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intro

This is one phase in a detailed analysis of the sequence of moves for Leg-push motions of "normal  push" method of skating. For more context and an overview of all the phases of the sequence, see the summary of normal-push phases.

Set-down path and Landing configuration of Normal-push are significantly different from the Double-push set-down -- see Set-down phase ip0 for in-push.

key points

  • Theme: Land foot early to start pushing early, and in position to extract the most power out of this leg's upcoming push.

  • "Overlap" Timing: Set down the next foot before the previous leg-push finishes pushing: (a) to allow the mass of skater's body to drop low without fear of falling over onto the ground; (b) to "catch" the maximum percentage of reactive-side-force energy from the previous leg-push and upper-body side-moves; (c) to support the last part of the push by the other leg; and (d) to add power and minimize any "dead spot" gap in the stroke-cycle, by starting this leg's push as early as possible.

  • Prepare for effective range-of-motion in Side-of-leg-Out phase of main-push:

[ ski: ] Land the ski with the minimum inward tilt needed to transmit force through the inside edge -- or possibly sometimes even a bit on the outside edge. Set down closer toward underneath the pushing hip, with knee pointed out, and with as little ankle-pronation as possible.

[ skate normal-push: ] Land the skate on outside edge further inside than its hip, with the knee pointed outward, and with the ankle supinated.

  • Prepare for effective aiming of push-force and range-of-motion in Extension phase of main-push, by flexing the ankle strongly forward, so the ankle joint is way back behind the knee joint.

  • Pelvis + hips do not face with aim of next ski or skate at set-down (which is different from shoulders). Optimal is to start with pelvis and hips facing somewhat toward the opposite side, so during the leg-push the non-pushing hip moves forward ("forward-abdomen-rotation" move). Then the pelvis + hips will have turned to face with aim of the skate or ski at the end of the leg-push. (Also, the most propulsively effective positions and rotations of the torso + shoulders are often opposite to those of the pelvis + hips).

  • The set-down move can do a little actual propulsive work in itself -- not just preparation for work in other phases.

definition of this phase

The Set-down phase starts when the knee and foot have reached their farthest inward position while from their recovery move in the air. It ends when the foot lands down on the ground.

If the Recovery move did not move the knee and foot further inward than the landing point, then the distance and time of the set-down are very short, nearly zero. But in many situations the elite racers first move their knee and foot further inward in recovery, so the set-down move has an outward component, and persists through a noticeable time and distance.

If the Recovery phase R included an optional (non-propulsive) backward move, then the set-down phase will include a forward move of the foot and leg. This can produce additional net positive work for propulsion -- provided that the aim-angle of the ski or skate at the stopping of this forward move is closer to straight forward than the aim-angle of the ski or skate at the starting of this forward move. (which normally does not happen for skis).

Set-down path and Landing configuration for Normal-push here are significantly different from the Double-push set-down for in-push in phase ip0.

drivers of propulsion

sideways moves

For normal-push, these are the key physical parameters that determine the amount of propulsive work added from sideways components of moves in this phase:

(a) the maximum outward speed attained by the mass of each part of the leg (upper leg, lower leg, foot with attached skate or ski).

Secondary driver: The maximum speed is higher if the sideways finish-position for the leg moves of the Recovery phase R is farther inward (other things being equal).

For double-push, no significant net propulsive work is added by sideways moves,  because both legs are pushing toward the same side during this phase.

But double-push already received significant benefit from sideways move in the previous Recovery phase R.

forward-backward moves

??

These are the key physical parameters that determine the amount of propulsive work added from forward-backward components of moves in this phase:

(a) how much larger is the aim-angle of the skate or ski during the propulsion-positive part of these moves (starting of the forward component) than is the aim-angle during the propulsion-negative part of these moves (stopping of the forward component).

(b) the maximum speed of the forward component of these moves.

Timing coordination is not a key decision point, because the starting comes during Phase 3 of the other leg and the stopping comes mostly during Phase 1 or Phase U1 of this leg. But there's still the question of how quick or soft the landing is:

(c) how much of the maximum speed is retained to the instant of landing (rather than slowing down the forward motion for a softer impact into the ground).

The crucial point is to get parameter (a) to be significantly different from zero.

For normal-push, there will be a difference in aim-angles, if the aim-angle on landing at the end of the Set-down phase 0 (and during the initial part of Phase 1) is nearly straight in the forward-motion direction.

