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kinds of skating techniques

There are different motion techniques for trying to skate fast, and some things about what to look for in video are different, so it's it good to be clear about which technique the skater it trying to perform.

two main skating techniques, each with variations:

Normal-push

This is the technique used by most people without special coaching. Each leg makes one push during the stroke-cycle, and that push is aimed diagonally to the outside (away from the other leg). The foot maintains roughly the same aiming direction throughout its push. Except there might be a slight tendency to aim less toward outside near the end of the stroke if "carving through the heel" is employed. There's a variation where the foot is set down aiming straight (without pushing), then pivots outside to make its push.

Three variations of Normal-push:

Advantages:

  • simplicity.

  • minimal frictional losses from changing the aim of the foot.

These frictional losses are more important if using a long ground-contact implement: e.g. long-blade ice speedskate. Very important for using a normal-length cross-country ski on snow, where simple Normal-push is usually the only relevant option.

Other possible advantages:

  • pushing can be made continuous through the whole stroke cycle: no gap.

  • allows focus more on the big strong leg Extension muscles (e.g. gluteus, quadriceps) for high-force situations.

Simple

The foot is set down already aimed diagonally toward the outside. It's helpful to think of the push in three phases:

(1) Sweep-out:  At first while the foot is closer to underneath its hip, the push is a Sweep sideways, directly outward from underneath. The line of its push is roughly along the ground surface, mostly sideways and partly backward. This push engages little-known muscles on the side of the leg, which could hip-abduction, medial knee-hip rotation, and ankle-pronation.

These muscles are typically substantially less powerful the leg-extension muscles. Serious racers, especially ice speedskaters, do special exercises to strengthen them.

(2) Tween:  A lower power segment between two phases. Not a good configuration for getting a high proportion of propulsive Work from either Sweep-out or Extension muscles. The objective for this phase is to somehow get through it quickly.

One strategy for getting through the Tween phase more quickly could to aim the foot a little more outward at the start of this phase, then more inward again at its end. A possible advantage of Double-push is that the skater goes into this phase carrying more sideways momentum.

(3) Extension:  When the foot is a significant distance out to the side away from underneath it's hip, the push is an Extension: aimed roughly along the line from the hip through the foot, so it includes a downward aspect. The Extension push can engage familiar muscles from walking and running: hip-extension (e.g. gluteus), knee-extension (e.g. quadriceps), and ankle-extension (e.g. calf).

The theme of the "simple" style of Normal-push is to get a good range-of-motion distance in each leg-push. Sometimes skater's with well-developed Sweep-out muscles set down their foot inside from vertically underneath its hip, to lengthen the range-of-motion in phase 1.

Extension focus

Another strategy is to skip most of Sweep-out phase 1, and emphasize the big strong leg Extension muscles. Set down with foot aimed outward to immediately begin pushing.

The theme of this "Simple with Extension focus" variation is to increase turnover frequency and force-magnitude from the big leg muscles. Used for high-force situations like climbing a steep hill, or sprinting, or skating into strong headwind, or on a slow surface (rough or soft).

Pivot-aim

This variation is recommended by some speedskating coaches. The foot is set down aimed roughly straight in the overall direction of forward motion, then pivoted to aim outward (away from the other leg) for its outward leg-push.

The foot does not push sideways in this variation. It only glides and supports body weight.

Pushing outward would contradict the push by the other leg and/or weaken the sideways weight shift which will add work to this leg's later push.

Pushing inward could add propulsive Work -- and by my definition that is one form of Double-push.

There are two possible timings for the pivot from straight to aiming outward:

  • At or before the time the other foot lifts up off the ground -- which implies that the foot was set down significanly before the other foot lifted up, and that the other (pushing) foot shares in supporting body weight the whole time the foot is gliding straight.

  • After the other foot lifts off the ground -- so there's time with the foot gliding straight supporting full body weight.

Advantage of setting down straight before lift-up of pushing:

  • helps emphasize a low hip position for the Extension push, because the foot gliding straight can support the weight of the upper body allows the hip to stay low for a longer time period.

This supporting of a longer lower hip position could also have been achieved with the foot already aimed outward and starting its own push (especially if it had been set-down further underneath inside). Or as part of Double-push.

