Ken Roberts

what's here:  secrets - - details - - more ideas

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Actually these ideas are for fast skiing.  Which might be different from joyful skiing, or pretty skiing.  

And these ideas are based on my analysis of the basic physics and biomechanics -- so they might be right for programming a skiing robot, but not the best way to develop the bodily perceptions and visualizations of a fully human skier who must learn to ski in a fully human way.

my secrets

  • There are many "right" ways to skate, on flat and gentle terrain. [ details ] 
  • Side-glide and side-weight-shift and sideways-leg-push are critical for effective skating. This single-ski balance and rhythm-coordination can be learned and practiced without snow. [ details ]
  • I go with the "magic" of skating. [ details ] 
  • Single-Poling rhythm keeps the poling simple enough for non-experts to manage. [ details ]
  • "Gear-shift" is one big advantage of skating over classic striding. [ how to ] 
  • There is no free lunch -- from glide or gravity. [ details ] 
  • There is a cheap lunch -- by using more different muscles. [ details ] 
  • First learn to skate from the hips. [ details ] 
  • Two-phase leg push:  I set the ski down close in underneath and start pushing outward early with my hip abductor and inward-hip-leg-rotator and ankle-pronator muscles. [ details ] 
  • Key prerequisite to use the big knee-extension muscles (quadriceps) in Phase 2 is to get the ankle way back behind the knee. So I set down with my ankle joint deeply flexed with my knee way down + forward, and my heel way back behind.

Not like stepping my foot forward in walking or running. I get my leg forward into the next push by reaching with its hip, not with its foot.

Deep ankle-flex is not the same as deep knee-bend. Can have either one without the other, or both together. Frequent mistake is to substitute knee-bend for ankle-flex.

  • Skating up a hill is a different game. [ details ] 
  • Extra glide time cuts my power and speed. [ details ] 

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Optimal racing technique is not the "right" way to skate.  There are many possible goals for skating:  short-term speed, long-term efficiency, the feeling of glide, the feeling of relaxation, the feeling of explosiveness, . . . 

Climbing up a hill, efficiency really counts, otherwise you're "burned up" when you reach the top, or you "stall out" before.  There aren't many choices for best efficiency.

[ more to be added ]

Actually this doesn't work from a standing start.  The "magic" only happens at speed:  because then most of the power of the push gets sucked into the direction of the speed.  So even though the ski is angled so the force is pointed mostly sideways, the power gets directed forward.

That just happens to be how the physics of power for motion works.  And it delivers wonderful results in skating -- if I give in to its superior magic, and resist falling back into the instinctive static concepts of classic striding.

(Actually in our normal skating we want the benefits of both sideways-push and backwards-push. But pushing backwards with our leg to make our body go forward is obvious and natural -- while the effectiveness of the sideways push is strange and unexpected. So it's a key learning stage to isolate the pure sideways leg-push and let our unconscious muscle control centers recognize it and master it.)

Some people avoid using Single-Poling Skate (or "coaches skate") because they never see elite racers doing it. And double-poling when skating on the flats can be fun.

But when we really need the help of the poling to climb up a steep hill, the coordination with the leg-pushes and other body motions gets tricky to manage effectively. So most skaters who do not have the sophisticated multi-part coordination and upper-body strength of an elite racer are better off keeping it simple with Single-Poling Skate.

I "shift gears" between low-speed and high-speed by choosing the angle of my ski on the snow as I push it partly sideways.

The more I aim my skate-push purely backward instead of significantly sideways, the more I fall back into the fixed-gear limitations of classic striding.

(On flat terrain, aiming the pole-push at an angle to the side doesn't help with gear-shift, because the two-dimensional plane of the poling motion is basically vertical.  To shift gears in poling, need to play games with angles in that vertical plane -- see further below) 

Some bodily motions may feel like I'm getting something for free -- by "falling" or "pendulum" to the other side, or by "extending the glide".

Physics says that it's true that some of these motions can increase the amount of force in a particular phase of the stroke-cycle.  But they can do that only by robbing power from some other phase -- or by wasting time and slowing down some other phase.

