what's here

   front or foot-aim view at Set-down

• ground-contact to hip vertical angle

• knee - ankle - ground relationship

• ankle - knee - hip relationship (front)

• pelvis hips side-tilt

• hip - torso-shoulder relationship

• hand-arm - shoulder relationship

   side view at Set-down

• gentle into setting down

• toe - ankle - knee relationship

• ankle - knee - hip relationship (side)

• pelvis hip rotation

   more on Set-down

• foot-aiming angle

• foot land how far out in front?

• hip height above ground

[ under construction ] 

front view or foot-aim view at Set-down

some of these checkpoints are better viewed from the "foot-aim" view (in the line through heel and toe) instead of a straight front view.

ground contact to hip relationship

priority: A

best observed in foot-aim view, but usually front view is good enough.

standard-form perceptual check

"neutral" = vertically underneath the hip joint (top of the femur bone).

There is a tendency to set down outside from vertically underneath the hip, while thinking that the foot-ground contact is vertically directly underneath the hip.

simple Normal-push

for simple Normal-push, this checkpoint is a key determiner of the range-of-motion of the Out-Sweep push, and of the proportion between Out-Sweep push and Extension push.

• in high-speed lower-force situations (e.g. flat or slightly downhill slope, low-friction surface, tail-wind, strong skater), tends to offer higher sustainable power rate if set down further underneath toward the opposite side from the new leg-push.

• in high-force situations (e.g. steep uphill, quick acceleration, head-wind, non-strong skater), tends to offer higher sustainable power rate if set down further outside on the same side as the new leg-push.

• skaters with strongly developed Extension muscles but not much developed Out-Sweep muscles (e.g. strong bicyclists who've done little skating), tends to offer higher sustainable power rate if set down further outside on the same side as the new leg-push.

Double-push

for Double-push, this checkpoint is a key determiner of the range-of-motion of the In-Sweep push.

There are two main strategy focus options for double-push: (a) emphasize higher turnover frequency from the big leg-extension muscles (e.g. hip-extension, knee-extension) also used in the main normal-push; or (b) emphasize engaging new muscles on the inside of the leg (e.g. hip-adduction, lateral-hip-rotation, ankle-supination) which are not used in the main normal-push. Typically those who cross-train with lots of bicycling and not as much Double-push-skate specific training should prefer option (a). Most skaters without time for lots of Double-push specific training should usually prefer option (a).

• those emphasizing D-p strategy option (a) higher turnover frequency of the big leg-extension muscles, should set-down with the foot-ground contact more toward the opposite side from the new leg-push, definitely not vertically underneath the hip of the new pushing leg.

• those emphasizing D-p strategy option (b) engaging new muscles on inside of leg, should set-down with the foot-ground contact closer to vertically underneath the hip of the new pushing leg.

• in high-speed lower-force situations (e.g. flat or slightly downhill slope, low-friction surface, tail-wind, strong skater), tends to offer higher sustainable power rate if set down further outside on the same side as the new leg-push -- but usually it is difficult to make this work farther than directly underneath the hip.

• in higher-force situations (e.g. substantial uphill, acceleration, head-wind), tends to offer higher sustainable power rate if set down further underneath toward the opposite side from the new leg-push.

• skaters with strongly developed Extension and Out-Sweep muscles but not much developed In-Sweep muscles (e.g. strong skaters who have not practiced effective Double-push much), tends to offer higher sustainable power rate if set down further underneath toward the opposite side from the new leg-push.

knee - ankle - ground relationship

priority: A

best observed in foot-aim view; for most higher-speed lower-force situations, usually front view is good enough, because foot is aimed fairly close to straight forward anyway. But for lower-speed situations like climbing up a hill, foot-aim view is often important, because it could be tricky to observe in front view, because the foot is aimed significantly to side at set-down. 

standard-form perceptual check

"neutral" = ankle joint in line between ground-contact and knee joint.

Many skaters have a strong tendency to set down with the ankle "pronated" -- so the ankle joint is inside the line from ground-contact to knee joint. Even after they work on avoiding it, they tend to keep reverting back to setting down with ankle-pronation.

simple Normal-push

• Typically set down "neutral".

• High-force situations:  might set down somewhat pronated (ankle joint inside the line from ground-contact to knee joint).