  • perhaps allows time get the edging of the newly set-down foot under control?

Advantage of continuing to glide straight after lift-up of pushing foot:

  • makes it possible to extract net positive propulsive work from a backward-forward arm-swing.

But it is difficult to imagine a situation in which this additional propulsive work from the arm-swing could outweigh the loss in Power (measured in Watts) from slowing the stroke-cycle and reducing turnover frequency. Especially with the alternative of getting propulsive work from sideways arm-swing which does not require any gap between leg-pushes.

Double-push

Fastest technique for expert inline speedskaters for non-short time durations on gentle terrain without sharp curves. The foot is set down and then pushed toward the inside, with the upper body getting pushed toward the outside (toward the side of the new pushing leg). Then the foot is pivoted to aim outward (away from the previous pushing leg, toward the new pushing leg), and pushed toward the outside, with the upper body stopping going toward the outside, then starting to go toward the inside (away from the pushing leg, toward the side of the next pushing leg).

Three variations of Double-push:

The key distinction between Double-push and Normal-push (in my definition) is that "Double push" has two pushes in a row by the same leg, the first toward the inside and the second toward the outside, with the other leg making no push in between those two. In Normal-push first leg makes one push, then the other leg makes one push.

Some people define "Double push" as when the foot makes something like an S curve on the ground, first a smaller arc to the inside, then a larger arc on the outside. By my definition, you can steer your foot around all the inward arcs you want -- but unless you actively push toward the inside, you're still doing Normal-push. And as long as you push to the inside and then the outside, there's no need for an "S curve" -- it counts as Double-push.

Advantages:

  • can engage new muscles on the inside of the leg (hip-adduction, ankle-supination, possibly lateral knee-hip rotation)

  • moves quickly through the less effective "between" phase into the more powerful "Extension" phase of the main outward push.

  • can use the big hip-extension and knee-extension muscles at higher turnover frequency.

  • can increase the speed and mass engaged in sideways weight shift.

Increase speed because there's an additional sideways push by the leg (provided this push is transmitted to the upper body, and provided the timing of the upper body moves is accurate), and because the leg recovering through the air can contribute its full sideways speed to propulsive work. In Normal-push, only the sideways speed that the leg retains thru Set-down contributes to propulsion.

Extension in-push -- (higher turnover of Extension muscles)

The obvious idea for the inward push of Double-push is to make substantial use of all the muscles available to help. The leg muscles available are the Extension muscles (familiar from walking and running -- and the normal outward push in skating), and the little-known Sweep-In muscles on the inside of the leg, which can include hip-adduction, lateral knee-hip rotation, and ankle-supination.

So the In-push is much like the outward main push in its angles and phases, except that the Sweep-In muscles are substituted for the Sweep-Out muscles, and the Extension phase 3 is cut short. The leg does not push to full extension.

There are two problems with going to full extension: (a) the upper body starts to fall downward to the outward side, where there's no other leg to catch its fall; and (b) the leg must be compressed again before it can make its outward main push, and too much compression required adds time to the stroke-cycle, which tends to reduce Power (measure in Watts).

Theme of this approach is to get higher turnover frequency of the big leg Extension muscles (more like the frequency used in bicycling). The balance and sideways motion required for this variation tends to feel pretty radical, so (as of 2007) it's only used by a few elite racers.

Sweep sideways focus -- (engage new leg muscles)

This approach focuses on the Sweep-In move (inward push phase 1), and cuts out much of the Extension move (inward push phase 3 and some of 2). The Sweep-In muscles on the inside of the leg are not used propulsively in Normal-push skating, and not used propulsively in walking, running, bicycling, or other effective ways to move across a significant distance on land. There are used for balance and stability, and for positioning moves in lots of sports.

Theme of this approach is to engage these little-known leg muscles for propulsion, and spread the work load across more muscles, which should enable the skater to sustain a higher rate of Power (measured in Watts) near their lactate-related threshold -- which is important for performances longer than sprints.

These Sweep-In muscles are not very big or strong in most people, so for this approach it likely makes sense to do some special training exercises to develop them.

Lots of inline speedskaters who are serious about outdoor racing use this approach, because it's easier to learn to perform reliably than the "Extenion in-push" variation. People who train on off-road trailways with limited width are more likely to take this approach.

