Freeman's forward foot thrust during double poling.

The discussions of Freeman recently made me think of a video I
downloaded off Jann's download page last winter. It's a relay race I
beleive from Austria. Kris leads the opening classic leg putting the
U.S. out in front. While entering the stadium it is apparent Kris is
thrusting his feet forward together while double poling. The Swedish
commentator is really impressed saying Freeman's technique is
excellent. I've experimented with this forward movement with the feet,
but for me it takes my attention away from double poling powerfully.
The last time I saw a skier using this technique was Lubov Egorova
during the Lillehamer Olympics. Who is to say this helps with speed,
or momentum?

--- Douglas Diehl wrote:
The discussions of Freeman recently made me think of a video I
downloaded off Jann's download page last winter. It's a relay race I
beleive from Austria. Kris leads the opening classic leg putting the
U.S. out in front. While entering the stadium it is apparent Kris is
thrusting his feet forward together while double poling. The Swedish
commentator is really impressed saying Freeman's technique is
excellent. I've experimented with this forward movement with the
feet,
but for me it takes my attention away from double poling powerfully.
The last time I saw a skier using this technique was Lubov Egorova
during the Lillehamer Olympics. Who is to say this helps with speed,
or momentum?

I don't think he is thrusting his feet forward, but pulling them
forward with the power of his stomach crunch. I teach this in my
clinics: Stand still on your rollerskis. Put your arms forward in the
poling position. Now try to pull your feet forward with just your
lower abs. That's the feeling you want in the DP. NOT thrusting the
hips back when you start to pole (like most of us do).

Rob Bradlee

I find the phrase, "pulling them forward with the power of his stomach
crunch," to be subject to more than one interpretation. The question is
whether Kris is actively supplementing the effect of his crunch with an
active ankle thrust, in the context of coming down the stretch of his
relay leg. I'd guess he better had been, if he didn't want to face
criticism for dogging it. For the past two seasons, some teachers
("coaches") here in TC have been teaching just such an active ankle
thrust, as a supplement to a good crunch. I think this came from
attending Subaru Factory team clinics.

Gene


Rob Bradlee wrote:

I don't think he is thrusting his feet forward, but pulling them
forward with the power of his stomach crunch. I teach this in my
clinics: Stand still on your rollerskis. Put your arms forward in the
poling position. Now try to pull your feet forward with just your
lower abs. That's the feeling you want in the DP. NOT thrusting the
hips back when you start to pole (like most of us do).

Rob Bradlee

I think forward thrust of the foot could add a little power to
double-poling, if it's timed right.

Douglas Diehl wrote:
While entering the stadium it is apparent Kris Freeman
is thrusting his feet forward together while double poling
. . . Who is to say this helps with speed?

From physics, quickly starting the feet moving forward relative to the rest
of the skier's body generates a "reactive" force in the backward direction
(by Newton's Third Law). The magnitude of work done by this force is equal
to the kinetic energy at the maximum relative velocity attained during the
thrust. To the extent that this work can be transmitted (see more below) to
the pole-tips and thru them into the snow, it helps with the skier's overall
speed.

But how the move _stops_ is just as important. Because Newton's Third Law
says that stopping also generates a force, this time a _negative_ force.
The critical point is _not_ to permit this negative force to be transmitted
thru the pole-tips into the snow -- otherwise it will exactly cancel the
good work done by the starting of the thrust. The simplest way is to _time_
the forward-thrust so that the feet are not slowing down again until _after_
the pole-tips come up out of the snow. The ideal is to have the feet attain
maximum velocity (relative to the rest of the body) at just before the
pole-tips lose firm contact with the snow, and only then begin slowing down.

Seems like it makes sense to time it so the start of the foot-thrust comes
later than the start of the pole-push (and maybe it's better to delay the
start of the foot-thrust to pretty near the end of the pole-push?)

Other ideas:
- - quickness: Sudden acceleration to max velocity, because amount of useful
work is proportional to max velocity. Perhaps the elite racers are doing
"plyometric" shin-muscle exercises?

- - position at start: The larger the range of motion distance the ankle
moves relative to the body, the higher max-velocity can be attained with the
same acceleration. So perhaps start with the ankle-joint strongly flexed,
positioned way behind the knee joint.

- - transmit the force "rigidly" thru the body to the ski-pole-tip.

