reactive force in Classic striding

I got thinking about how to deal with the reactive forces from moving body
parts not connected to the snow in Classic diagonal striding. Key things I
found:

- - reactive down-force from raising the torso + head with good timing will
improve kick-grip. It's what the elite racers do (instead of the
"leg-stomp" taught by some American instructors). see
http://roberts-1.com/xcski/classic/secrets/back_lift

- - delaying the crunch-down of the chest + shoulders onto pole until after
start of the pole-push transmits positive reactive force to forward-motion
work.

- - timing the forward-recovery moves of the leg and arm can help forward
propulsion. The key is to delay the start of these moves until after the
tip of the other pole is planted in the snow and starting to push.

- - negative impact on forward-propulsion work is another source of
inefficiency in Classic striding compared with Skating. Yet another reason
why even at slow speeds up steep hills, V1 Skate is usually faster than
Classic.

- - sideways and vertical reactive forces have no significant direct impact
on forward-propulsion work in Classic (though they may have an important
impact on the kick-grip which is a prerequisite for much forward-propulsion
work).

So I'm going to focus the rest of this post on forward and backward moves
and forces.

More detail below.
Warning: Way too much detail. Stop reading here.

Ken
_______________________________________
Details on forward and backward reactive forces:

The physics of "reactive" or inertial forces is described by Newton's Third
Law (as in like "every action has an equal and opposite reaction"). Key
things to know about them:
(a) They can help or hurt a skier's speed or kick-grip, depending on their
direction and timing.
(b) A reactive force is generated by stopping the move of any body part,
just as much as by starting the move. Reactive forces in skiing always come
as a pair: one from starting, one from stopping -- in opposite and
cancelling directions, but with different timing. So the critical thing to
manage is the timing.
(c) A reactive force only impacts a skier's overall speed if it gets
transmitted to the snow. In Classic skiing this happens whenever either the
pole tip or the kick-zone of the ski is stopped on the snow, but not while
simultaneously the ski is gliding and the pole tips are in the air.
(d) So the only way a pair of reactive forces can impact the skier's overall
speed is by special timing: if one comes while the pole-tip or ski is
stopped in the snow, and the other one comes during a pure "passive glide"
phase.
(e) The biggest factor in determining the amount of positive (or negative)
Work done by a reactive force is the peak velocity attained by the body part
being moved. (Work = 0.5 * Mass_of_body_part * peak_Velocity *
peak_Velocity * inefficiency_factor).

Implications for Classic striding technique:
(1) Time the forward or backward moves of major body parts so that its
positive reactive force comes while the pole-tip is stopped in the snow, and
its negative reactive force comes during the "passive glide" phase (while
simultaneously the ski is gliding and the pole-tips are in the air).

(2) For a move with a net positive impact of its reactive-force pair, hold
back the start of the move, and then make it very quickly, for maximum peak
Velocity and therefore maximum physical Work added to the stroke-cycle. For
a move with a net negative impact, make it softer and slower.
________________________
Summary analysis of forward-backward moves of six major body parts:

- - recovering Arm: lots of freedom for different timing possibilities
here -- see discussion below.

- - recovering Leg: the Stopping is negative for forward-propulsion, but
it's hard to prevent it from being transmitted into the snow. Good timing
on the Starting can cancel this negative -- see discussion below.

- - pushing Arm: Starting a direct push backward into the snow is
inevitably negative in terms of reactive force, but it's the direct push is
so much larger that it's worth accepting it. If the slowing and stopping of
the "follow-through" of the arm is done passively by gravity, or all comes
after the end of the active leg-push, then the positive reactive force from
the stopping is not transmitted into the snow, so it's a net negative.
Though there _might_ be an opportunity in using bicep and forearm and
front-shoulder muscles to earlier _slow_ the follow-through of the arm from
the pole-push -- make some of this slowing come before the leg-push
finishes. The reactive force from this slowing would then add positive
reactive force to the leg-push, but trying this also has negative
consequences which I'd suspect are larger.

- - pushing Leg: same impact of Starting as with the arm-push. The
reactive force from the Stopping of its follow-through is positive, but I'm
not seeing any reasonable way to avoid having it come in the passive glide
phase. So this move is inevitably a net negative impact on propulsive Work.
At lower speeds, the positive work from the direct push backward is so much
greater that it's worth enduring the negative. But the higher the velocity,
the higher the negative -- another reason that Classic striding is so much
less effective than skating for high speeds.