So if perform normal-push in the simple obvious way, with a constant aim-angle throughout the push, there's little need to worry about forward-backward leg motion during the Recovery and Set-down phases. Which is why there is usually little or no propulsive benefit to forward-backward leg motion in the Recovery phase with skis.

One obvious way to get propulsive benefit from the forward-backward leg motion components in Recovery and Set-down is to start the push with the aim-angle of the skate small during the early part of Phase 1 just after Set-down, then pivot the skate outward to a larger aim-angle before the start of the Central push of Phase 2.

For double-push, there will be a difference in aim-angles, if the aim-angle on landing at the end of the Set-down phase U0 (and during the initial part of Phase U1) is nearly straight in the forward-motion direction.

Small aim-angle on landing typically comes with the "lite" style of double-push.

muscle moves

??

for forward propulsion:

propulsive only in special cases:

  • hip flexion
  • knee extension

Bringing the foot and leg forward during set-down is net propulsive only if aim-angle of the other ski or skate is pivoted outward during its contact with the ground. Of course this forward move must be prepared for by the optional extra backward move during Recovery phase R. The first problem is that the starting of this extra backward move is counter-productive for propulsion, so the stopping of it delivers no net gain in propulsive work (except to the extent that the backward component of leg and foot motion was already necessary as part of the main leg-push).

The second problem is that while the starting of the forward move delivers reactive force to the other leg which is positive for propulsion, the eventually required stopping of the forward move is negative for propulsion. So the way to obtain a net gain in propulsion is to change the aim-angle of the ski or skate to pivot more outward during its time on the ground -- so the stopping comes during Phase 1 when the aim is more straight forward, so the backward component transmitted to the ground is lower than in Phase 3 when the aim-angle is more out to the side.

It is difficult to pivot a ski while its pushing on the ground without large frictional costs, and it's difficult to swing the ski backward very far in recovery without it getting tangled in the ground.

[ ice + inline: The backward-recovery move and pivot while pushing are much easier on ice or inlines, so this forward move is often used by speedskaters in normal-push (non-double-push) stroking. But in double-push stroking it's more difficult to get the timing exact to exploit differences in aim-angle, so the focus in recovery and set-down is more on side-motion and on getting the skate down on the ground to start into direct pushing. ]

details + hints

??

  • Set down the next foot before the previous leg-push finishes pushing -- during Phase 3 of the previous leg-push.

Even if the next foot is not visible pushing yet, just having it on the ground and trying to hold its position relative to the other foot is already adding force to Phase 3 of the other foot's push. The force needed to "try to hold" the next foot (net of some across-the-body transmission losses) is added to the pushing force through the other foot.

Another way to look at it is that having the next foot down already gives the other foot something to push against. If the next foot were not on the ground, more of the work of the other foot would be going into moving the mass of the upper body faster sideways, instead of immediately propulsive. The faster sideways motion can be converted into propulsion is future phase, but there is some power lost in that conversion.

Another advantage of double-push over normal-push stroking is that the aim-angles of the two feet during the period of overlap is closer (because they're both aimed to the same side), so more force can be transmitted for a longer period between the two feet, especially at high speeds. If the feet are aimed further apart while they're both on the ground, then they "split" apart much quicker for the same amount of force between them.

[ ski: Early set-down might not work so well in icy conditions. It might be necessary to delay the landing of the next ski until the end of the previous leg-push -- so that the previous ski retains full down-force for maximum edge-grip into the hard snow. ]

  • Foot and knee are started back outward a little ways, just before the foot lands.

To add beneficial reactive side-force to the Main-push of the other leg.

?? [ double-push is different: the knee does not come outward, but instead the foot comes way outward. ]

  • ?? The foot moving forward in the air passes close by the other foot on the ground.

?? Some skate coaches say: "the toe of one kisses the heel of the other".

?? [ double-push is different: the foot goes way outside, but the knee comes forward close. Some skate coaches say: "the knees kiss". ]

  • ?? Knee and toe of foot start to rotate inward just before the foot lands.

?? To generate rotational momentum and kinetic energy to help the propulsive inward-knee-roll move in Phase 1.

?? [ double-push is different: knee and toe hold steady, or perhaps start to rotate outward just before the foot lands. ]

  • ?? At the instant of landing, the aim of the Knee is in line with the aim of the ski or skate -- or perhaps a bit more outward.