Straight only in-push

This variation keep the foot aimed roughly straight in the direction of overall forward motion thru the whole in-push. There is an active push toward the inside, but there is no aiming or arcing of the foot inside. So there is no direct propulsive work from this push, but it does increase the speed of the sideways weight shift, which can be converted into additional propulsive work by adding force to this leg's outward push which it makes after the foot pivots to aim toward the outside.

The difference between this and the Pivot-aim variation of Normal-push is that this Straight only variation of Double-push does make an active push (toward the inside) while gliding straight, and the Pivot-aim Normal-push does not.

It is not unusual for other Double-push variations to set down with the foot aiming straight (to help support the weight of the upper body to sustain a lower hip position longer for the Extension push by the other leg), then pivot to aim inward and push inward. This pivot is separate from the Aim-switch pivot at the finish of the In-push.

Advantages of this variation:

  • Balance is simple.

  • Simple support for the low hip position enables the Extension push of the other leg to contribute a higher proportion directly into propulsive Work.

This support could also be provided by the other Double-push approached, but this variation keeps it simpler and more focused.

  • Adds some propulsive work of its own (unlike Pivot-aim Normal-push).

  • Less frictional loss and possible edge-control difficulty than the full Aim-switch of the other Double-push approaches.

  • Good for maintaining the rhythm of Double-push while climbing up a hill, where taking the time to aim the foot inward would leave too large a gap between the high-force outward pushes.

Theme of this variation is to increase sideways shift of upper body weight and to support the Extension phase of the other leg.

For an example of an elite ice speedskater using it, see Chad Hedrick's medal-winning performances in the 2006 Olympics in Torino.

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skating situations

Different skating situations call for different adjustments and variations of the moves -- or sometimes a very different technique style. The analysis of a video depends on what is the situation. Below are the main ones I'll be considering in these pages about video analysis.

standard-form perceptual check

This is a "situation" not of performance, but for developing more accurate perceptions of how different positions and rhythms should feel -- and identify mis-perceptions.

For example, lots of skaters feel that they are setting down with their ankle straight, when actually they're setting down with their ankle significantly pronated.

The idea of the observations for this "standard-form" style is that they are either: (a) fairly simple to observe in video; or (b) often misperceived by skaters; or (c) difficult to perform but important to learn.

The positions and moves and rhythms are not necessarily the most effective for power or speed. Sometimes they are good to expose to the conscious mind -- even if the neuro-muscular control module in our unconscious brain does something different in a performance situation.

higher speed

The standard higher speed situation is skating at a steady speed on flat terrain with a low resistance surface and no wind.

The typical strategy for skating in this situation is:

  • engage more muscles (including less-strong muscles) -- which more widely spreads the "load" of handling lactate-related stress.

  • increase the range-of-motion of muscles, to get more Work out of each push -- which reduces the percentage of Work wasted as "overhead" in setting up each push.

higher force

Two situations which require higher force are climbing up a steep hill, and sprinting acceleration from a standing start. Others are skating a headwind, or on a soft surface.

The typical adjustments to skating motion for a higher force situation are:

  • stronger muscles, better developed for high force, deliver a higher percentage of Power output.

Some muscles have Slow Oxidative (SO) fibers which are better developed for higher force. Or perhaps just having a greater quantity of SO fibers which have lots of "practice" of neural coordination for working together to deliver a higher total force without too much stress on any single fiber. Or perhaps there are other ways that muscle fibers close together can help each other out in demanding situations (? e.g. share fuel ?)

  • less-strong muscles are engaged less, or only hold static to transmit the force of other muscles.

Holding static ("isometric") normally engages mainly SO fibers, which produce less lactate-related substances.

  • turnover frequency is higher.

That's how to get a similar Power rate out of a smaller total of muscle mass engaged. By focusing more on SO fibers, you can utilize a larger percentage of your central cardio-vascular capacity without generating so much lactate-related substances.

  • for a longer-duration performance in a higher force situation, the range-of-motion is shorter, limited to the segment of a muscle's range which is better suited for sustaining high force.

