Double push
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Double push is an inline speed skating technique. Its major advantage over the previously practised "classic" technique is that it allows the skater to do useful work during part of the skating stride that was hitherto "wasted", and therefore to go faster. However, opinions differ on its usefulness for non-elite skaters as it requires much practice before a skater is faster using doublepush than without.
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History
It is uncertain who first invented the doublepush, but Chad Hedrick is generally acknowledged as having brought it to the public eye. Prior to Chad's arrival on the scene circa 1992, inline speed skaters used the classic speed skating technique derived from ice speed skating. Hedrick's alternative approach rapidly proved itself to be faster and was soon adopted by many elite skaters.
"Edges"
Wheels used in speed skating are usually round or elliptical in profile, so do not have actual edges. However, the terminology is carried over from ice skate blades, which do. In inline skating, being "on an inside edge" refers to skating with the wheel of the skate leaning inwards (right skate leaning left, and vice versa). An outside edge is the opposite, and a center edge implies that the wheel is vertical.
Classic stride
The classic speedskating technique is an alternating cycle of pushes and glides. The cycle starts as the skater is gliding on one (let us suppose the right) leg with his knee bent. He then straightens the knee to push outwards (rightwards) away from him. When the pushing leg reaches full extension he sets down the left (support) leg and glides on it while picking up the right leg and bringing it back behind and underneath him (recovery) ready to be placed on the ground (set down) and become the new support leg. Once the pushing leg has been recovered, the support leg pushes out in the other direction and the cycle repeats. Note that the left leg pushes exclusively left, and the right leg right.
This will naturally result in a push that starts on the center edge or on a slight outside edge and "rolls over" onto the inside edge as the foot moves away from the skater's center of mass.
Double-Push
The double-push eliminates the "glide" phase from the above cycle, replacing it with an "underpush". The recovery skate is now set down on an outside edge and pushed underneath the body (right leg pushes left, and vice versa) while maintaining that outside edge, before being steered or pulled back across the centerline for the regular inside-edge push that follows.
By its nature the double-push is less stable than classic technique, making it difficult to learn: if the skater should lose traction during an underpush there is no support leg to fall back on.
The double-push also allows for the application of force through both legs simultaneously. The skater does this by transferring his weight from the inside-edge foot to the outside-edge foot gradually, rather than transferring all of his weight to the outside-edge foot instantly. Some hold that this is essential to the DP, but this opinion is not universal.
Advantages
The principal advantage of the double push is that it allows the skater to generate propulsive force during a part of the stroke cycle which was previously wasted. In the glide phase of the classic stroke, friction from road and bearings tends to slow the skater down, yet the deep knee flexion during this part of the cycle fatigues the muscles. In DP he can do useful work in this time instead. This is beneficial to his muscles and also helps him to keep his speed more nearly constant instead of surging ahead and then slowing down repeatedly.
The DP also extends the effective stroke length: one of the best ways to go faster is to push further in each stroke, but whereas traditionally this requires sitting lower (restricting blood flow and increasing lactic acid production in the muscles), in DP the same leg at a higher bend angle can achieve a longer effective stroke by pushing both inward and outwards.
[ TODO: Publow also claims "reflex potentiation" and "gear selection" as advantage of DP ]
Double-Push On Ice
Some claim that it is possible to use the double-push in ice skating, but probably not to the same extent as in inline skating. The physical mechanics of applying the outside-edge of a thin strip of steel against frictionless ice, at a wide angle, are less forgiving than the physical mechanics of applying a rubber wheel against a high-friction road surface at a similar wide angle. This is due in part to the fact that ice blades do not cut into the ice when the blade is perpendicular to the ice, this means that when moving the blade from outside to inside edge, there is very little bite available at the point where the blade is perpendicular unlike roller blade wheels. Basically, an ice blade can never be perpendicular to the ice, it must always be on either the inside or outside edge (note, this does not apply to ice hockey blades or figure skating blades, they have a recess in the centre of the blade allowing for stability when the blade is perpendicular to the ice) . As difficult as it is for a layman to see and recognize exactly when an inline skater is performing a double-push, it is even more difficult to recognize the subtle outside-edge motion when performed on ice, which has often led to the conventional wisdom that it's not possible at all. Time will tell whether the double-push technique will be adapted to and impact ice speedskating the way it has impacted inline speedskating.
References
- Publow, Barry (1999). Speed on Skates. Human Kinetics. ISBN 0-88011-721-4