Ideas about Ski Skating -- cross country skiing -- Roberts

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Ski Skating in cross country skiing
Ken Roberts

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My main questions and ideas

I'm not sure which I'm doing: V1 or Open Field skate (V2 Alternate)?

My story: I was confused about this for myself for several months. Fortunately it didn't stop me from having fun skating. I thought I was doing V1, but I finally I figured out that all along I had really been doing Open Field Skate ("V2 Alternate"). I think the reason for my confusion is that "V2 Alternate" to me sounded complicated and advanced -- and "V1" sounded simple. Since what I was doing felt simple and natural, it fit for me to choose the term that sounded to me like that.

Hope this helps you sort it out:

If you're making one double-pole-push for each full both-leg-strokes cycle, and it feels like a natural rhythm and even timing on both sides ( push on Left / glide on Right / push on Right / glide on Left / push on Left . . . ), and you never had any special instruction -- then you're probably doing Open Field Skate.

If you feel like you're rushing the pole-push side a bit, and you have to be careful about how you plant your pole tip on your non-push-side so you don't trip your ski on it, then you've probably gotten into V1 / offset.

See the descriptions under "skating techniques and names" for more detail.

The key difference is that Open Field Skate has a distinct glide-with-no-push phase after landing the new ski and before starting the pole-push, so there is a distinct "passive" glide phase on both sides. The main point of V1 offset is to eliminate this glide-with-no-push phase on (at least) one of the two sides.
Another distinguishing feature is that if you find that you're tripping over your pole when you use a normal double-pole-push motion with both hands at the same vertical level and pushing straight back with both poles, that's a sign of V1 offset. If you haven't noticed a problem with tripping over your pole, and you're comfortable starting your pole-push with both hands at the same height off the snow, that's a sign of Open Field Skate. The usual solution to this problem with V1 is to hold one hand lower and plant that pole out to the side -- "offset" hand positions.
It may be possible to have a pole-push timing which is somewhere in between exact V1 offset and exact Open Field Skate.

For non-racers, there is a continuous "transition range" of useful motion-technique variations between Open Field Skate and V1 offset.

On flat terrain with hardpack snow, there's usually a substantial passive-glide phase that can be used to prepare for a maximum pole-push. Up a gentle-to-moderate hill, most skiers shorten the passive-glide phase so they don't slow down as much before they start the pole-push.
As the hill gets steeper, the passive-glide phase gets even shorter -- until there isn't time to get the non-pole-push-side ski recovered back to the center before pushing on the pole -- so the two collide. So the pole has to be planted a little ways out to the side so it doesn't get in the way of the recovering ski -- that's when it gets into the range of V1 offset. When the passive-glide phase is completely eliminated, the non-push-side pole has to be planted way out to the side, and that's "full" V1 offset.

If you watch elite racers going up steep hills, they're almost always doing V1 offset. And on gentle terrain they usually do V2, so it's not easy to find examples of Open Field Skate in videos from a World Cup race or the Olympics.

For them the "transition range" is usually not continous: "big" V2 on the flats, to "quick" V2 on moderate hills -- but then to V1 with no passive-glide and "full" offset poling up steep hills. Elite racers have such stable balance gliding long on either ski, and such strong arms to push on both sides, that they have little use for Open Field skate ("V2 Alternate" / 2-skate).

What is V1 ("offset") actually good for?

My story: Once I figured out that I was not doing V1, of course I wanted to learn V1, but this proved trickier than I expected. Perhaps that's because I made it more complicated than it had to be. But even if you can learn it easier than I did, why bother?

Open Field skate (V2 Alternate / 2-skate) offers a lot of advantages:

the fun of the passive-glide phase (you lose at least one of them)
a little rest interval during the passive-glide phase
time to recover your ski back to the center without tripping over your pole
the full pushing power of both poles
natural rhythm betwee the two sides

Why go to the trouble of learning a new technique that gives all those away?

The big problem with Open Field Skate is that the distinct passive glide-with-no-push phase is a "dead spot" from the perspective of power -- because useful power comes from pushing against the snow with something. No pushing-against-the-snow work, No power. To make up for that no-power phase in the stroke cycle, some muscle group(s) needs to push harder in the other phases to make up for it.