- - torso + head crunch-down: crunching down onto the pole with
front-abdominal and chest muscles also has a forward aspect to it, so
reactive force from the Starting of this move is aimed backward and
potentially positive. But it would not get transmitted into the snow to
help forward-propulsion work if it started during the "passive glide" phase.
So the best elite racers delay its start until after the pole is planted in
their classic striding and kick-double-pole (but not in their pure
double-poling -- why?). When the crunch-down stops, the forward-component
of the reactive force exactly cancels the earlier positive for
forward-propulsion (assuming the skier was first careful with timing to
_get_ that positive -- otherwise the net impact is negative). And by timing
the Stopping of the crunch-down to come partly after the start of the
leg-push, the downward-component of the reactive force has a positive impact
on kick-grip.

- - torso + head straighten-up: The move from using the back muscles to
straighten the upper body also has a backward aspect. The forward-component
of the reactive force from starting this move has a negative impact on
forward-propulsive work, but its downward-component has a positive impact on
kick-grip, so elite racers are glad to start this move in the middle of
their leg-push. The reactive force from the slowing and Stopping of this
move is positive for forward-propulsion but negative for grip -- but the
proportions are different from at the Starting, because the angle of motion
is different (so uncertain implications for timing).
________________________________
more on Recovery of Arm + Leg

Implication 1 above explains why it helps to delay the recovery moves of the
arm and leg. The starting acceleration of those moves generates a backward
force, which will help push the skier forward _if_ transmitted into the
snow -- but that transmission won't happen if the moves are started during
the passive glide phase -- so it's key to delay them until after the
pole-tip is planted and pushing.

I was coming to think based on Implication 2 that it would also help to hold
the arm and leg way out back _behind_ while waiting to start the
forward-move. Because then there's more distance available to accelerate to
maximum peak velocity and therefore higher positive work added to the
stroke-cycle. Or perhaps delay the start of one or both recoveries even
later, but then make the move extra-quick.

The problem with this is that elite racers seem to _stop_ these
forward-recovery moves of the arm and leg while the other leg is still
pushing backward into the snow. And the reactive-force from this stopping
deceleration is aimed forward -- so it cancels some of that good leg-push
force aimed backward. The higher the previous peak velocity, the greater
the negative force-cancellation. So right now it's looking to me like
there's no benefit in trying to increase the peak velocity, because the
starting and stopping forces are largely self-cancelling.

So why don't the elite racers make one or both recovery moves a little
slower? (so the stopping would not come until the passive glide phase, after
the end of the pushing by the other arm and leg). My answer is that the
leg-push stops by coming off the ground, so the other recovering leg has to
already be there in position to support the weight of the skier's body, and
"be there in position" implies "not still moving forward relative to the
rest of the skier's body".

The stopping of the recovering easily be delayed to come during the passive
glide phase, to avoid transmission of its negative impact. So this is
simple a simple way to increase the total forward-propulsive work of the
stroke-cycle. But it's not what elite racers do, because the key driver of
overall skier speed is the Power _rate_ of work per minute (not work per
stroke-cycle). Delaying the stopping of the arm-recovery means delaying the
starting of the next arm-push, which means delaying the whole next
stroke-cycle. So this delay end up increasing the total _time_ duration of
the stroke cycle, so fewer stroke-cycles per minute, so less work per
minute. Since the mass of the arm is not that large, the gain in work is
not large, so for elite racers it's not worth increasing "dead spot" time
gap of the passive glide phase. (For a skier with goals other than speed,
might be worth considering).
_______________________________________

Hey Ken - I think you're really on to something with this focus on reactive
force. However, I think you've once again identified a good issue and then
overanalyzed it to a ridiculous level, and along the way gotten yourself
irretrievably screwed up. I'd be interested to see some video of you putting
all of this into action. I think I have a fairly clear idea of what it'll
look like, and it's not pretty. It's also not effective.

I have a fairly significant batch of skis to grind today, so I can't spend a
great deal of time on this. However, I'll make a couple of notes. I don't
know that I really want to invite further conversation, but I guess that's
what I'm doing.