The idea is to choose a segment where the muscle has better "leverage" -- able to apply a higher force to the ground and mass of the rest of the body while exerting a lower force within itself.  E.g. the hip-extension and knee-extension have better leverage when the leg is straighter.

There are two main ways to generate problematic lactate-related substances which limit longer-duration performance: (a) Engage SO fibers without delivering enough oxygen to them; or (b) Engage a substantial percentage of FG fibers even if there is lots of oxygen available.

The problem with exerting higher force within the muscle is that it engages more of the Fast Glycolitive (FG) fibers -- which produce Work only anaerobically, so they inherently produce lactate-related substances -- which are a problem to manage and become a limiter on performance for longer duration. (Not that lactate itself is bad, but it's an easily-measured marker of anaerobic process.) So even if there is enough oxygen available to the muscle, there is still substantial lactate-related problem.

Also the FG fibers are more limited in the kinds of fuel they can "burn" (only glucose and glycogen -- unlike SO fibers which can also "burn" lactate and fatty acids) -- so once their stored glycogen is depleted their future effectiveness on that day is limited (because it takes a long time to restore glycogen). So for a longer duration performance, it's better to use a motion style which can be executed mostly with the Slow Oxidative (SO) fibers, and not use the motion style (e.g. larger range-of-motion including lower leverage configuration) which engages the FG fibers until near the finish.

  • for a longer-duration performance, find (clever?) ways to reduce the required Power output.

more . . .

Some other situations which might be somewhat different:

  • very low gliding resistance (especially ice)

  • high gliding resistance (especially snow)

  • using ski poles or rollerski poles to help push more through the arms

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checkpoints in stroke-cycle

A checkpoint is a point in time in the stroke-cycle when one or more observation are made. Since observations are changing throughout the stroke-cycle, in order to get an accurate observation it's important to be clear about which video frame to choose to observe it. 

Checkpoints shared by Normal-push + Double-push:

  • Set down of foot

  • Midway thru leg-push

  • Finish of leg-push

  • Recovery of leg thru air 

Double-push: additional checkpoints

  • Max-In-Push (Front view)

  • Finish of In-push (Side view)

  • Crossing from inside to outside (Front view)

Overall: There are also important observations not tied to a single checkpoint:  See Overall observations through the stroke-cycle.

Details

  • Set-down: the moment the foot lands down on the ground. [ observations ]

Before Set-down the foot was somewhere up in the air. After Set-down the foot is gliding on the ground.

  • Midway: positionally about half-way between Set-down and Finish of stroke. [ observations ] 

In foot-angle view, at Midway a line between the center of the hip and ground-contact is slanted about 20-25 degrees away from vertical.

  • Finish stroke: just before the foot lifts up off the ground. [ observations ]

If getting very detailed, there could be two different moments: (a) one when only the heel lifts off the ground (for a non-klap inline skate) or lifts off the frame + blade for a klap-frame on ice, or off the binding + ski for snow; and (b) another just before everything comes off the ground. Unless stated otherwise, this paper will use the more obvious definition (b).

  • Recovery (in the air): the "Recovery" moves are how the foot gets from Finish to Set-down. [ observations ]

In order to make repeated strokes, after each leg-push is finished, all the skater's body parts must be "recovered" back to the same configuration they had at the start of the push.

Normally the Recovery moves take place while the foot is up in the air, but on inline or ice skates it's also possible to "recover" while the foot is gliding on the ground, sometimes called "sculling". This method usually does not produce as high a power output or overall forward skating speed as recovery thru the air -- and it's instructive both to try it in actual skating, and to figure out the physics of why the difference in forward speed.

Double-push additional checkpoints

  • Max In-push -- maximum inward reach (Front view):  The frame in Front view that shows maximum inside reach by the foot in inward leg-push. [ observations ]

Likely this checkpoint is pretty close to the same time as the Finish In-push checkpoint. But in Front view it's difficult to know in which frame the inward push finishes. So we offer a slightly different checkpoint which is easier to detect.

  • Finish In-push (Side view):  The finish of the inward push. The frame in Side view when the foot is farthest forward relative to its hip. [ observations ]

  • Crossing from inside to outside (Front view):  The frame in Front view when the foot passes vertically under its pushing hip. [ observations ]

more . . .

 

?? [ more to be added ]

 

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more . . .

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