Don't get fooled by the easy promises:  feeling the "glide" or the "fall" can be fun -- but it really helps only comes together with the muscular work of a "push" or a "lift" -- though that muscular work might come at different time than the "free lunch" feeling.  Better to learn which of your muscles can really help you go -- in clever unexpected ways.

The obvious major muscles to use for skating are:

(a) big leg muscles for skate-push:  quadriceps and rear butt (gluteus maximus)

(b) arm muscles for pole-push

But there are some less obvious muscles which can contribute power to skating, if I learn how to use them.  Using these muscles is real work, so not a free lunch.  But there's not much else for these muscles to be doing, so they can help take some load off the obvious muscles which are getting tired, or contribute that extra power needed to make it over the top of a hill.

(c) hip abductor (gluteus medius) muscles -- to push the whole leg out to the side -- to give some power to the "phase 1" of the skate-push.

(d) swing-the-shoulder-sideways muscles in the abdomen:  starting a quick "swing" or rotation of the weight of the shoulders (and chest and head) sideways -- away from the ski edged into the snow -- automatically generates a "reactive" force (by Newton's Third Law) toward the direction of the skate-push (phase 2).  Then after stepping onto the other ski, the stopping of this swing motion generates another "reactive" force to help the next skate-push (phase 1).

(e) abdominal forward-bend muscles -- to add down-force to the pole-push (phase 1)

(f) front-chest muscles -- to add a "crunch" motion to the pole-push (phase 1) -- so it's a forward "curling" of the upper body, not just a "hinging" at the waist.

(g) back muscles -- to lift the weight of the torso + shoulders + head upward (in the pole-recovery phase) -- so it can then be dropped down onto the pole-push.

(h) quadriceps leg muscles -- to lift the hips up -- along with the weight of the entire upper body -- and push the hips on forward, to "launch" the upper body forward (in the pole-recovery phase) -- so the weight of the entire upper body can drop down onto the arms and poles -- to add major force to the pole-push (phase 1).

(i) calf muscles -- to make a "toe-push" that adds some force and range-of-motion to the end of the skate-push (phase 2b).

More?  There could be a couple more muscles that can help sometimes.

Revisit the "obvious" muscles -- how do they coordinate with all these other muscles?

(a) big leg muscles (quadriceps and rear butt) -- are engaged primarily in "phase 2" of the skate-push, while the (c) hip abductors and (d) stop-the-shoulder-swing muscles give the power to "phase 1".

(b) arm muscles -- most of their push can be delayed until "phase 2" of the pole-push, so that "phase 1" of the pole-push can focus on the abdominal and chest crunch (e) + (f), with extra power from dropping the upper body weight, previously lifted by (g) + (h) muscles during the pole-recovery phase.

Though there are many muscles that can contribute to forward motion in skating in many ways, trying to learn them all at once is too complicated.  For most people it works best to start with the "foundation" -- focus on the hips and below.  First learn how to balance and push effectively with the legs and keep the upper body simple. 

Then add the fancier stuff.  But even later, keep going back practicing to that simple foundation of the hips and the legs.

I can start my skate-push earlier by consciously initiating with my hip abductor muscles.  Its effective range of push is roughly the same regardless of the height of the hip.

The main leg-extension push with the quadriceps gets a longer effective-push range when initiated from a low hip position.  So if the motion-technique includes up-and-down motion of the butt and upper body (like for more pole-push power on gentle terrain), time the main leg-extension so it starts at the down-point (which would come after at least half the pole-push).  If down-motion of the butt has been mostly eliminated (like climbing a steep hill), then the main leg-extension with the quadriceps can start pretty early.

Coordinating with the pole-push:  I'm making a pole-push that is overlapping with my skate-push, I need to be even more careful to delay extending my knee and ankle joints until after much of the pole-push (especially until after any upper-body crunch and butt-drop actions in the pole-push) -- because the extension of the leg naturally tends to raise my butt and upper-body -- and that's in opposition to effective pole-push power (see below under pole-push for up-and-down work).

