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   front or foot-aim view at Midway

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front view or foot-aim view at Midway

some of these checkpoints are better viewed from the "foot-aim" view (in the line through heel and toe) than the Front view.

The other leg is often almost (or already) landed down on the ground at the Midway point for the main pushing leg -- see more detail.

When does the active pushing start? How much of the time between Set-down and Midway is passive gliding?  see more below.

knee - ankle - ground relationship

priority:  B-

best observed in Foot-aim view. In the Midway and Finish phases it is difficult to observe this accurately in the Front view.

Ankle-pronation is often criticized, but from a perspective of physics it's more complicated: Usually bad for propulsive work at Set-down, usually good for propulsive work at Midway and Finish (provided that it is transmitted effectively thru the knee and hip to mass of the upper body).

Most skaters seem to do it without conscious control (or even in spite of conscious belief that pronation is bad) -- so doesn't seem important to monitor it closely.

A higher proportion of the force is applied directly to propulsion if the pronation move is made when the hip to ground-contact line is more vertical -- so it's better to make the move earlier during the main outward leg-push -- soon after Set-down in classic Single-push, soon after Aim-switch in Double-push.

It's normal to observe the positive ankle-pronation configuration already at Midway in the leg-push.

Key thing with it in video analysis is not to worry about observing it at Midway or Finish of leg-push. Instead use the observation of ankle-pronation as a reminder to check on stability of medial hip-knee rotation (and hip-abduction) -- see details.

Typically fast skaters use more pronation at Midway and Finish in high-force situations (sprinting, climbing steep hills) and less pronation at Midway and Finish in high-speed situations (cruising flat and gentle terrain). I'm not sure what the physics or biomechanics is to explain this difference.

Ski-skaters on snow (cross-country ski racers) tend to use more medial knee-hip rotation and less ankle-pronation than ice or inline speedskaters. The design of their boots (as of 2006) tends to favor this. I'm not sure what the physics or biomechanics is to explain this difference. In 2007 some elite ski-skaters (especially sprinters) were starting to use lower-cut boots, which tend to better enable ankle-pronation.

standard-form perceptual check

Ankle somewhat pronated at Midway and Finish is typical and good.

Some would say that a good perceptual check would be to finish with no pronation. I fail to see what the benefit of educating this perception would be.

Another idea might be that it's good to educate the perception of exaggerated pronation.

simple Normal-push

Ankle somewhat pronated at Midway and Finish is typical and good -- usually more pronation in high-force situations like sprinting or climbing up a steep hill.

Double-push

Ankle somewhat pronated at Midway and Finish of main outward push is typical and good.

Pronation at the finish of the inward push would usually be bad for propulsive work.

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ankle - knee - hip relationship (front)

priority: C  (or B for ski-skaters on snow)

best observed in Foot-aim view.

The motions and positions of "medial hip-knee rotation" with the knee in a flexion position is not talked about much.

"medial" hip rotation moves the kneecap toward the inside toward the other leg.

"lateral" hip rotation moves the kneecap toward the outside away from the other leg.

From a perspective of physics of skating it's usually:

  1. bad for propulsive work at Set-down

  2. good for propulsive work at Midway (provided that it is transmitted effectively thru the hip-abduction muscles to the mass of the upper body)

  3. bad for propulsive Work at Finish (because it would be a sign that knee is still in a flexion position -- which is bad for the finish of the big Extension push)

Key difference between the roles of ankle-pronation and medial hip-knee rotation is that ankle-pronation is definitely compatible with full extension of the knee joint, while medial hip-knee rotation is not compatible with full extension of the knee when the foot is used for strong pushing in skating. It only works for skating when the knee is in a flexion position.

So the hip-knee rotation move must be in the medial sense (propulsive) in the early phase of the leg-push, then in the lateral sense (anti-propulsive) in the later phase of the leg-push. It can be net positive for propulsive Work because the effect is larger when the leaning angle of the leg is small (close to vertical) and smaller when the leg is tilted strongly. So the positive early phase outweighs the negative later phase, and the result for the overall stroke-cycle is positive.

Key thing with it in video analysis is not to worry about observing it at Midway. Instead use the observation of medial knee-hip rotation as a reminder to check on the stability of hip-abduction (see knee hip relative sideways motion) -- since it is frequently associated with "wiggly" motion of the pushing hip sideways opposite from the pushing knee.

