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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.
-
Theme: Add reactive side-force and perhaps
forward-force to the finish of the other leg's push, and land the
foot in position and with momentum to extract the most power out of
this leg's upcoming push.
-
Set down the next foot before the previous
leg-push finishes pushing: (a) to stop the mass of skater's body
from falling too low; (b) to support the last part of the push by
the other leg; and (c) to minimize any "dead spot" or "low
power" gap in the stroke-cycle, by starting this leg's push as early
as possible.
-
Go for maximum effective range of motion in the
start of the main-push: [ ski: ] Land the ski with
the minimum inward tilt needed to transmit force through the inside
edge. Set down as close toward underneath the pushing hip and with
as little ankle-pronation as possible. [ skate
normal-push: ] Land the skate on outside edge further inside
than its hip, and with the ankle supinated to reach across the
centerline a little further.
-
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 pelvis rotation" move). Then the pelvis + hips
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 actual propulsive
work in itself -- not just preparation for work in other phases.
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.
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.
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??
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. ]
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overview of phases | R 0
1 2 3 | d-p
phases | more Leg
??
- 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. ]
?? 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". ]
?? 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)
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