But I'm thinking that the mass of the feet is so small that the max kinetic
energy and work generated directly in the basic forward thrust is not much.
And there's so many body parts in between to absorb the force before it gets
transmitted thru the ski-pole-tip usefully into the snow. Maybe worth
optimizing for elite racers, but there must be lots of other technique
points for the rest of us to learn with higher priority.

Classic striding and kick-double-pole:
Same basic physics principles apply to the forward thrust of other body
parts in other Classic techniques, notably classic striding, where you bring
the recovering leg and arm forward thru the air, while the other leg is
pushing back and down against the snow. The reactive-force benefit there is
real, but in the videos I don't see the elite racers doing anything tricky
to manipulate it (like say, delaying the start of their forward-recovery
move). Except perhaps in kick-double-pole with the pre-positioning of the
non-pushing leg.

Ken
____________________________________
P.S.
But this forward-foot-thrust work does not add speed unless it can be
transmitted into snow. The force initially goes into the skier's body:
first the leg, then the hips, then the abdomen and chest ("core
stability"?), then to shoulders and arms. To the extent that all these body
parts transmit the force in a "rigid" way, instead of absorbing it by
collapsing or shifting away -- that's what percentage of the work will get
transmitted thru the pole-tip to the snow to help with overall speed.

So like if the hips are dropping back at the same time as the feet are
thrusting forward, they could be absorbing the work from the feet intead of
transmitting it thru to the chest and shoulders -- of course that hip move
might be doing something else that helps propulsion. Lotsa trade-offs to
consider (and measure?)
____________________________________

Whoa ... I try to keep quiet, but hang on ....

:^]


From physics, quickly starting the feet moving forward relative to the rest
of the skier's body generates a "reactive" force in the backward direction
(by Newton's Third Law).

His feet moving forward is already a result of force applied through the poles
and body, not the other way around.

The magnitude of work done by this force is equal
to the kinetic energy at the maximum relative velocity attained during the
thrust. To the extent that this work can be transmitted (see more below) to
the pole-tips and thru them into the snow, it helps with the skier's overall
speed.


???


But how the move _stops_ is just as important. Because Newton's Third Law
says that stopping also generates a force

Stopping is a result of force, this time a _negative_ force.
The critical point is _not_ to permit this negative force to be transmitted
thru the pole-tips into the snow -- otherwise it will exactly cancel the
good work done by the starting of the thrust. The simplest way is to _time_
the forward-thrust so that the feet are not slowing down again until _after_
the pole-tips come up out of the snow. The ideal is to have the feet attain
maximum velocity (relative to the rest of the body) at just before the
pole-tips lose firm contact with the snow, and only then begin slowing down.

If your feet are going faster than your center of gravity at the release of the
poles from the ground, then what makes the body's center of gravity catch up to
the feet???

[snip]

I think it's got more to do with reducing friction of the kick wax ... why
don't you scoot your feet ahead like that when you V2 skate?


jw
Milwaukee

jw wrote
I think it's got more to do with reducing friction of the kick wax

Creative suggestion -- and that might be why some people do it.

But I'm not yet convinced, because . . .
in double-poling only about half the skier's body weight is riding on each
ski, so the kick grip wax should not be making much contact with the snow
anyway.
And actually the kick wax zone of the ski has no way of "knowing" where it
is forward/backward in relation to the skier's center of mass. All it
"knows" is how much pressure is coming thru front and rear of the binding,
from the toe and middle and heel of the skier's foot. So as long as the
skier presses more with the heel of the foot than with the toe and ball of
the foot, there will be a reduced friction of the kick wax. No need to
thrust the foot forward, just

why don't you scoot your feet ahead like that when you V2 skate?

Most obviously because it might interfere with using the feet optimally in
the skate-push, and the useful propulsive work from the main leg push is way
bigger than the reactive force from foot-thrust -- so who would want to risk
any loss in the big thing for such a small potential gain?

Also because in skating the general problem with forward-backward reactive
force is that it's difficult to time in the _stopping_ part of the move at a
moment in the stroke-cycle when the negative reactive force would _not_ be
transmitted into the snow (similar problem with the backward recovery move).
That's why skating technique focuses mainly on side-to-side reactive forces.

His feet moving forward is already a result of force
applied through the poles and body, not the other way around.

Yes, some of the feet's forward motion is due to the current pole-push --
and even more is just inertia because they were already moving forward at a
pretty good speed from 37 previous pole-pushes.