That might be OK on flat and gentle terrain, but going up a steep hill that muscle group is already pushing pretty hard to near their limit in another phase of the stroke cycle. In that situation, forcing it to push yet harder might put the muscle group over its "quick recovery" limit, so its performance after that is reduced.

By eliminating the "dead spot", it spreads the push-force demand more evenly through the stroke cycle, and reduces harmful "peak forces".
Also on a steep hill there's the possibility of "stalling out" during the passive glide phase, even sliding back a little -- which sounds like a bad idea.

So then why bother to learn Open Field Skate?

First, lots of people enjoy gliding with no push. Some say it's the main reason they choose to ski at all -- V1 offset to get up a hill is only a necessary evil until they can get back to that beautiful rhythm of push / glide / push / glide.
And on gentle terrain, Open Field Skate allows a more powerful pole-push to help go faster -- because there's more time to set up for the pole-push.
V2 allows even more pole-push help for going faster, but full V2 requires much better balance, so lots of skaters prefer a solid strong Open Field Skate to a V2 which is precarious and rushed.

Why not use Herringbone to go up hills?

My story: Hanging out with racers, I came to feel that using herringbone to go up a hill was an admission of failure. But when I forced myself to use other techniques, I found that I had to stop and rest before I reached the top -- or that I made it, but my legs were burning so much that I felt slow and tired for the next hour. For me, herringbone works as a way to "skate slow" and not get burned up.

So my opinion is that sometimes I should use Herringbone skate. Or even the Herringbone "walk".

It allows me to operate with lower energy output if that’s what I need for a while. And each skate-push gets immediate synchronized assistance from a pole-push. So if my big leg muscles are burning, this is a way to limit the load on them.

A big disadvantage of herringbone is that it does not provide an effective way to use the powerful abdominal muscles -- versus offset V1 and Open Field skate which do use the abs. (Of course if you have not been training for powerful abs, you may not care about this.)

Also, since there’s no grip wax on skating skis, Skate herringbone is harder than Classic herringbone. I have to angle your skis out further to be sure of not slipping back.

Is more glide good?

See the discussion of "Is more glide good?" for Classic striding.

Many of the arguments also hold for ski skating.

Reactive side-force

What is reactive side-force?

"reactive force" is my term for an "inertial" force: a force on a physical object that results from accelerating (or de-accelerating) the mass of some other object. I call it "reactive" because it arises from Newton's Third Law: "Every action has an equal and opposite reaction".

In a propulsive motion which is repeated, each object connected to the skier must have the same relative velocity (and position) at end of the stroke-cycle as that same object had at the start of the stroke-cycle (which is the end of the previous stroke-cycle). Therefore every acceleration must be paired with a de-acceleration. So all the reactive forces must always come in opposite pairs somewhere in the overall stroke-cycle -- one reactive force from accelerating an object and another from de-accelerating that same object. The two reactive forces in each pair are opposite in direction and separated in time.

Since they are in opposite directions, typically there is no net gain in useful forward-propulsion work over the whole stroke-cycle. Unless there's some clever trick for "losing" one force in the pair, which of course must be the "bad" one which would have had a negative impact on forward-propulsion work.

It happens that with skating motions it is typically pretty difficult to find a clever way to "lose" the negative members of Up-Down or Forward-Backward reactive force pairs.

But with some Side-to-Side reactive force pairs, the skating leg-push motions make it possible to apply both members of the pair positively for forward-propulsion work. And the inclined-plane simple-machine of the skating ski angled out to the side on the surface of the snow provides the "magic" of converting push-force aimed out toward the side into power pushing the skier forward.

Exploiting reactive side-force is an important aspect of the most effective skating techniques.

[ under construction ]

Since all side-force must be transmitted thru the leg muscles, how can reactive side-force from other muscles actually help?

The answer to this question is tricky.

short answer

The muscles of the leg can transmit a wide range of force magnitudes. In normal skating their peak push-force is significantly lower than their maximum capability. So they are capable of transmitting the additional force total from reactive side-force moves by other muscles (such as from torso-swing or arm-swing moves).

But the requirement for the leg muscles to transmit additional force also reduces the power contribution they would otherwise make from their own push-force.

Because they make their leg-extension move with lower relative velocity, and Power = Force * Velocity -- but there's other compensating adjustments (range-of-motion, turnover frequency, set-down-position) they can make so that the Velocity does not have to be too much lower.