• Very low gliding resistance:  often set down somewhat supinated (ankle joint outside the line from ground-contact to knee joint).

Double-push

• Typically set down "neutral".

• High-speed situations, or for those emphasizing D-p strategy option (b) engaging new muscles on inside of leg -- might set down somewhat pronated (ankle joint inside the line from ground-contact to knee joint).

• High-force situations:  might set down somewhat supinated (ankle joint outside the line from ground-contact to knee joint).

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ankle - knee - hip relationship (front)

priority: A

foot-aim view is normally clearly better than front view for observing this.

standard-form perceptual check

"neutral" = knee joint in line between ankle joint and hip joint.

simple Normal-push

• Typically set down "neutral". I have not noticed any variations from this.

• If foot is aiming straight forward at Set-down:  then might consider setting down with some medial knee-hip rotation (i.e. knee already "rolled" inward -- knee joint inside the line from ankle joint to hip joint), to increase range-of-motion for a lateral knee-hip rotation move before the foot-pivot, which (if transmitted thru hip-adduction muscles) would add sideways kinetic energy to upper body mass -- which could then be "caught" after the foot had pivoted to aim diagonally outward, and converted into (propulsive) kinetic energy.

• If very low gliding resistance:  then might possibly consider setting down with some lateral-hip-rotation (i.e. knee pointed outward -- knee joint outside the line from ankle joint to hip joint), to increase range-of-motion for medial-hip-rotation moves.

But there might be a range-of-motion competition between the medial knee-hip rotation move and the hip-abduction move, in which case this wouldn't help much.

Double-push

• Typically set down "neutral".

• High-speed situations, or for those emphasizing D-p strategy option (b) engaging new muscles on inside of leg -- then might consider setting down with some medial knee-hip rotation (i.e. knee already "rolled" inward -- knee joint inside the line from ankle joint to hip joint), to increase range-of-motion for a lateral knee-hip rotation move during In-push.

pelvis hips side-tilt

priority: B (or A)

view: best observed in front view, not foot-aim view.

See discussion of different approaches to this move on Finish page.

At the Set-down for simple Normal-push this implies that the hip of the leg being set down should be higher than (or roughly level with) the other hip which is in the final phase of its push -- not because it's good for the Set-down leg, but because it's good for the other leg.

Set-down for Double-push has trade-offs.

standard-form perceptual check

The approach that makes sense to me for educating percerption is to have two perceptual checks for this: one for staying level with no tilt, and one for moving to maximum tilt:

(a) Pelvis should be level: both hips at the same height off the ground.

(b) Pelvis should be tilted sideways toward side of Set-down.

simple Normal-push

Pelvis should be tilted sideways toward side of Set-down (or perhaps level):  Hip of the leg being set down should be lower than other hip (or perhaps level with it).  But the Set-down hip should definitely not be higher than the other hip.

If pelvis is tilted toward side of Set-down, the torso + shoulders should not already be over on the side of set-down.

Double-push

At the Set-down for Double-push, there's conflict between the desire to have the pushing hip higher to better engage hip-extension muscles versus the desire to land the set-down foot further away toward its own side.

So likely there will not be much distinct pelvis sideways tilting observable at the moment of Set-down.

hip - torso-shoulder relationship

priority: B  (or A)

This is a priority B if the question is between gaining power from a torso-shoulder side-swing move or not gaining. It's priority A if the skater is actually losing power due to mis-timing of torso-shoulder motion.

view: best observed in front view, not foot-aim view. 

standard-form perceptual check

if practicing torso-shoulder side-swing style:  Position of shoulders should be roughly centered over hips (ideally somewhat toward the side of the pushing leg which is finishing) -- but moving quickly (relative to the hips) across to the side of the set-down.

if setting down with foot aiming straight forward, the position above should not be reached until the foot pivots to aim angled significantly toward the outside.

if practicing the "quiet upper body" style: Shoulders centered over hips, chest facing straight forward.

simple Normal-push

Position of shoulders should be roughly centered over hips (ideally somewhat toward the side of the pushing leg which is finishing) -- but moving quickly (relative to the hips) across to the side of the set-down.

if setting down with foot aiming straight forward, the position above should not be reached until the foot pivots to aim angled significantly toward the outside.