Typically fast ice + inline speedskaters do not show much medial knee-hip rotation at Midway, even though physics suggests it should add propulsive Work. Perhaps their hip-abduction muscles are already near their effective limit of Force transmission for other reasons. Or perhaps it's that medial hip-knee rotation has trickier physics than ankle-pronation, so they emphasize the simpler move.

But elite racers skating snow in cross-country skiing typically use medial knee-hip rotation a lot, and ankle-pronation less.  I'm not sure what the physics or biomechanics is to explain this difference.

standard-form perceptual check

For ice + inline speedskaters: Straight line in foot-aim view thru ankle + knee + hip. No medial hip-knee rotation.

For snow skaters: Substantial medial hip-knee rotation at Midway of leg-push (but none at start of push). Knee in foot-aim view should be obviously inside the line thru ankle + hip.

simple Normal-push

For ice + inline speedskaters: Either slight medial hip-knee rotation or None is OK at Midway of leg-push

For snow skaters: Substantial medial hip-knee rotation at Midway of leg-push (but none at start of push). Knee in foot-aim view should be obviously inside the line thru ankle + hip.

Double-push

For ice + inline speedskaters: Either slight medial hip-knee rotation or None is OK at Midway of main outward push.

relative sideways motions + stability

Priority:  A

No "wiggly" in the motion between Set-down and Midway:  Every joint moves at least as far sideways as the joint immediately below it.

Physics:  Newton's Third Law of action/reaction says that forces always come in pairs, so the skater must manage both "ends" of each pushing move. The obvious "end" is the transmission of the push force thru bones and joints thru the foot externally against the ground. It's fairly clear that if there's collapse or absorption of force in some link between the pushing body part and the ground, there's a lower amount of effective propulsive Work than could have been obtained from that body part's pushing. Consistent "rigid" transmission is good, inconsistent "wiggly" linkage is bad. But that's not the only place to lose out on propulsive Work . . .

The other less-obvious "end" for the pushing move is the transmission of the force to the mass of the skater's body which is away from ground-contact. Each push needs something to push against. The more mass gets started moving quicker away from the direction of push toward the ground, the more to push against, so the higher the force toward the ground.

Newton's Third Law says that both "ends" add to propulsive Work. So failures and weaknesses in transmission of force for either "end" of the pushing do reduce propulsive Work. So it's helpful for video analysis to examine key links in the transmission chain.

This is trickier to observe than other video checkpoints, because it requires comparing sideways distances in different video frames from different times in the stroke-cycle. Doing this reliably might require having an accurate way to measure distances between two parts in the same frame, and record each measured distance as a number -- but there might be a way to compare two distances without measuring each of them as a scalar number.

Here's what to compare (and measure?) . . .  

Motions from Set-down to Midway:

  • Knee vs Ankle -- foot-aim view:  pushing Knee joint moves at least as far sideways (relative to the ground-contact point) away from the leg-push direction as its Ankle joint moves.

  • Hip vs Knee -- foot-aim view:  center of pushing Hip moves at least as far sideways (relative to the ground-contact point) away from the leg-push direction as its Knee joint moves.

Above the hips it's different -- because with good timing the shoulders (and hands if using arm-swing) should be moving toward the new pushing leg at the time it is set down. So they cannot just instantly be moving the other way. First they have to slow down and stop -- and that's their contribution to giving the leg muscles more to push against during the time between Set-down and Midway.

After they've stopped, then they start moving in the other direction away from the push, but most of that should be happening later, after Midway. So the sideways motion of the shoulders (and arms) should be lagging behind the hips.

These same key observation points for "consistent" transmission versus "wiggly" apply to all motion styles -- except that with outward push in Double-push for the main outward push, substitute "Aim-switch" for "Set-down"

For the inward push of Double-push substitute "Maximum inward move of In-push" (or "Finish of In-push") for "Midway", and note that all the directions are reversed: outward versus inward.

If a higher part of the body moves sideways away from the leg-push direction farther than the body part immediately below it, it adds propulsive Work of its own -- beyond transmitting work from moves of other body parts.

This is good provided that the additional Work is transmitted to the upper body and down to the ground, and that other moves are not "softened" in order to make that transmission capacity available. A typical pitfall is that adding sideways motion in one link is combined with subtracting it from or absorbing it in another link.

pelvis hips side-tilt

priority: B

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

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

standard-form perceptual check

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

simple Normal-push

Pelvis sideways configuration should likely be roughly level: both hips at roughly the same height off the ground.