But pushing out to the side with the hip abductor muscles actually drops the butt a little, so it actually helps the pole-push.  So the "secret" of coordinating the skate-push with the pole-push is to start pushing early with the hip abductor muscles, and delay the extension of the knee and ankle.


(1) gravity increases the minimum force required to keep the skating motion going, and not "stall out".

(2) force in the vertical direction dominates the problem, but the "magic" of the skate-push isn't as effective against this force, because the vertical direction is mostly outside the plane of the skate-push on the surface of the snow..

(3) using body-weight as a trick to maximize pole-push force gets inefficient.  Once I've used my leg muscles to accomplish the critical work of lifting my butt and upper body vertically, the point is to "lock in" my gain.  It's wasteful to drop it down again to add power to my pole-push. 

The key strategies for success skating up a steep hill are: 

(a) Eliminate phases with no or low force.  Instead place each "pusher" into position to start pushing immediately, in a biomechanical configuration that uses the strongest muscles in their strongest sub-ranges.  And as soon as any pusher gets into that position, actually start pushing on it.  (This results in fewer shorter phases -- so what do I do when my stroke-cycle finishes in a shorter time?)

(b) Find ways to more directly lift the weight of my body in the vertical direction -- and then hold onto each vertical gain.  It's still good to use my back muscles to lift my chest and head and then drop their weight onto the pole-push -- but it's inefficient to use my critical leg-power that way in a vertical-focus environment.

This is the main hidden cost of making sure to include some passive glide after each skate-push -- or phases with low work.  They waste stroke-cycle time.  Physics says that if my goal is to go faster, wasting time on phases with low force or no force reduces my power and speed.  Because: 

(1) the time used for those phases results in fewer stroke-cycles per minute, so my average work per minute is lower -- so my power and speed are lower.

(2) I can try to make up for one low-force phase by applying stronger forces with muscles in other phases, to keep the average work output from dropping.  But this puts higher peak-force stresses on those other muscles, so they will fatigue faster and risk injury.  (If those other muscles could handle those peak force levels without fatigue and injury-risk, why wouldn't I already be using them at those higher levels?)

Some say that taking a longer rest gap between harder pushes is a way to improve endurance performance of muscles.  But if that were true, pro bicycle road racers would be using longer crank-arms and pedaling at a slower cadence frequency -- or figuring out how to insert gaps into their stroke cycle.  And elite mountain runners would be doing "moose-hoofs" and hill-bounds.

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more physics principles for ski skating

So dropping my body weight is only worthwhile if it's to directly push the pole-handles down and back.  

On gentle terrain, raising my upper body weight can add to my forward-motion only if done before the start of the pole-push, and in preparation for soon dropping it into the next pole-push. Raising my upper body is counter-productive during the pole-push, and wasteful if it is then dropped without getting applied to a pole-push.

Up-and-down work is not very effective in helping the skate-push, because physics says that the butt and upper body must drop down vertically in order to apply power -- but the big power of the skate-push is from applying the big leg muscles to extend the leg in the knee and ankle joints -- which tends to raise the butt and upper body.

Instructors recently have told me to practice eliminating up-and-down motion when skating without poles -- even though I rarely skate without poling otherwise.  I think the reason is because if my pole-push is overlapping with my skate-push, I need to learn to eliminate the natural tendency of the skate-push leg-extension to raise my butt and upper-body -- because that's in opposition to effective pole-push power. 

The high gear is the more vertical angle (further back), the low gear is the more horizontal angle (further back).  It's the geometry of the triangle that enables a slower muscle speed pushing through pole handle to best match a high forward speed at the pole tip in the snow when the pole is closer to vertical.  

Then at high speed, there's a little "magic" like with the skate-push:  Most of the power of the push gets sucked into the direction of the speed.  So even though pole is angled so the force is mostly downward, the power gets directed forward.

So at low speeds up hills, I plant my pole tips back, and extend the push longer back -- to focus on the low-gear angle range.  At high speeds on the flats, I plant my poles closer to vertical, and cut off my follow-thru near my hips -- to focus on the high-gear angle range.

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see also


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concept words: ski skiing cross country cross-country xc snow skier skiers skis nordic roberts

skating: skate skates skater skaters push glide