So there is some cost to adding force from other muscles. But the extra power from the additional muscles is much larger than this loss -- so you skate forward faster by using reactive side-force (even though it might be less "efficient").

long answer

To make up a semi-realistic numerical model of how much the Power = Force * Velocity changes as a result of adding more force from a torso-swing move is very complicated.

Such a model requires many assumptions and parameters about Force / Velocity / Position / (etc) trade-offs -- perhaps with different trade-offs at different times during the same leg-push motion -- of which it would be very difficult to get very accurate data.

So I'm thinking it's better just to try some personal time-trial measurements to see if adding reactive side-force is making you go faster -- and what sort of adjustments work better for you to get the most out of it.

I find that it helps to shorten the range-of-motion of my leg-extension move -- and select the "straighter" end of this range-of-motion, for best transmission of higher total force.

Torso-swing move: bigger is better?

The torso-swing move produces a reactive-side-force benefit to forward-propulsive power.

But the amount of this benefit is not based on the position or distance of the move. Rather the amount of reactive-side-force benefit is based on the quickness or maximum speed of the torso-swing move. Greater distance in the move should be only a result of focus on quickness and accurate timing.

When non-elite skiers try to imitate the range-of-motion of the torso-swing of elite racers, they often get counter-productive results.

The big trap is that focus on generating and feeling the benefits of torso-swing comes to "drive" the rhythm of the whole stroke-cycle. But torso-swing and reactive side-force must remain in a role of a secondary aid to other moves which are the primary drivers: the legs in V1 and perhaps the double-pole-push in V2.

For more details of the problems, see is NKT a good idea?

Big torso-swing that effectively delivers power because it results from quick acceleration and accurate timing is good. But that only comes from year-round training of specific muscles (e.g. lower back and obliques) and tricky coordination of timing with other moves. It's all different from and irrelevant to any other sport, so most non-elite skiers are not going to achieve the coordination or maintain the special-muscle-training pre-requisites.

Most skiers will do better to keep their torso-swing move limited and secondary.

Torso-swing move: hips and shoulders?

Frequent error:

A frequent error is to rotate or swing or tilt the hips and shoulders together in the same direction.

But actually the two joints have very different roles in the biomechanics and physics of skating. For reactive side-force, the motions of the hips and of the shoulders have nearly opposite impacts on forward-propulsion benefit.

For details see Is NKT a good idea.

Weight-transfer

What is Weight-transfer in skating?

[ under construction ]

Weight-transfer = Falling?

It is true that the skater's hips are usually moving downward at the moment that the old ski comes off the snow and the new ski lands into the snow.

Some skiers and coaches think this has great significance.

So we ask:

Does this falling help weight-transfer?
Does this falling help forward-propulsion power?
Is it a "free lunch" (or even a "cheap lunch")?
Would it help forward-propulsion to try somehow to increase the falling force of the skier's weight onto the next ski?

Analysis of skating with no poles:

The leg-push force always has both an upward component and a sideways component. The proportion of these components depends on the vertical angle of the line from the hip joint to the foot and ski.
In human skating motion, the vertical angle of the line from the hip joint to the foot and ski varies during the leg-push. So the proportion of these components varies during the push motion.
In order to have a repeatable motion, the vertical position of the hip joint must be the same at the start of the stroke-cycle as at the end.
The gravitational force of the weight of the upper body on the hip joint is constant through the stroke cycle (assuming there is no poling).
Therefore sometimes during the stroke-cycle the vertical component of leg-push force must be greater than the gravitational weight of the upper body, and sometimes it must be less than.
In the initial segment of the leg-push, the ski and foot is more underneath the hip, so the angle of push is more upward than sideways, and the vertical component of leg-push force is greater than the weight of the upper body, and the hip joint must rise up.
In the final segment of the leg-push, the ski and foot are way out to the side relative to the hip, so the angle of push is more sideways than upward, and the vertical component of leg-push force is less than the weight of the upper body, and the hip joint must fall down.

Therefore

The observed "falling" of the hips is simply an inevitable result of the fact that the vertical angle of push varies during the leg-push of a human skater (with no poles).

Does this falling help forward-propulsion in direct push?