?? If the knee is already rolled partway inward at set-down, then the inward-knee-roll muscles have less range-of-motion to add propulsive work to Phase 1. So better to err toward having the knee aiming a little toward the outside at set-down.

?? [ double push is different: the knee comes more inward close to the other knee (some coaches say "the knees kiss", ready for a little outward-knee-roll move at the start of the first (inward) push of the double-push stroke. For normal-push stroking, better to err on the side of the knee pointing more outward. ]

  • Pelvis and hips are not facing aimed in line with the aim of the skate or ski.

Actually the pelvis optimally faces slightly away toward the side of the previous leg, because of the forward-hip-rotation move. Second best is to land with the Pelvis aimed straight in the line of the skater's overall average forward motion.

This is different from the alignment of the shoulders at set-down.

Optimal is to start with pelvis and hips facing somewhat toward the opposite side, so during the leg-push the non-pushing hip moves forward and then the pelvis + hips have turned to face with aim of the ski or skate at the end of the leg-push.

You cannot get the benefit of power from the Forward-hip-rotation move, if the opposite hip is already forward at the start of the leg-push. Rather you get a negative impact on power from a "backward-hip-rotation" move -- which makes the leg-push feel easier, because it is less effective for propulsion.

Nose alignment? NKT? The aim of the Nose at set-down is irrelevant to the physics and biomechanics of skating propulsion. The concept of aiming the Nose toward aim of the next ski or skate at set-down can be negative for propulsion, because it offers a temptation to get the Pelvis aimed that same way at set-down -- which requires a backward-hip-rotation move which actually reduces propulsive power.

  • [ ski: ] Land the ski slightly tilted onto its inside edge -- to transmit reactive side-force from the previous leg-push.

and sometimes to begin the next leg-push as early as possible (especially when climbing up a steep hill).

Some instructors and books say that the ski should be set down flat in order to minimize friction in gliding. But there is not that much extra friction from tilting the ski just a little bit on edge -- and usually this is more than compensated for by the opportunity to transmit and apply forces as in Phase 1.

[ inline: Inline skates can be pushed effectively even if the wheelframe is tilted the "wrong" way, so in normal-push stroking with inline skates, I can get a longer range of outward push by setting the skate down across the centerline to the inside of the pushing hip, with the wheelframe tilted toward the outside. ]

[ ice: Speedskaters on ice also can set down on the outside edge and start pushing outward through the outside edge. ]

  • [ ski: ] Land the ski the minimum edging needed to transmit force to the snow without the ski edge slipping.

Many skiers "over-edge" -- tilt the ski more onto its inside edge than necessary. Sometimes this is because they have not yet developed solid edging control. But other times it's because they have not tried to out how little edging is actually needed. Anyway one important way to develop better edging control is to practice holding edge-grip with less tilting.

A key reason that minimum edging is important is because more edge-tilt usually requires setting the ski down further out toward the side -- which results more strain on leg muscles and joints -- see next point below.

Using minimal edging enables starting the leg-push close in underneath the hip, which enables the hip to stay higher and the next knee straighter, so less strain for the same size of leg-push.

In soft snow, more edging results in the ski digging down more into the snow -- which increases friction -- another disadvantage of over-edging.

Icy snow is the typical situation where more tilting on edge might be needed in order to maintain edge grip.

  • Pronation of ankle at set-down?

[ ski: It is possible to edge the ski by pronating the ankle (i.e. bending the ankle joint sideways toward the inside of the knee-heel line).

The downside of this approach is that if the ankle is already pronated at set-down Phase 0, then it the ankle-pronator muscles have less range-of-motion available to add work in later phases of the leg-push. Therefore ...

Most skiers most of the time should set-down with the ankle aligned vertically in the line between the heel and knee.

