[ under construction ]
[ more to be added ]
January 2006 ideas
Double-push stroking is faster than normal-push in some situations
especially the "Inside-of-leg-In" muscle
moves, including hip adduction, lateral-hip-rotation (with bent knee),
especially the forward component of
leg-recovery. Perhaps also the possibility of using forward arm-swing.
Double-push does a better job of exploiting the forward-backward
reactive-force (at set-down for In-push) separately and without
disrupting side-side reactive-force (through the Aim-switch).
Double-push moves more quickly through the
non-optimal "in-between" leg-lean angle, to better exploit the most
effective leg-extension propulsion configuration in phase 3 --
and without missing out on the best configuration for the
Side-of-leg-Out muscle mores in phase 1a.
Double-push builds more sideways kinetic energy,
because the in-push has a strong sideways component.
Not clear to me that "avoiding deep knee bend" is an
advantage of double-push -- or if it's even desirable to avoid that.
Maybe the point is not to avoid deep knee bend, but to get more
propulsive benefit from it.
May 2005 ideas
Excerpts from Ken Roberts post to rec.sport.skating.inline group:
The key advantage I'm seeing is biomechanics:
double-push enables a higher percentage of the pushing (and a higher
percentage of any passive-gliding support) to to be performed with a
straighter leg. Applying force with less knee-bend puts less strain on
the big leg muscles and tendons.
I learned this "avoiding deep knee bend"
explanation from Barry Publow's book, Speed on Skates (Human Kinetics,
1999) ISBN 0-88011-721-4. This question is addressed on pages 36-37.
The problem with trying to get a bigger stronger
classic push stroke is that it requires bending the knee more at
set-down, and during the first phase of the main leg-extension push.
Double-push escapes from this problem by re-using the straighter
less-bent-knee segment of the leg-extension range-of-motion. Another way
to think of this is that the skater's hips are on average not a low as
they'd be with "classic" stroking at the same speed.
The two key "lessons" I'm getting so far from the
analysis for my own skating are:
push all the way to full leg-extension. (otherwise
miss the main benefit)
pushing on the outside edge with strong
leg-extension directly out toward the side can help
forward-propulsion power even if the skate is aimed straight ahead
(in the right context). So there's no need to cut off the
outside-edge push early.
Still the puzzle remains that there are surely some
drawbacks to double-push -- say from the complexity (overhead?) or the
single-ski-crossover phase (dead spot gap?). So how does double-push
generate enough power to make up for that?
What's the underlying physics + biomechanics of why the "double-push"
stroke technique takes less strain than simpler "classic" stroking to go
the same speed?
(Of course in some situations double-push imposes more strain, like
for climbing up a steep hill, and simpler "classic" stroking is more
effective there. A correct and useful theory ought to explain that
Phases of the leg stroke
(1) Set-down of right skate.
In race videos I've analyzed, the leaders set the right skate down on
the pavement -- somewhat on its outside edge -- and while the previous
left skate is still making its main regular push on its inside edge. Not
sure how this helps or hurts, or if it's even different than simpler
"classic" stroke (if nothing else, it gives the leg-recovery-lift
muscles a rest). I wondered if the leaders were starting to push on the
outside edge of the right skate already simultaneous with the left
skate's main push, but I saw no evidence of that in the videos (? if we
put force sensors in their wheelframe, then we could know better ?)
There is no non-pushing Recovery phase gap in double-push. The right
leg is already pushing thru its outside edge -- already using its big
muscles in leg-extension -- while the Left leg is tracing its path thru
the air. Now I'm not saying that there is or should be any non-pushing
phase in the simpler "classic" stroke-cycle (see discussion under phase
4) -- but there's at least a "light-push" phase where any pushing is
being done by smaller weaker muscles (e.g. hip abductors / gluteus
medialis) than the big main leg-extension quadriceps and gluteus
Push with leg-extension toward the left thru outside edge of right
skate, with the skate aimed somewhat toward the left side.