A serious problem is if the shoulders positioned roughly over the hips, but are moving away from the side of the set-down (since this is absorbing propulsive work from the leg-push).

Typical problem is that the shoulders are already over on the set-down side relative to the hips, roughly near the end of their sideways move.

I suspect this timing problem results from two things: First is a desire simplify the coordination by making the start and stop of the torso-shoulder swing simultaneous with the start and stop of the leg-push.

Fortunately this does not absorb any work from the leg-push, but it does not add any either -- so it's using muscles to move the upper body around, without getting anything for it. The reason it does not add any work is because both the acceleration and deceleration of the mass of the torso have taken place while the leg is pushing sideways at roughly the same angle in the same direction, the the Newton's Third Law positive from the acceleration is exactly cancelled by the deceleration.

I suspect this might be because our (unconscious?) perceptions are good at sensing the benefit from accelerating a body part, but not good at sensing the loss from decelerating (because the loss is irrelevant to critical tasks like throwing a stone, and to a key phases of human running).

The key "fix" is to learn to hold back the start of the torso-shoulder swing, to "de-synchronize" it from the leg-push.

Another problem is that the shoulders are moving sideways in the good direction, but only slowly.

Physics says that what determines the amount of propulsive benefit from the torso-shoulder side-swing move is the speed at which the shoulders are moving sideways at the instand of set-down (for classic Normal-push).

Double-push

Shoulders roughly centered over hips, and not moving (much) sideways -- is what is usually seen in the fastest inline speedskaters in videos around 2004-2006.

What physics says might add more propulsive Power (in Watts) would be to have the shoulders somewhat off to the side away from the set-down, and not moving at all (relative to the hips).

One possibility is that the fastest inline speedskaters are already so close to some "limit" on how much force their legs can deliver that this style would not add much Power for them. Another is that since the amount of added Power is determined by the speed of the move, not the range-of-motion distance, starting the move with more distance would only slow down their turnover frequency.

Typical problem observed is that the shoulders are already moving across toward the set-down side. But this is the correct timing for classic Normal-push. For Double-push the shoulders should be moving across center at the Aim-switch.

Key "fix" is hold back the start of the torso-shoulder side-swing in Double-push until after set-down. Use set-down as the "trigger" perception to start the torso-swing move.

But holding the torso + shoulders all the over on one side for so long gets strenuous -- which could explain why even the fastest skaters tend to allow their torso to "drift" back into the center, and then they start their quick torso-swing move from there.

hand - arm - shoulder relationship

priority: C

This is priority C if the question is between gaining power from an arm-swing move or not gaining. It's priority B if the skater is actually losing power due to mis-timing of arm-swing motion.

view:  best observed in front view, not foot-aim view. 

If arm-swing is not being used to add propulsive Power, then this observation can be whatever fits with or helps other aspects of the skater's motion -- e.g. balance or rhythm.

If swinging the arms and hands from side to side is being used to add propulsive Power, then the observations should be like for the position and motion of torso + shoulders under hips - torso-shoulder relationship -- e.g. for simple Normal-push, arms roughly in front of body, perhaps a little to one side -- but not way off to one side or the other, and moving definitely toward the set-down side.

If swinging the arms and hands forward and backward is being used to add propulsive Power, then the observations should be different from that.

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side view at Set-down

ground contact to hip relationship: 

Best as I can tell, all the important aspects of this for the physics and biomechanics of propulsion are determined by other more specific observations. See discussion under more on Set-down.

Gentle into setting down

Motion speed and direction of the foot immediately before Set-down should be roughly the same as after Set-down:

• No stomping downward.

The proportion of leg Extension force which is directly propulsive is lowest just after Set-down. An explosive move puts more stress on muscles and their support + transmission structures. When skating on a softer surface (notably snow) extra down-force increases gliding friction by making the skate "plow" deeper into the surface, or compress the surface more inelastically.

The only skating situation where a strong down-force can make sense is very high-force for a short time (e.g. sprinting or climbing a short steep hill).

• No quick forward move from Recovery into Set-down.

I don't know any way this kind of move can add net positive propulsive Work to the overall stroke-cycle, without obviously larger negative impacts on other propulsive moves. I suspect skaters who like this kind of move strongly perceive the positive work from starting and accelerating this move, but fail to perceive either the negative work from decelerating or stopping it, or miss the other big negative side-effects on Work and Power -- see more detail.