The loss in potential energy of the weight of the upper body falling is converted partly into a gain in kinetic energy whose direction can be converted through the inclined-plane machine of the current skating ski into forward-motion.
What goes down must come up (in order to have a repeatable motion).
When the weight of the upper body rises up in the initial phase of the next ski's push, its gain in gravitational potential energy is removed from the kinetic energy of the skater's forward motion.
Over the full stroke-cycle with both rising and falling of the weight of the upper body, there is no net gain in overall forward-motion power or speed.
Indeed, because all energy-conversion processes are inefficient, there will normally be a net loss in power due to the rising and falling of the hips and upper body (in skating with no poles).

Does this falling help forward-propulsion through reactive-side-force?

The loss in potential energy of the weight of the upper body falling is converted partly into a gain in kinetic energy of the skier's upper body sideways across toward the next ski, where it can then help the forward-propulsion of the next ski's push.
But the downward fall must be slowed and stopped (in order to have a repeatable motion).
Stopping the downward fall requires that the ski be landed more underneath the skier. So push-force is aimed at a more vertical angle, so there is less push-force available during the initial segment of the leg-push to push the ski away out to the side, and to push the skier's upper body mass sideways across toward helping the next ski's push.
The more the work from "falling" helps the final phase of the previous ski's push and the initial phase of the current ski's push, the more work is subtracted from the final phase of the current ski's push and from the initial phase of the next ski's push. So over the whole stroke-cycle there is no net gain in overall forward-motion power or speed.
Indeed, because all energy-conversion processes are inefficient, there will normally be a net loss in power.

Would it help forward-propulsion to try somehow to increase the falling force of the skier's weight onto the next ski?

No, not likely. Since the net result over the whole stroke-cycle is at best no gain (in ideal conditions), and a loss in power due to conversion inefficiencies (in any real conditions), thus increasing the falling force is likely going to increase the amount of the power-conversion losses.
The greater the force and velocity of falling, the more the next ski must be landed underneath the skier to stop the fall, and so the lower the proportion of force is left-over to push the ski out to the side and back against the snow for forward-propulsion.

But . . .

adding the complexity of poling allows clever combinations of motions which might possibly extract some net gain in power out of the rising and falling of the skater's upper body -- either from the use of additional muscular work, at the cost of something else.
the up-down motion of the upper body may also have associated vertical reactive-force pairs. Normally these reactive-force pairs will cancel each other out over the whole stroke-cycle when skating with no poles. But adding the possibility of "jump-skate" might make it possible to extract some net gain from a vertical reactive-force pair which does not cancel because one of the pair occurs while both skis are in the air. But "jump-skate" has additional performance costs of its own.

NKT a good idea?

"NKT" means having Nose-Knees-Toes aligned vertically after weight-transfer, at the start of the leg-push on the next ski.

It's an old coaching idea, and it might still have some value:

possibly to help beginners to learn single-ski balance?
possibly to help get skiers get started on the torso-shoulder-swing move?
the concept of Knees-Toes alignment is helpful: pointing the knee in the direction of the ski (instead of collapsing the knee inward). Perhaps should change the abbreviation to "HKT", and focus on Hip-Knee-Ankle alignment.

But there are several reasons why the NKT concept is a bad idea:

(1) NKT says that the amount of reactive-side-force benefit from the torso-swing move is based on the position or distance of the move.

Actually the amount of reactive-side-force benefit is based rather on the quickness or maximum speed of the torso-swing move. Greater distance in the move should be only a result of focus on quickness and accurate timing.
Most non-elite skiers cannot sustain their best benefit if they try to imitate the Range-of-Motion of the torso-swing of elite racers. Their torso-swing muscles (e.g. lower-back and obliques) will quickly tire and become slow and ineffective.
Maximum reactive-side-force benefit also requires special timing: The start of the torso-swing move should be held back. The NKT approach completely misses this aspect of timing.

(2) NKT typically leads to a slower turnover frequency for non-elite skiers.

because it is too much Range-of-Motion for their level of specific torso-swing training -- since the torso-swing muscles are not used propulsively in summer sports.
Slower turnover frequency is bad for most skiers in some situations -- especially for climbing up steep hills -- the most critical problem in skating for most skiers.

Slow turnover is bad for climbing up hills because it requires either (a) each leg-extension push to go through a larger range-of-motion, which requires more knee-bend and thus more fatigue-strain on muscles and joints; or (b) insert non-pushing gaps between leg-pushes, which means either that the legs are not being used to deliver their full power capacity, or that they are taking on higher peak-force strains in order to make up for the gaps.