But there should be motion toward tilting away from the pushing leg.

Double-push

Pelvis sideways configuration should be roughly level: both hips at roughly the same height off the ground; though pelvis might still be tilted slightly toward the pushing leg.

Anyway it should be moving toward tilting away from the pushing leg: non-pushing hip dropping lower.

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 definitely over to the side from the hips, toward the side of the pushing leg. Ideally, the shoulders should not be moving much sideways in either direction.

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

simple Normal-push

Position of shoulders should be definitely over to the side (relative to the hips), toward the side of the pushing leg. Ideally, the shoulders should not be moving much sideways in either direction.

Double-push

Midway thru the main outward push, the position of shoulders should be definitely over to the side (relative to the hips), toward the side of the pushing leg.

Perhaps the shoulders will still be moving toward the side of the pushing leg, but that should be only the "follow through" from the quickness of their move thru the Aim-switch phase, not continuing the motion farther just to increase their relative sideways position.

During most of the In-push, the shoulders should definitely be moving toward the side of the pushing leg.

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 Midway

 

toe - ankle - knee relationship

priority:  A

"Knee vertically over Toe" is a key configuration for the skating leg-push. But for the main leg-push outward, it's not necessary to be in this configuration at Set-down. Rather the key time for it is just before the main Extension push later in the leg-push.

This timing leads to the "sideways banana" path of the foot relative to its hip. The foot lands in a neutral position, then moves backward (relative to its hip) just after Set-down (as the knee joint moves down + forward), and then (at least in "standard form") the foot "slices" forward (relative to its hip) as the knee-extension muscles activate later in the stroke.

Physics:

  • A key propulsive move is to "slice" or "carve" the foot forward in the later Extension phase of the push. The more you prepare for that by starting the ankle farther behind the knee, the more range-of-motion distance (and the more propulsive Work) you get out of the knee-extension muscles in the "forward slice" move.

If the knee starts roughly over the ankle, then the push from the knee-extension torque is aimed only forward parallel to the ground or upward away from the ground. In order to add propulsive work to skating, its line of force must be aimed partly downward and toward the foot. The only way to get that is if the knee joint is substantially forward from the ankle joint.

Otherwise virtually all the work of the main Extension push must be supplied by the hip-extension muscles (perhaps with a little help from ankle-extension). If you want the knee-extension muscles to help, you have to prepare by getting the knee into position.

Cross-country skiing: When poles are being used to help, the knee-extension muscles can do Work other than by contributing to the Extension phase: instead by raising the hips and the mass of the upper body, which builds gravitational potential energy, which can be converted into propulsive work by dropping it onto the ski poles. But if a skier wants the knee-extension muscles to be able to contribute to both the "raise upper body" phase and the Extension phase, then Toe-ankle-knee relationship needs to be prepared.

  • There is propulsive Work available from the ankle extension (a.k.a "plantar flexion") muscles -- pushing the ball + toe of the foot away from the knee. The closer the ball + toe are toward the knee at the start of the push, the more range-of-motion available to add propulsive work in pushing from the starting angle to the finishing angle.

Normally this ankle-extension push comes only as a quick "flick" at the very end of the stroke. Before then the heel stays low for best transmission thru the foot to the ground of the big forces from the big hip-extension and knee-extension muscles.

  • High-force situations: There's a more critical need for knee forward over toe in situations like climbing up a steep hill: Getting the knee forward relative to the ankle is needed in order to get the hip as far forward as possible relative to the foot-ground contact -- so the knee-extenstion muscles are best able to handle high force load in the upcoming Extension phase.

If the hip is significantly behind the ankle or foot-ground contact, then the weight of the upper body puts substantial force on the knee-extension muscles -- just to support resisting the force of gravity -- in addition to the force those muscles need to apply to push actively to extend the leg. In a high force situation, that extra force load has a substantial impact on performance in the Extension phase.

There are two key points to avoiding having the hips too far back: (a) avoiding too much flex in the hip-knee-ankle angle; (b) getting a sharp enough "forward" flex of the knee-ankle-toe angle. In high-force situations, it's easy to remember (a) but forget (b). I suspect the reason is that our instinct from walking + running is to stretch our foot forward, so when we're under stress and unconsciously want to "do more" to help, we step farther forward.

standard-form perceptual check

toe-ankle-knee angle definitely less than 90 degrees. Pushing knee is vertically over its toe.

simple Normal-push

toe-ankle-knee angle definitely less than 90 degrees. Pushing knee close to vertically over its toe.