• sideways motion can start (gently) before Set-down

If it makes sense to start pushing sideways immediately on Set-down -- which it usually does if the foot is already aiming diagonally, then it can also make sense to have the foot moving sideways already immediately before Set-down. This provides a little physical momentum going into the push, and it helps trigger the neural control to start pushing immediately -- so no precious stroke-cycle time is lost.

For simple Normal-push, it definitely makes sense to bring the recovering leg closer sideways toward the other leg -- further inside than it will be at Set-down -- then move the foot outward while lowering it onto the ground. Because the starting of the outward move adds sideways reactive force to the push of the other leg.

For Double-push there's no propulsive benefit, since the sideways reactive force is self-cancelling for the final move thru the air into Set-down. But in a higher-force situation where it is desired to start the In-push immediately, then it makes sense to trigger the neural control for that early.

toe - ankle - knee relationship

priority: A  

Physics:

• This angle gets important for the Extension phase (especially in High-force situations) see details under Midway checkpoint. So the sharper angle is not necessary until the start of the Extension phase -- but at Set-down it needs to at least be on its way to that.

cross-country skiing: There's a special case where it helps to get the knee far forward relative to the ankle at or immediately after set-down: on the Poling-side of V1 skate. The reason is that getting the knee forward gets it lower, which allows the hip to go lower,  drops the weight of the upper body lower -- which can transmit more gravitational potential energy into the pole-push which converts that energy into additional propulsive work. So the need for the early forward ankle-flexion move derives from the special timing of the pole-plant.
(In some elite racer videos it sorta looks like the move is not only from active force of the shin muscles, but also from allowing the weight of the upper body to press the knee down. Sometimes combined with a medial-hip-knee-rotation move, so it looks like a "collapse" downward and inward.)

• cross-country skiing: If the foot is set down with the knee not so far forward, but then the shin muscles apply force to move the knee forward relative to the ankle after Set-down (during the initial Sweep-outward phase), and the knee-extension muscles transmitted this motion to the hips, this would lift the weight of the upper body -- which does work by increasing the gravitational potential energy of the body -- which could be converted into propulsive work by pushing down on the skating foot -- or likely more effective, push down on the cross-country ski poles.

But in order for the shin muscles to apply this force to the upper body, there needs to be a long "tail" behind the ankle pressing against the ground. On a normal inline skate the rear wheel is too close to the ankle, so force from the shin muscles only lifts the toe wheel off the ground, doesn't move hip anywhere. With a long ski (or perhaps long rollerski) the toe cannot rise, so the force goes to the upper body (if the knee transmits it).

There's no work done if the knee is moved down and forward by the weight of the upper body pushing down on it, or by some momentum carried from some other move. There's only work if the shin muscle applies force. So just seeing the knee move on video does not prove that the move is propulsive.

This is a rather small effect compared with the impact on Toe-knee-ankle to prepare for the Extension phase, so it's better to err toward having the knee more forward at Set-down. Nevertheless elite XC ski racers are observed to sometimes set down with the knee only partly forward, then move it further forward.

standard-form perceptual check

Knee vertically over slightly behind toe, and well in front of ankle -- say about 75% of the way from ankle to toe - (though it's OK if knee is already vertically over toe).

simple Normal-push

Knee vertically over slightly behind toe, and well in front of ankle -- say about 75% of the way from ankle to toe - (though it's OK if knee is already vertically over toe).

Then soon after set-down the knee will move down and forward over the toe (during phase 1).

When climbing up a steep hill, the knee might already be vertically over the toe at set-down -- to be able to start the "carving" forward move sooner.

Double-push

Knee close to vertically over the toe already at Set-down -- to be able to start the "carving" forward move sooner in the In-push.

Except . . .
For the "use new muscles on the inside of the leg" strategy of Double-push, it could be OK to set down more like for simple Normal-push,

ankle - knee - hip relationship (side)

priority: B+

The ankle-knee-hip angle is very important in the physics of skating Power, but it's more a matter of muscular training than technique. The typical problem is that skaters forget to work on getting a sharper more compressed angle -- perhaps because it's not fun, or perhaps there's no simple right or wrong answer, just "keep playing with more".