(3) NKT says that the amount of torso-swing should be the same as the amount of ski side-angle.

but the physics and biomechanics of torso-swing and ski-side-angle are completely different and unrelated.
NKT would say that there should be more torso-swing at slower speeds when climbing up a steeper hill, and less torso-swing when going up a gentler hill.
but for most non-elite skiers, climbing up a steeper hill is exactly when they need higher turnover frequency in order to survive, and therefore less torso-swing.

(4) NKT implies that the hips and shoulders and head all turn together uniformly in the torso-swing move.

Actually the roles of the hips and the shoulders are completely different in generating reactive side-force benefit.
When pushing off the left ski, the shoulders should rotate clockwise (as viewed vertically from above) to face more toward the right side (which moves body mass away from the direction of the skate-push, good for reactive-force). But rotating the hips clockwise reduces the reactive-side-force benefit (because it moves some body-mass toward the direction of the skate-push, bad for reactive-force).
The hips should be held stable, or even better should rotate slightly counter-clockwise (as viewed vertically from above) to face a bit toward the left side, toward the currently-pushing ski (which results in the right hip getting advanced a bit forward, so the next ski lands a bit forward of the previous ski).
The motion of the head and nose is largely irrelevant to the amount of benefit from the torso-swing.
So another danger of the NKT concept is that the skier will unconsciously make strange counter-productive tilts of the head and/or bends of the torso + abdomen in order to force the "alignment of nose-knees-toes".

(5) NKT implies that the ski should be landed vertically underneath the body, and the upper body should be in static balance over the ski.

This implies a pause in between leg-pushes, because the ski is not (easily) edged when it is vertically underneath the body.
This pause may be OK on flat or gentle terrain, like for V2 skate, and then the loss in leg-push is compensated by the stronger pole-push.
But for V1 skate ("offset") in climbing up a steep hill, this pause is a power-loss "dead spot" -- because the pole-push is already being exploited to maximum. The only way to make up for this dead spot is to increase the peak-force and/or range-of-motion of each leg-push -- but these compensations put more stress on the leg muscles and joints, so the skier fatigues sooner.
Instead the key trick in V1 skate is land the next ski somewhat out to the side from the hip, so the ski is already edged at the instant it hits the snow. And start the next leg pushing down and out before the ski lands, to guarantee full continuity of leg-push force (which permits minimum peak-force and minimum range-of-motion, and therefore minimum fatigue-stress on the leg muscles and joints).

(6) Elite racers pay no attention to NKT. Close analysis of World Cup videos with slow-motion and pause shows that sometimes the elite racers swing their head and shoulders less than their ski-side-angle, sometimes the same, and sometimes more.

Implications

Beginning skaters should learn solid single-ski balance -- by balancing on a single ski (usually with no poling) -- not by looking for some irrelevant alignment of upper and lower body parts.
Intermediate skaters who already have some decent single-ski balance: If they practice no-poles skating, they usually make some torso-swing move instinctively anyway without being told. The hard thing for them to learn is hip stability.
Skaters who want to use torso-swing to help survive climbing up a steep hill should focus on quickness, not range-of-motion. First find the best sustainable turnover frequency and range-of-motion for the leg pushes. Then fit the torso-swing moves into the rhythm of the legs.

The big trap of NKT is that it's easy to feel the reactive-side-force benefit of a big torso-swing move. However non-elite skiers can only sustain that big range-of-motion at a slower turnover frequency -- but it's difficult for them to feel the connection between slower turnover frequency and earlier leg-muscle fatigue in climbing up steep hills.

Is complete side-to-side weight transfer necessary?

[ under construction ]

[ this section needs to be revised to take into account the advantage of torso-swing and the difference between upper-body and lower-body weight transfer. Also need to define "weight transfer" and "complete weight transfer".]

Complete weight transfer has several benefits:

It's fun to do.
It's a good practice exercise.
It's a satisfying achievement.
It's useful in many situations.
It's a good mental image for lots of skaters.
The ability to do it is fundamental to optimal technique.