But in icy snow conditions the extra edging help of early ankle-pronation might be necessary at set-down. Or perhaps if you've got strong hip-abductor muscles for Phase 1, but weak ankle-pronator muscles, then it might be better to use early ankle-pronation to enable an effective set-down more underneath the hip and less toward the outside. ]

[ inline: Since the amount of grip from inline skate wheels is not much dependent on the precision of how the wheelframe is tilted, there can be no benefit from pronating the ankle at set-down. Better to save all the range-of-motion of the ankle-pronator muscles to add active push work to later phases when the wheels are already on the ground. Indeed a skater with strong ankle-pronator muscles could add further range-of-motion to the normal push stroke by setting down with the ankle in a supinated position (ankle joint bent sideways toward the outside of the knee-heel line). ]

[ ice: Like for inlines, can also land with ankle in supinated position, then use the ankle-pronation for a propulsive move. Also perhaps on ice there is another consideration: an ice speedskater might make an ankle-pronation move just as the skate tips from outside edge onto inside edge -- to get the blade decisively onto the new edge, more quickly through the "no man's land" of straight-up vertical. That's because ice skates have real edges, so precision with them really matters sometimes. (while inline skate wheelframes do not have edges, so straight-up vertical is not a "no man's land" to be rushed through.) ]

  • [ ski: ] The foot lands roughly underneath its hip joint.

Except that skiers who do not have hip abductor muscles developed to stably transmit side-forces might need to land the snow a little outside its hip joint -- to use bone-joint alignment to make up for the lack of muscle strength.

The problem with landing the ski further outside the its hip joint is that this results in the hip dropping lower at the end of the leg-push -- if the leg pushes out all the way. Because the weight of the upper body falls farther if the leg is not as much vertically from underneath it.

Lower hip means that the knee of the next leg must start lower and more bent, so more strain on the big leg muscles and knee joint of the next-pushing leg.

This lower hip result could be avoided by shortening the leg-push -- by not pushing all the way out. But not using the straightest segment of the leg-push range-of-motion requires putting more strain on the big leg muscles and knee joint of the currently-pushing leg. So the only way I see to avoid the strain is to weaken the main leg-push.

[ inline: Inline skates can be pushed effectively even if the wheelframe is tilted the "wrong" way, so in normal-push stroking with inline skates, I can get a longer range of outward push by setting the skate down across the centerline to the inside of the pushing hip. Speedskaters on ice also normally set down to the inside of the pushing hip. ]

  • Aim the next ski or skate as far out toward the side as is consistent with the current forward speed.

It is true that as the speed increases, the ski or skate must be aimed more foreward in order to "keep up" with high speed in the leg-pushes.

But the straighter foreward angle of the skate or ski is a result of higher speed, not its cause.

The cause of higher speed is higher forward-propulsion power. Forward-propulsion power is generated by the backward-component of the leg-push -- which is readily transmitted through a ski or skate that is aimed out toward the side.

And foreward-propulsion power is generated when the sideways-component of the leg-push is converted into the direction of forward-motion. Physics says that this directional power-conversion from sideways into foreward is most direct and efficient the more the skate or ski is aimed out toward the side.

Of course there's a limit to how wide an angle can be handled at a higher speed. But it's good to keep playing with how far can aim the ski or skate out toward the side for a given speed of foreward motion -- to try to "err" on the side of more angle out to the side.

The situation where this strategy could be disadvantageous is in soft or slow snow.

[ inline: Speedskaters on inline skates sometimes land the skate aimed more forward than they angle they intend to use for the main push with in Phase 2, or sometimes it's even aimed toward the inside (in "double-push" technique). That way it can better and longer support the final phase of the push of the other skate (because it stays closer to the other skate longer if its angle is closer to the other skate), or even make its own positive push toward the other side (in "double-push"). Then the skate is pivoted to the optimal angle for Phase 2. A ski could also support the other ski longer, or even actively push toward the other side -- but then too much power would lost during the pivot -- so the ski must be set down already aimed at an angle pretty near ready for Phase 2, even though this angle is sub-optimal for Phase 1. ]

  • The foot is moving outward before the foot lands down in the snow.

This "pre-push" generates a little reactive side-force into the leg-push.

The landing of the foot also generates a little reactive down-force into the leg-push, but not much of this down-force gets converted into side-push or forward-push, because in this Phase the ski or skate is too close underneath the hip. So much of this down-force just goes into compressing the snow -- which might also increase the frictional resistance against gliding, by making the ski "plow" deeper.

This "pre-push" also helps assure continuity of pushing across the two legs -- no gap or "dead spot" between the two pushes (a key strategy theme for skating up a hill).

(note that ski-skating V2 and OFS leg motions might be different from this)

more . . .

see also

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concept words: skating skate skates skater skaters push glide inline inlines ski skiing snow roberts

skating: skate skates skater skaters push glide

inline inlines ice speed speedskate speedskating speedskater speedskaters roller

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