Anybody with a rowing background will understand the importance of reactive
forces. I'm not much of a rower but I spent a couple of summers getting out
once or twice a week. When you're rowing your body weight is by far the
greatest concentration of mass in the boat. And it's on a slide. As you can
imagine, it's possible to use the slide effectively to help the boat move
along. It's also possible to just about bring it to a halt. In skiing we're
generally not talking about moving such large masses (in proportion to the
whole) that reactive forces are enough of a consideration to screw with too
much. As Ken has noted, timing is the key.

Here's the trick. Natural timing and smooth motion are not a product of
continual on-the-fly analysis and adjustment. They exist as motor engrams -
motions that are so ingrained as to be automatic. If you want to make a
meaningful change to technique you've got to identify the fundamental motion
and "drum it in" through mind-numbing repetition. Most skiers with
relatively good athleticism and a basic skill set use something very close
to the optimal timing for their technique. Very infrequently I run into
somebody with screwed up timing, and it's usually because they've been told
to change it. You don't make effective changes to timing (or to reactive
forces) by thinking about each individual component and trying to adjust
them cognitively.

If you really want to try to test the use of reactive forces there are a
couple of drills that can be very helpful, and that don't require extensive
analysis. My favorite - one that my Uncle John has used to embarass a lot of
pretty good skiers - is to diagonal stride with no kick wax. If you really
want the sum of your reactive forces to propel you forward, take away the
platform for your more conventional active forces. This drill is frustrating
at first, but after an hour or so you'll find that you can ski up hills with
almost no slipping, if your timing and position is good, and if you've got
decently strong arms.

The other interesting drill for working with reactive forces is to try
double poling with no poles. If you can use your body weight and compression
to actually move yourself forward, you've learned something. This is pretty
easy on roller skis where you've got a ratchet to lean against. Much harder
on snow.

Finally, I'll not that the emphasis on ensuring good kick through a "back
lift", or any other "secret" move points to fundamental problems. If your
skis fit right and you're even in the ballpark on wax choice you just
shouldn't have to do anything too special to get good grip. If you do it is
a clear indication of a problem with your weight shift and/or your body
position and position over your skis. It's not terribly uncommon to find
skis that are poorly fit and won't work right. It's much more common to find
skiers trying lots of tricks to get kick because they don't have sound
fundamentals.

Well, as tempting as a point-by-point rebuttal may be, I don't have the
time. I also expect that it would serve no purpose. So I'll leave it at
that!

Zach



"Ken Roberts" wrote in message
news
I got thinking about how to deal with the reactive forces from moving body
parts not connected to the snow in Classic diagonal striding. Key things
I
found:

- - reactive down-force from raising the torso + head with good timing
will
improve kick-grip. It's what the elite racers do (instead of the
"leg-stomp" taught by some American instructors). see
http://roberts-1.com/xcski/classic/secrets/back_lift

- - delaying the crunch-down of the chest + shoulders onto pole until
after
start of the pole-push transmits positive reactive force to forward-motion
work.

- - timing the forward-recovery moves of the leg and arm can help forward
propulsion. The key is to delay the start of these moves until after the
tip of the other pole is planted in the snow and starting to push.

- - negative impact on forward-propulsion work is another source of
inefficiency in Classic striding compared with Skating. Yet another
reason
why even at slow speeds up steep hills, V1 Skate is usually faster than
Classic.

- - sideways and vertical reactive forces have no significant direct
impact
on forward-propulsion work in Classic (though they may have an important
impact on the kick-grip which is a prerequisite for much
forward-propulsion
work).

So I'm going to focus the rest of this post on forward and backward moves
and forces.

More detail below.
Warning: Way too much detail. Stop reading here.

Ken
_______________________________________
Details on forward and backward reactive forces:

The physics of "reactive" or inertial forces is described by Newton's
Third
Law (as in like "every action has an equal and opposite reaction"). Key
things to know about them:
(a) They can help or hurt a skier's speed or kick-grip, depending on their
direction and timing.
(b) A reactive force is generated by stopping the move of any body part,
just as much as by starting the move. Reactive forces in skiing always
come
as a pair: one from starting, one from stopping -- in opposite and
cancelling directions, but with different timing. So the critical thing
to
manage is the timing.
(c) A reactive force only impacts a skier's overall speed if it gets
transmitted to the snow. In Classic skiing this happens whenever either
the
pole tip or the kick-zone of the ski is stopped on the snow, but not while
simultaneously the ski is gliding and the pole tips are in the air.
(d) So the only way a pair of reactive forces can impact the skier's
overall
speed is by special timing: if one comes while the pole-tip or ski is
stopped in the snow, and the other one comes during a pure "passive glide"
phase.
(e) The biggest factor in determining the amount of positive (or negative)
Work done by a reactive force is the peak velocity attained by the body
part
being moved. (Work = 0.5 * Mass_of_body_part * peak_Velocity *
peak_Velocity * inefficiency_factor).