But there's no "correct" angle. The best ankle-knee-hip angle to use depends mostly on the strength and training of the skater, and the requirements of the performance situation -- see below in this section.  If a skater tries to copy the angle of some elite racer, but does not have the strength and training to handle it, that skater is mainly going to get tired out quickly (and possibly injured).

Here's some ideas about ankle-knee-hip angle:

• Sharper more compressed angle at Set-down is something to keep remembering to work on and train

• It's good to measure and monitor this angle in a serious coaching program.

• For performance, the important thing is for each skater to learn to feel what height is appropriate for different situations (perhaps by playing with different heights in practice).

Though the ankle-knee-hip angle is critical for determining the range-of-motion for the big leg-extension muscles, there's no right or wrong way to do it. Here's some considerations for what's appropriate:

• skaters with stronger leg muscles can use a sharper angle (because it's strenuous to support body weight)

• higher speed will tend to use a sharper angle (to get lower for less air resistance).

• higher force situations (e.g. climbing up a steep hill) will tend to use a less sharp angle, less knee bend.

The leg muscles and joints can handle higher forces sustainably if the knee is not bent so much. Of course in a finish-line sprint there's little need to "sustain", so more knee bend can be used.

• a stroking style more dependent only on a single push by each leg will tend to use a sharper angle (to "get the most" out of that single push)

Double-push can use a wider angle, because each leg gets two pushes to get power out of its big muscles.

Skating with Poles can use a wider angle, because the poles are also available to deliver power, also because the power to the poles sometimes uses the vertical dropping of the hips to add work.

standard-form perceptual check

ankle-knee-hip angle of 90 degrees.

simple Normal-push

Some guidelines I've heard for the ankle-knee-hip angle:

• serious ice speedskaters often use an angle at least as sharp as 90 degrees.

• serious inline speedskaters for classic Normal-push often use an angle of 90 degrees or wider.

• less serious or less strong inline skaters use a wider angle, less knee bend.

• most aspiring speedskaters do not use enough knee bend for maximum power, and should keep practicing to use a sharper angle more of the time - (but have to work up to it slowly over weeks and months)

Double-push

Some videos I've seen (not easy to find them with a good side view):

• Pascal Briand, pushing pretty hard, looks to me like about 90 degree angle at set-down.

• for cruising at marathon distances, it's clear that most fast skaters use much less knee bend than 90 degrees.

pelvis hip rotation

priority: B-  (or for climbing a steep hill: A)

Rotation of the pelvis about the axis of the spine is a key determiner of the "gearing" of skating, especially for climbing up a steep hill. It can also add a little propulsive Work, but this can get a little tricky -- if in doubt, just leave the hips "square" to the direction of overall forward motion.

standard-form perceptual check

pelvis square to the line of overall forward motion. Neither hip is ahead of the other.

simple Normal-push

It many situations advancing the set-down side hip ahead of the pushing hip can add a little propulsive power. Ideally the maximum rotation position should be reached exactly at the time of the Finish of the leg-push. But there's a limit to how much range-of-motion is helpful on this "leading" kind of pelvis rotation move: It's helpful at least until the line thru the two hips is perpendicular to the foot-aiming direction, and perhaps a little bit beyond that -- but then it becomes counter-productive (because the point is to move mass away from the leg-push direction). So there's no point in exaggerating this "leading" rotation.

If the "leading" rotation move looks real obvious in video, then it's likely getting to the point of counter-productive.

But when climbing up a long steep hill, advancing the set-down side hip also requires a higher level of Power to be expended to sustain a rate of climbing without "stalling out". In that situation, it can be helpful to face the pelvis more toward the set-down side, to move the set-down hip behind the other hip, to slow the overall vertical rate of climbing, and reduce the rate of power required to sustain climbing without stalling out.

There's no "limit" on ability of the range-of-motion of this "lagging" kind of pelvis-rotation to reduce the demand for power when climbing up a hill. If the "lagging" rotation move looks obvious in video of climbing up a very steep hill, that's OK.

Double-push

In typical non-low speed Double-push situations, it adds a little power if the set-down-side hip is advanced a little ahead of the other hip at the Finish of the main outward push.

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