The right hip is not as low at the start of this push as it would be
in the simpler "classic" stroke, because the phase-5-Left inside-edge
push that preceded it was not as long as the "classic" stroke would have
been -- so there wasn't as much time for the hips and upper body to
"fall". (Actually the hips might not literally fall much or at all, but
there's still a necessary physical relationship between
stroke-length-time, average vertical-leg-angle, and average
Since there is a backward-directed component to the push-force vector
during this phase, some of the work done clearly adds to
forward-propulsion power. And since Power (in Watts) partly follows
Velocity, and Velocity is mostly forward not sideways, the component of
Power going directly into forward-propulsion is typically larger than
the component going into side-to-side motion of the skater's
center-of-mass, even though the Force is aimed mostly sideways. The
puzzle is that when I look at race leaderr videos, this phase seems so
short. (whereas for many of us non-elite skaters the problem is that
though we've learned to ride beautifully on that outside edge, we're not
actually pushing much on it with the big leg-extension muscles -- so we
get all the overhead of the double-push, but miss out on most of the
Right skate curves thru aiming straight forward.
But as the skate curves to nearly straight forward, there is little
or no direct forward-propulsion benefit, because there is little or no
backward-directed component in the leg-push. Yet in my video analysis of
leading racers, I see them continuing their push into this
aiming-straight-forward point. How could this help?
My answer is in this case side-push does help, indirectly -- after
delay -- by generating sideways momentum of the skater's center-of-mass
toward the right side. Then in phase 5 when the skater makes the main
push toward the right side (which then does have a backward-directed
component), much of this side-momentum gets "caught" and converted into
forward-propulsion power. (and maybe some already in phase 4). And this
point about delayed feed of power into forward-propulsion also applies
to the sideways-component of power from phase 2.
One thing I notice in the video clips I've analyzed so far of both
Eddy Matzger and Chad Hedrick doing double-push in actual races is major
side-to-side movement of the hips and shoulders -- but can't tell if
it's more than they would use for simpler "classic" stroking, because
nowadays it's hard to find any video clips of an elite racer not using
double-push in any comparable situation.
(4) Right skate continues curving and aiming toward the right side.
During this phase the right leg must get flexed again as it comes
underneath the right hip. The hip is not as low as it would be in the
simpler "classic" stroke, because the phase 2 outside-edge push that
preceded it was not as long as a regular classic stroke, so there wasn't
as much time for the hips and upper body to "fall". The skater can
choose to flex the hip and knee even lower during this phase, to make
the main phase 5 push a little longer -- which makes sense. (Or could
even sink the right knee and hip so low that the phase 5 push was as
long as the simpler "classic" push would have been for the same speed --
but I'm not yet seeing why it's good to mix that with double-push).
Is this phase a "dead spot" for forward-propulsion power? Without
force sensors in the wheelframe, it's hard to know for sure what the
elite racers are doing.
Here's some ways to do a little work in this phase: (a) It's possible
to push out thru the inside edge with the hip abductor / gluteus
medialis muscles before the big leg-extension muscles are in their
configuration for pushing. (b) Could also get do some forward-propulsion
work by moving the opposite left hip forward (by rotating the pelvis
and/or twisting the abdomen about its vertical axis). (c) If the knee is
angled inward from the plane of the hip-ankle-toe, then flexing the shin
muscle to drive the knee down and forward would add a little
forward-propulsion force. (though perhaps the best way to improve Power
is to get past this phase: Make the hip-flex knee-flex move as quick as
possible.) -- Anyway most of these same moves could also be used during
the Recovery phase of the simpler "classic" stroke.
(5) Push with leg-extension
Push with leg-extension toward the right thru inside
edge of right skate.
Not as long a push as the simpler Classic stroke, so the knee does
not have to be bent as much at its start.
(other) Upper Body moves, etc.
There's various side-to-side and up-and-down and backward-forward
motions that the skater can do with the recovering left leg, and with
the shoulders, and with the arms. As far as I can tell, many of these
are irrelevant, though some can help if they're properly timed, and
leading racers do some of the ones that help. But the same motions can
equally well be performed with similar effectiveness (or irrelevance)
which simpler classic stroking -- so they're not relevant to a theory of
why double-push is better.