Double-push

Midway thru main outward push, the toe-ankle-knee angle should be clearly less than 90 degrees. Pushing knee close to vertically over its toe.

ankle - knee - hip relationship (side)

priority:  A

Physics: The main Extension push of the leg contributes a higher proportion of its Force directly into propulsive Work when the hip to ground contact angle is larger, with the leg leaning farther away from vertical. Starting the Extension push earlier while the pushing foot is still close to underneath its hip is less effective -- the force goes mostly down into the ground instead sideways outward.

So it's valuable to hold back the Extension push until later in the stroke. Also . . .

To prepare for more range-of-motion in Extension, must first get the leg into a configuration with more "flexion" -- more compressed.

But holding a more compressed leg position is more strenuous for the knee-extension muscles. The time of maximum compression is shorter if have a little less compression of the leg at Set-down (or Aim-switch), then add a little compression (knee-flexion) during the first phase of the leg-push, so it's ready "just in time" for the later Extension push.

standard-form perceptual check

ankle-knee-hip angle should exactly be the same as at Set-down -- or perhaps a little smaller, more compressed.

simple Normal-push

ankle-knee-hip angle should be roughly the same as at Set-down -- or perhaps a little smaller, more compressed.

Except that in high-force situations (e.g. sprinting, climbing a steep hill), the big strong hip-extension and knee-extension muscles must be engaged quicker at higher turnover frequency. So the Extension phase needs to start earlier, so by the Midway time, the ankle-knee-hip angle might have started increasing. Sometimes this increase is so explosive that reaction to the downward component of the push sends both of the skater's legs simultaneously up into the air.

Double-push

Midway thru the main outward leg-push, the ankle-knee-hip angle should be roughly the same as at Aim-switch.

But in the In-push, the Extension push starts earlier, so the ankle-knee-hip angle is increasing through most of time between Set-down and Aim-switch. Then during Aim-switch there's an opportunity to re-compress the leg a little before the main outward push.

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 hills. It can also add a little propulsive Work.

But at Midway through the leg-push, hips should appear roughly square to the direction of the skater's overall forward motion.

see discussion on Set-down page.

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more . . .

active push versus passive glide

Sometime between Set-down and Midway -- but when? The foot could just glide without any sideways outward force for a ways, or it could start pushing sideways outward immediately after Set-down. It can even start pushing outward while the other foot is also pushing.

How can you tell from video analysis what proportion of the time between Set-down and Midway was just passive gliding and how much was active pushing?

Double-push: The outward push doesn't start with Set-down, but rather after the "Maximum inward push" point (in Front view) -- so for Double-push substitute the "Max-In-push" for "Set-down" in this discussion.

The answer is important for increasing Power (measured in Watts) and forward speed, because "dead spots" (or even low spots) in the stroke-cycle can be "wasted time" which reduces the overall average Power rate. Sometimes it's not really wasted, because it's a necessary step to getting much more Work in a different phase which more than outweighs the time spent in the "dead spot". (Or sometimes it's not really a dead spot: rather an indirect way of adding propulsive work which will be transmitted to the ground later.)

The are two ways to address a dead spot in the stroke-cycle, to increase overall Power: (a) Find a way to use it to add propulsive Work (directly or indirectly), or (b) Find a way to get through it quicker. Often the two go together: the move that adds Work also gets it over with quicker.

For the phase just after Set-down, the obvious way to directly add propulsive work is to start the Sweep-Out push using some of the hip-abduction, medial knee-hip rotation, and ankle-pronation muscles -- and also make sure to transmit that work to the mass of the upper body.

The easy way to detect and measure that Sweep-Out push would be to have force / torque sensors in the skate somewhere. But if all we have it video, then what? How detect if some of those muscles are actually pushing? and if magnitude of force is a little or a lot?

I think the answer is some modification of the

relative sideways motions + stability

approach -- but I'm not sure what should be modified.

Missing:  I think the missing piece is active work by the hip-abduction muscles -- how to accurately observe how much range-of-motion angle or distance they move through.

I think I've already got the Work from ankle-pronation and medial knee-hip rotation moves covered in the knee - ankle - ground and ankle-knee-hip (front view) sections on this page -- combined with the relative sideways motions + stability for the transmission of their force to the mass of the upper body.