Implications for Classic striding technique:
(1) Time the forward or backward moves of major body parts so that its
positive reactive force comes while the pole-tip is stopped in the snow,
and
its negative reactive force comes during the "passive glide" phase (while
simultaneously the ski is gliding and the pole-tips are in the air).

(2) For a move with a net positive impact of its reactive-force pair, hold
back the start of the move, and then make it very quickly, for maximum
peak
Velocity and therefore maximum physical Work added to the stroke-cycle.
For
a move with a net negative impact, make it softer and slower.
________________________
Summary analysis of forward-backward moves of six major body parts:

- - recovering Arm: lots of freedom for different timing possibilities
here -- see discussion below.

- - recovering Leg: the Stopping is negative for forward-propulsion, but
it's hard to prevent it from being transmitted into the snow. Good timing
on the Starting can cancel this negative -- see discussion below.

- - pushing Arm: Starting a direct push backward into the snow is
inevitably negative in terms of reactive force, but it's the direct push
is
so much larger that it's worth accepting it. If the slowing and stopping
of
the "follow-through" of the arm is done passively by gravity, or all comes
after the end of the active leg-push, then the positive reactive force
from
the stopping is not transmitted into the snow, so it's a net negative.
Though there _might_ be an opportunity in using bicep and forearm and
front-shoulder muscles to earlier _slow_ the follow-through of the arm
from
the pole-push -- make some of this slowing come before the leg-push
finishes. The reactive force from this slowing would then add positive
reactive force to the leg-push, but trying this also has negative
consequences which I'd suspect are larger.

- - pushing Leg: same impact of Starting as with the arm-push. The
reactive force from the Stopping of its follow-through is positive, but
I'm
not seeing any reasonable way to avoid having it come in the passive glide
phase. So this move is inevitably a net negative impact on propulsive
Work.
At lower speeds, the positive work from the direct push backward is so
much
greater that it's worth enduring the negative. But the higher the
velocity,
the higher the negative -- another reason that Classic striding is so much
less effective than skating for high speeds.

- - torso + head crunch-down: crunching down onto the pole with
front-abdominal and chest muscles also has a forward aspect to it, so
reactive force from the Starting of this move is aimed backward and
potentially positive. But it would not get transmitted into the snow to
help forward-propulsion work if it started during the "passive glide"
phase.
So the best elite racers delay its start until after the pole is planted
in
their classic striding and kick-double-pole (but not in their pure
double-poling -- why?). When the crunch-down stops, the forward-component
of the reactive force exactly cancels the earlier positive for
forward-propulsion (assuming the skier was first careful with timing to
_get_ that positive -- otherwise the net impact is negative). And by
timing
the Stopping of the crunch-down to come partly after the start of the
leg-push, the downward-component of the reactive force has a positive
impact
on kick-grip.

- - torso + head straighten-up: The move from using the back muscles to
straighten the upper body also has a backward aspect. The
forward-component
of the reactive force from starting this move has a negative impact on
forward-propulsive work, but its downward-component has a positive impact
on
kick-grip, so elite racers are glad to start this move in the middle of
their leg-push. The reactive force from the slowing and Stopping of this
move is positive for forward-propulsion but negative for grip -- but the
proportions are different from at the Starting, because the angle of
motion
is different (so uncertain implications for timing).
________________________________
more on Recovery of Arm + Leg

Implication 1 above explains why it helps to delay the recovery moves of
the
arm and leg. The starting acceleration of those moves generates a
backward
force, which will help push the skier forward _if_ transmitted into the
snow -- but that transmission won't happen if the moves are started during
the passive glide phase -- so it's key to delay them until after the
pole-tip is planted and pushing.

I was coming to think based on Implication 2 that it would also help to
hold
the arm and leg way out back _behind_ while waiting to start the
forward-move. Because then there's more distance available to accelerate
to
maximum peak velocity and therefore higher positive work added to the
stroke-cycle. Or perhaps delay the start of one or both recoveries even
later, but then make the move extra-quick.