But from the viewpoint of physics, complete weight transfer does not give optimal speed and efficiency (mostly a concern for racers). It is not the "be-all and end-all" of skating technique. It is not the driver of skating power.

the Pendulum deception

Here's why -- the Pendulum deception: e.g. Lee Borowski says that complete weight transfer is like a "floating pendulum" (page 13 of "The New Simple Secrets of Skating"). That's a seductive image, because with a pendulum you get true side-to-side weight transfer for free -- without expending extra energy -- by using gravity. But it's a deception: because true side-to-side weight transfer when skiing requires real work from the skier's leg and arm muscles.

Some skaters may feel that they are using gravity to help, but physics says that gravity can only be used to shift the timing of the real muscular work (or make it less efficient). The only way to move the skier's complete center-of-mass from side to side is to push against the snow.

The justifications I've heard for claiming that complete weight transfer is so fundamental are:

1 - it feels good ("relaxed balance")
2 - it's the biomechanical configuration of joints in which muscles can apply the most power.
3 - gliding on a flat ski has lower friction.

The problem with #2 and #3 is that they are not a full picture of the physics of speed and efficiency. There is also:

4 - complete weight transfer absorbs muscular energy that could otherwise be applied to forward motion
5 - cadence / turnover: The more times per minute I can push with my most
powerful muscles in their most powerful ranges of motion, the faster I will
go.

Therefore, physics says that the best technique for speed and efficiency is
not just #2, but rather a optimal compromise among #2 and #3 versus #4 and
#5.

Puzzles for me

Why is V2 better on gentle terrain -- but V1 up steep hills?

It seems pretty clear that elite racers prefer V2 on gentle terrain and V1 for steep hills. What's the physics behind this choice?

Some ideas:

V2 is a "high-gear" motion technique in the sense that some phases of the stroke cycle are conducted in biomechanical configurations whose leverage geometry favors higher speeds and lower forces.
V2 is a "high-gear" motion technique in the sence that some phases or sub-phases of the stroke cycle are "dead spots" -- so other phases must handle higher peak forces. Not a problem at normal power requirements -- but could become a bottleneck when subjected to higher power demands. A stroke cycle without dead spots can handle a wider variety of power demands.
Elite racers are strong enough so that they could climb up most hills using V2 while staying within their normal not-too-anaerobic power capability -- if they were doing a solo time trial. But in mass-start races they need to stay with the skiers ahead of them in order to "get the draft", get help blocking the force of air resistance. So a key game in mass-start races is to go hard up some hills to try to "lose" competitors who are hanging on in the "draft". So the trailing skiers also have to go hard up hills to stay "in contact" with the pack leaders.

Therefore power demands get pushed to the limit on the steeper uphills. Therefore peak force gets pushed to limits. V1 focuses only on the big muscles that can handle high peak forces -- and let's the arm and abdominal and leg muscles work together to deliver peak force on one side.
Dead spots also raise peak force requirements -- and V1 is the best technique for eliminating dead spots.

Maximum-work V2 gets the most out of the double-pole-push by using the big leg muscles to lift the butt and upper body in preparation. But that style of double poling becomes less efficient up steel hills -- because lifting the weight of the upper body through the same range of motion twice is not efficient.
The leverage of some V1 motions are better set up for delivering vertical motion component up against gravity. V2 has better geometry for delivering horizontal components of motion.

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what's here

Questions

more Questions

Reactive side-force

Weight-transfer

Puzzles

see also

My main questions and ideas

I'm not sure which I'm doing: V1 or Open Field skate (V2 Alternate)?

What is V1 ("offset") actually good for?

Why not use Herringbone to go up hills?

Is more glide good?

more questions

Knee bend -- aggressive versus light? how much? why?

"butt down in the bucket"

compare with inline and ice speedskaters

Ankle flex? how much? why?

Forward lean of shoulders? how much? why?

Forward Fall

Parallel forward-lean versus ankle-flex angle?

Reactive side-force

What is reactive side-force?

Since all side-force must be transmitted thru the leg muscles, how can reactive side-force from other muscles actually help?

short answer

long answer

Torso-swing move: bigger is better?

Torso-swing move: hips and shoulders?

Weight-transfer

What is Weight-transfer in skating?

Weight-transfer = Falling?

NKT a good idea?

Implications

Is complete side-to-side weight transfer necessary?

the Pendulum deception

Puzzles for me

Why is V2 better on gentle terrain -- but V1 up steep hills?