But I think the point of the foot-thrust move is to give the feet some
_extra_ forward speed by using like maybe the quadriceps muscles.

If your feet are going faster than your center of gravity
at the release of the poles from the ground, then what
makes the body's center of gravity catch up to the feet???

The usual way -- just pull them back underneath using your calf muscles.

Ken

"Ken Roberts" wrote in message
...
And actually the kick wax zone of the ski has no way of "knowing" where it
is forward/backward in relation to the skier's center of mass. All it
"knows" is how much pressure is coming thru front and rear of the binding,
from the toe and middle and heel of the skier's foot. So as long as the
skier presses more with the heel of the foot than with the toe and ball of
the foot, there will be a reduced friction of the kick wax. No need to
thrust the foot forward, just


You're ignoring the fact that the toe is also connected to the ski. If you
lean back at the end of the poling motion, you can not only press down at
the heel, but lift up with the toe. This changes the shape of the wax
pocket considerably. You can't do this if you are just putting your weight
straight down on the heel.

-Nathan

Wow -- this leg thrust move works way better than I expected. I tried it on
rollerskis on a rail trail over the weekend and measured a substantial gain
in a personal time-trial: 1:35 versus 1:55.

Douglas Diehl wrote:
While entering the stadium it is apparent Kris Freeman
is thrusting his feet forward together while double poling

The video (less than 1 Megabyte) of Kris Freeman doing it is
http://avari181.mt.luth.se/Technic/KrisFreemanpoll.mpg

Even if half of my gain was from skier enthusiasm rather than physics +
biomechanics, that's still like an 8% gain in speed. My first time trying
it, after only fifteen minutes of practice (preceded by an hour of other
rollerski poling drills).

Should be even bigger gain on snow, where I'd additionally gain as much
reduction in ski-glide friction as jw and Nathan are talking about. (unlike
on rollerskis, where I think the big difference in weighting between the
rear and front wheels slightly _increases_ the total rolling friction.)

Reactive force?
Yes I think so. Seems to me there's a choice he
(a) a skier _could_ do all the same arm + shoulder + abdominal crunch +
forward-hip-flex moves exactly the same as Kris Freeman -- and also still
lower his butt exactly the same vertical distance -- by bending his knees
and flexing his ankles underneath his butt (using hamstring + shin muscles).
or
(b) a skier could do with his legs what Kris Freeman is doing: Thrusting his
whole lower leg forward using his quadriceps and calf muscles.

Move (b) results in more useful propulsive work than (a), because of the
reactive force backward from accelerating the mass of the lower leg forward.

I note that Kris Freeman accurately executes the critical _timing_ required
to exploit a reactive-force pair: so the forward motion of his lower leg
stops just _after_ his pole-tips leave the surface of the snow.

Ken

P.S. There might also be a "mental image" effect -- that my _thinking_ about
thrusting my legs forward somehow helped me better engage my abdominal
crunch muscles like Rob Bradlee suggested. Pretty good if I were somehow
getting _three_ benefits from trying this one forward-thrust move: reactive
force, improved glide on snow, and better engagement of crunch-muscles.

On Tue, 10 Aug 2004 18:38:32 +0000, Ken Roberts wrote:

Wow -- this leg thrust move works way better than I expected. I tried it on
rollerskis on a rail trail over the weekend and measured a substantial gain
in a personal time-trial: 1:35 versus 1:55.

A word of caution. You can get significant forward thrust of your foot by
just keeping your hips back, and as a consequence weight the back of the
ski. This definately does not speed you up. Thanks to Bryan Fish for the
tip.

Philip Nelson wrote
A word of caution. You can get significant forward thrust
of your foot by just keeping your hips back, and as a
consequence weight the back of the ski.
This definately does not speed you up.

I find that with any clever move that includes lowering my hips or some part
of my upper body, there's always the temptation to get lazy on the
_recovery_ move needed to get it back up high again (especially when I get
tired or distracted).

Not only does an incomplete upward recovery result in less useful work out
of future repetitions of that clever move -- also typically the resulting
lower upper body position messes up some other move which is more important.

Which confirms the general advice given some good coaches to focus my
attention on strong execution of the main power moves from a good body
position for those, and not to get distracted by the clever extras I see in
videos of elite racers.

. . . as a consequence weight the back of the ski.
This definately does not speed you up.

Really? even in soft snow conditions?

Ken