The problem with this is that elite racers seem to _stop_ these
forward-recovery moves of the arm and leg while the other leg is still
pushing backward into the snow. And the reactive-force from this stopping
deceleration is aimed forward -- so it cancels some of that good leg-push
force aimed backward. The higher the previous peak velocity, the greater
the negative force-cancellation. So right now it's looking to me like
there's no benefit in trying to increase the peak velocity, because the
starting and stopping forces are largely self-cancelling.

So why don't the elite racers make one or both recovery moves a little
slower? (so the stopping would not come until the passive glide phase,
after
the end of the pushing by the other arm and leg). My answer is that the
leg-push stops by coming off the ground, so the other recovering leg has
to
already be there in position to support the weight of the skier's body,
and
"be there in position" implies "not still moving forward relative to the
rest of the skier's body".

The stopping of the recovering easily be delayed to come during the
passive
glide phase, to avoid transmission of its negative impact. So this is
simple a simple way to increase the total forward-propulsive work of the
stroke-cycle. But it's not what elite racers do, because the key driver
of
overall skier speed is the Power _rate_ of work per minute (not work per
stroke-cycle). Delaying the stopping of the arm-recovery means delaying
the
starting of the next arm-push, which means delaying the whole next
stroke-cycle. So this delay end up increasing the total _time_ duration
of
the stroke cycle, so fewer stroke-cycles per minute, so less work per
minute. Since the mass of the arm is not that large, the gain in work is
not large, so for elite racers it's not worth increasing "dead spot" time
gap of the passive glide phase. (For a skier with goals other than speed,
might be worth considering).
_______________________________________

On Sat, 7 Aug 2004, Zachary Caldwell wrote:

If you really want to try to test the use of reactive forces there are a
couple of drills that can be very helpful, and that don't require extensive
analysis. My favorite - one that my Uncle John has used to embarass a lot of
pretty good skiers - is to diagonal stride with no kick wax. If you really
want the sum of your reactive forces to propel you forward, take away the
platform for your more conventional active forces. This drill is frustrating
at first, but after an hour or so you'll find that you can ski up hills with
almost no slipping, if your timing and position is good, and if you've got
decently strong arms.

The other interesting drill for working with reactive forces is to try
double poling with no poles. If you can use your body weight and compression
to actually move yourself forward, you've learned something. This is pretty
easy on roller skis where you've got a ratchet to lean against. Much harder
on snow.

Hey, now this is interesting!

I'm going to try it tonight. I'll take the chain off my bike and see
if I can ride up the hill to my house!

Ok. Seriously, there was a Cornell coach 15 or 20 years ago who used
to demonstrate diagonal stride without kick wax. Gary somebody. He
could do it amazingly well. To watch him you'd never know he had no
wax. Frustrating as can be to try to copy him. He was from Vermont,
too. Probably a connection there to Uncle John I bet.

-Mitch

Looks like Zachary and I have some common ground:
- - reactive force can be significant for propulsive moves
- - timing the moves is important, and
- - most people have a basic feel for an appropriate timing of
reactive-force moves

Zachary Caldwell wrote
I'd be interested to see some video of you putting all of this into
action.

Well I'm not saying that "all this" is a good way for anybody to ski,
because managing reactive force is only one ingredient of skiing
performance -- and likely down around number 5 on the priority list in the
physics + biomechanics of Classic striding. So all those ways I gave to
manipulate reactive force are only "other things being equal", not "the
right way to ski".

Also, some of those moves are difficult to learn (as Zachary pointed out) --
and learn in a _way_ that does not distract or obstruct other more important
moves. So instead let me suggest these videos of two far better skiers:
- - Kris Freeman: http://engineeredtuning.net/KrisClassic.mpg (less than a
Megabyte to download, thanks to Zachary for compressing to the essence).
- - Mika Myllyla: http://avari181.mt.luth.se/Technic/Mika3.mpg (3.9 MB)

I'm seeing all three of the first three reactive force moves in my original
being performed in those two videos, when I use Pause and Slow-motion to
view them -- I find the Apple Quicktime Viewer better than the Microsoft
Windows Media Player for that. One thing to try is Pausing the video at the
instant the pole is planted, and see that the recovering leg and arm are
still out behind, and the torso and head are still in a high position. Then
watch how the torso comes forward _after_ the pole plant, then rises back up
during the next leg-push -- all in harmony with the physics of exploiting
reactive force in my original post.

