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High hips forward for learning
It's reasonable to think that lots of progressing beginners
and intermediates can be helped by the mental image of "high hips
forward" -- as a way to help them avoid having part of their weight
back on the previous ski, while they're aready starting the next leg-push with
the other ski.
The idea is that the hips tend to drop down and back a
bit during the leg-push phase, so bring them up and forward during the
glide phase.
If their hips are way behind their ankle joint, most skiers do not
have the leg-strength to quasi-isometrically support their whole upper
body weight on the next kick ski, without some support from the previous
leg-push ski. So they need to bring their hips forward to roughly
over their ankle joint in order to commit full body weight to the
leg-push ski.
The obvious normal way to bring the hips forward is to use the
quadriceps upper leg muscles to extend the knee joint -- which raises
the hip joint up as well as forward. So the normal way for
most skiers to achieve committed side-to-side weight transfer is to
bring the hips forward "higher", at least higher than they
were at the end of the previous kick. See
more on Balance and
Weight Commitment
Recently (in 2002) some racing coaches have been saying that
moving the hip high is a hindrance to speed or efficiency -- instead
the more knee-bend, the more leg-push power.
Here are some of my thoughts on that, based on physics:
-
It is true that with a lower hip, the possible horizontal
length of the leg-push against the snow is greater. Since this
length is a key bottleneck to the speed of classic striding, an
approach to increasing this length is worth considering.
-
But when using this approach of an overall
lower hip, even if the hip is not high in
absolute measurement, it can still be relatively higher at the start
of the next leg-push than
it was at the end of the previous leg-push. That's what I've
observed in videos of some of the very best racers from 1997.
-
This lifting of the hip during the passive glide
phase is not wasted energy, since the elite racers can convert much
of the gravitational potential energy into forward-push force, by
applying it to the pole-push or to the next leg-push.
-
It's a way to use the powerful knee-extension leg muscles
(e.g. quadriceps ) to
add power to classic striding. Because after the weight of the
hips and upper body has been lifted higher, it adds more power when
it drops back down to help drive the start of the pole-push and/or
when it continues to drop down during the next leg-push.
-
The movement of extending the knee joint to lift the
hips forward helps pump more blood through the big leg muscles, so
there is less lactic acid build-up than if they held a static position
with their legs.
-
Many elite racers tend to drop lower the hip lower at the end of the leg-push
stroke anyway, so the only possible gain in stroke length from a low-hip
strategy is by reaching further foward at the start of the
leg-stroke.
-
It would be possible to recover the hip forward without
raising it higher, if the ankle were simultaneously flexed to bring
the knee joint lower. I have not yet checked more recent videos
to see if elite racers are now doing that. But if the ankle is
flexed more, then the new leg-push foot is not reaching further
forward, so the horizontal length of the leg-stroke is not
increased.
-
So the only way remaining to keep the hips low and reach the
new leg-push further forward is to not bring the hips
forward. That is, start the new leg-push while the hip is still
"sitting back" substantially behind the ankle.
-
This is theoretically possible, but it requires:
(a) more
quasi-isometric strength in quadriceps;
(b) more quasi-isometric
strength in calf and foot muscles to press toe and ball of foot;
(c)
more stress on the knee joint.
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