But even though that bent-knee position is optimal the next skate-push, it's very strenuous to hold that position statically until the skate-push actually starts.  So it is necessary for me to un-bend that knee some to get out of that position.  But I can also derive benefit by using that move to raise my hips and large body mass above them, then using that raised mass to drive the pole-push.  This same problem occurs on both sides, so making a pole-push on each side has another reason. 

But climbing up a hill, my speed is slower, so it's easier to set my new ski down on its edge and start pushing on it virtually immediately.   Another reason for making a pole-push on each side goes away.

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How could Open Field Skate be better for high speeds than V2?

Seems like the more times I get more help from more muscles, the faster I should go.  So two pole-pushes ought to be faster than one (or none).

Two reasons why this obvious argument breaks down: 

  • "Gearing" problem:  The velocity of the pole-push is limited by the fact that the pole can only apply force while its tip is stopped on the surface of the snow.  So the relative velocity of the tip of the pole versus the skier's body must be the same as the skier's forward speed.  So the faster the speed, the higher the skier's muscle-speed must be.  At some point the muscle-speed gets less than optimal part of that muscle's Speed-Power curve.  Eventually it the speed gets so high that the muscle cannot deliver hardly any effective contribution to forward motion at all.  The skate-push is not subject to this limitation. [ see more on this ] 
     
  • Pole-angle "gearing":  I can fight this gearing problem to some extent by focusing my pole-push on the "high-gear" sub-range of its motion -- the first part where the pole is more nearly vertical.  But eliminating the second part of the pole-push cuts the range of motion, so I get less benefit from the cost of setting up for each pole-push. [ see more on this ] 
     
  • Single focus on poling speed:  Perhaps by focusing all my pole-push muscles on only one pole-push, with more time for best set-up, and more recovery time in between, I can get less reduction in range-of-motion than with two pushes, and more time in between allows my poling muscles to recover better from the stress of higher muscle-speeds.  So the poling in Open Field Skate could continue to be worthwhile at some higher speeds, when V2 is not.
     
  • Aerodynamics:  The cost of air resistance drag goes up roughly as the square of the skier's speed.  The skier has more air resistance when raise upper body to start the pole-push.  So the air resistance cost, and the time wasted in preparing for the push, starts to outweigh the benefit.

Then at some higher speed, even the single pole-push per cycle of Open Field Skate can no deliver enough muscle-speed and range-of-motion to outweigh the cost of higher air resistance. 

Then the better aerodynamics of the consistently low position of Skate-No-Poles takes over.

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