You don't make effective changes to timing (or to
reactive forces) by thinking about each individual
component and trying to adjust them cognitively.

I'm not claiming to know how to _learn_ these moves (? different approaches
work better for different skiers ?). For now I'm playing with the physics
to try to lay out a range of possibilities for _what_ things to consider
even _trying_ to learn.

Like that forward-leg-thrust in pure double-poling. Until Douglas Diehl
told us on the newsgroup about what he got from Kris Freeman from that
Swedish video, I never even _thought_ that reactive-force move. Then once I
actually _tried_ it I started getting some measurable benefit in just
fifteen minutes (though of course not optimized like Kris and Daehlie and
the World Cup winner).

Most skiers with relatively good athleticism and
a basic skill set use something very close to the optimal timing for their
technique.

Yes, once I had a clue what to even try.

But one aspect of reactive-force that did _not_ come naturally for me in a
different move was the gain from holding back the start of the move, then
performing the move quicker to attain a higher peak velocity. Which so far
as I can tell is not very relevant for Classic striding, since I haven't yet
found in the physics any non-cancelling reactive-force pairs which are
positive for forward propulsion work.

Ken
____________________________________
Zachary Caldwell wrote
Hey Ken - I think you're really on to something with this focus on
reactive
force. However, I think you've once again identified a good issue and then
overanalyzed it to a ridiculous level, and along the way gotten yourself
irretrievably screwed up. I'd be interested to see some video of you
putting
all of this into action. I think I have a fairly clear idea of what it'll
look like, and it's not pretty. It's also not effective.

I have a fairly significant batch of skis to grind today, so I can't spend
a
great deal of time on this. However, I'll make a couple of notes. I don't
know that I really want to invite further conversation, but I guess that's
what I'm doing.

Anybody with a rowing background will understand the importance of
reactive
forces. I'm not much of a rower but I spent a couple of summers getting
out
once or twice a week. When you're rowing your body weight is by far the
greatest concentration of mass in the boat. And it's on a slide. As you
can
imagine, it's possible to use the slide effectively to help the boat move
along. It's also possible to just about bring it to a halt. In skiing
we're
generally not talking about moving such large masses (in proportion to the
whole) that reactive forces are enough of a consideration to screw with
too
much. As Ken has noted, timing is the key.

Here's the trick. Natural timing and smooth motion are not a product of
continual on-the-fly analysis and adjustment. They exist as motor
engrams -
motions that are so ingrained as to be automatic. If you want to make a
meaningful change to technique you've got to identify the fundamental
motion
and "drum it in" through mind-numbing repetition. Most skiers with
relatively good athleticism and a basic skill set use something very close
to the optimal timing for their technique. Very infrequently I run into
somebody with screwed up timing, and it's usually because they've been
told
to change it. You don't make effective changes to timing (or to reactive
forces) by thinking about each individual component and trying to adjust
them cognitively.

If you really want to try to test the use of reactive forces there are a
couple of drills that can be very helpful, and that don't require
extensive
analysis. My favorite - one that my Uncle John has used to embarass a lot
of
pretty good skiers - is to diagonal stride with no kick wax. If you really
want the sum of your reactive forces to propel you forward, take away the
platform for your more conventional active forces. This drill is
frustrating
at first, but after an hour or so you'll find that you can ski up hills
with
almost no slipping, if your timing and position is good, and if you've got
decently strong arms.

The other interesting drill for working with reactive forces is to try
double poling with no poles. If you can use your body weight and
compression
to actually move yourself forward, you've learned something. This is
pretty
easy on roller skis where you've got a ratchet to lean against. Much
harder
on snow.

Finally, I'll not that the emphasis on ensuring good kick through a "back
lift", or any other "secret" move points to fundamental problems. If your
skis fit right and you're even in the ballpark on wax choice you just
shouldn't have to do anything too special to get good grip. If you do it
is
a clear indication of a problem with your weight shift and/or your body
position and position over your skis. It's not terribly uncommon to find
skis that are poorly fit and won't work right. It's much more common to
find
skiers trying lots of tricks to get kick because they don't have sound
fundamentals.

Well, as tempting as a point-by-point rebuttal may be, I don't have the
time. I also expect that it would serve no purpose. So I'll leave it at
that!

