why double-push stroking is sometimes faster than normal skating -- Roberts

[ Print this | help ]

what's here

intro
January 2006 ideas
May 2005 ideas
more

Introduction

[ more to be added ]

January 2006 ideas

Double-push stroking is faster than normal-push in some situations because:

Double-push uses more muscle moves for propulsion . . .

especially the "Inside-of-leg-In" muscle moves, including hip adduction, lateral-hip-rotation (with bent knee), and ankle-supination.

Double-push can get more propulsive work in more ways out of leg-recovery.
Double-push can get more propulsion out of small additional forward-backward moves . . .

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?

Details

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 also.)

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 maximus.

(2) In-push

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 vertical-hip-height).

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 push benefit.)

(3) Aim-switch

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.