Zach

Ken Roberts wrote
I got thinking about how to deal with the reactive forces from
moving body parts not connected to the snow in Classic
diagonal striding. Key things I found:
- - reactive down-force from raising the torso + head with
good timing will improve kick-grip.
- - delaying the crunch-down of the chest + shoulders onto
pole until after start of the pole-push transmits positive reactive
force to forward-motion work.
- - timing the forward-recovery moves of the leg and arm
can help forward propulsion. The key is to delay the start
of these moves until after the tip of the other pole is planted
in the snow and starting to push.
. . .

Zachary Caldwell wrote
Hey Ken - I think you're really on to something with this
focus on reactive force. However, I think you've once
again identified a good issue and then overanalyzed it to a ridiculous
level . . .

If the real experts would publish or post something which analyzed this at
an _appropriate_ level, then I would not be tossing out my attempt. So
please just point me at some other publicly-available information in the
last fifteen years related to reactive force and classic striding, at _any_
level of sophistication or detail, and I'll be grateful and go off and see
what I can learn from that instead.

I bet the Soviet team fifteen years ago had a similar (but more accurate)
memo in their technique collection, written by some assistant coach, as part
of their systematic search for a winning edge -- and probably a couple of
other countries' national ski teams. Who knows -- even the U.S. national
team? It is rumored there are skiers on the current U.S. team who know
quantum physics and abstract algebra. Perhaps they have lowered themselves
to the old "classic" physics and mere vector linear algebra -- and figured
out all this reactive force stuff, surely three times better than my
attempt -- then focused in on the only two "nuggets" in it that really help.

But I have not yet found where they disclose those findings to _me_. Seems
like I have to learn them from video clips offered by our newsgroup hero in
Sweden.

Cross country skiing seems like an _abnormal_ sport to me. Contrast with
bicycling, where the basic physics and biomechanics have been understood for
decades, and I have a book on my shelf that explains it all. Different
coaches recommend different mental imagery or training strategies or
slightly different optimazations of force distribution over the
pedal-stroke-cycle, but it's not like major gaps in publicly-available
knowledge of how it works. Same with downhill Skiing: the physics is
complicated and non-intuitive, but there are several books on my shelf that
explain how it works -- and I don't see big debates and confusion about
technique on the discussion forums I'm involved with for non-XC skiing
(except when some die-hard telemarker tries to claim that not only does the
telemark turn look cool and feel cool, but it's also more effective than
christie or parallel turns). Same with Running, even Swimming (though the
physics of swimming to me seems _very_ tricky).

So I'm just trying to do something to help cross country skiing become like
the other human-propulsive sports I enjoy.

Please send me more and better public stuff that fills in the knowledge
gaps, and I'll learn it and use it and publicize links to it that other
folks can benefit from.

Ken

There's an aspect of reactive force physics where my understanding seems
completely different from what
Zachary Caldwell wrote:
My favorite drill is to diagonal stride
with no kick wax . . .
. . . The other drill for is to try
double poling with no poles.

My problem is that my understanding of the physics is that the only forces
that help forward propulsion are those that are transmitted to something
outside the skier's own body. So unless there's a favorable wind, or you're
going downhill, or you've got some kind of air-paddles -- or nearby
tree-branches to pull on -- that means the force must be transmitted into
the snow or ground. And the only ways I know are thru a pole-tip stuck into
the snow or by a ski base that's currently getting some significant friction
with the snow-surface.

If you really want the sum of your reactive forces
to propel you forward, take away the platform
for your more conventional active forces.

But reactive forces must use the same "platforms" as active forces (pole-tip
or ski-base-grip-zone) if they're going to make a substantial ongoing
difference in the forward speed of the skier's whole body.

Since the reactive forces that arise from moving a body part that stays
connected to the skier's body come in opposite pairs, the only way to get a
substantial net gain in forward propulsion is to substantially _transmit_ at
least one force out of the pair into the snow or ground.

So I'm not seeing how drills that substantially reduce the ability to
transmit _any_ force to the ground or snow are going to help learn about how
to exploit reactive force (or at least the "Newton's Third Law" variety of
reactive force). To me the essence of exploiting reactive force is just the
opposite: cleverly selective _strong_ transmission of force.

They're both good drills, but they don't seem to focus mainly on reactive
forces -- unless it's like "absence makes the heart grow fonder".

What am I missing here?

Ken