- Longer leg-push with smoother force can deliver more total push
for less cost.
It's tempting for athletic skiers like me to get
more leg-push by emphasizing the explosiveness. This feels powerful,
but it has undesirable side effects. It's more energy-efficient and
better for muscle stress and endurance to try to lengthen and smooth
the "kick".
what's here
- - Explosive versus
Smoother
- - Making the stride
longer :
in front |
behind |
heel-drive? |
knee-drive
- - Making the stride
smoother
see also
I was told long ago that the key to getting good grip was to
"stomp" on the ski sharply. Why else do they call it
the "kick"?
And it sure made sense to me that if I stomped the ski into the
snow that it would stick better. I found that I could get even
more stomp if I hopped into the air a little and landed on the
ski.
When I started getting serious about training for racing, there was
buzz about "plyometric" training: explosive motions.
And I definitely could see why that was important for Classic technique
-- since the impact of the stomp only lasts a short instant, so I need
to push as much forward-motion power through that instant as quick as I can.
Therefore I practiced that explosive "kick" all summer
and fall on rollerskis.
Then December came, and I went to XC ski camp on snow at Silver Star
in British Columbia, Canada. There I got to see myself and other
fellow amateurs on video, and we all found it amusing how different my
approach looked. Then the instructor and other skiers told some
different approaches they used to get better grip. That was the
start of my journey away from the "stomp-and-glide".
In late December and early January I spent a lot of time practicing
climbing up hills on snow, often without poles, and measured my
performance. Then I had to try to figure out why I was so much
slower on skis than running up hills on dry land -- and feeling like so
much of the leg-power I had trained in hill-running over the summer was
getting wasted now on snow.
I posted some analysis and questions to the rec.skiing.nordic
newsgroup and got some helpful responses. Jay Wenner especially
pointed out the importance of skiing smooth and relaxed, and to be able
to "feather" the leg-push within the limits of the
grip.
At the same time I worked out a simplified physical model of the
classic stride and grip forces. The ranges of results I got from
that showed numerically the value of smoothness, avoiding "dead
spots" and loss of momentum -- and the "double cost" of
explosive down-force. And then I started seeing it
when I watched videos of elite racers.
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Advantages of Explosive
For athletic skiers on gentle terrain, the leg-push
phase must be a small percentage of the total stroke cycle. For
the details of why, see the Glide
and Explosiveness page.
Disadvantages of Explosive
Increasing my forward-push force intensity runs
straight into that limit.
The first basic idea of the physics is called
"impulse", which is force multiplied by time. So I can get more forward-pushing impulse either by pushing stronger for
the same amount of time, or with the same force for a longer period of
time.
The second basic idea of the physics is called
"static friction", which says that there is a maximum on the
level of forward-pushing leg-force without slipping, for a given level
of downward-pushing force on the grip zone of the ski.
Putting these two ideas together, once I've
reached the maximum for forward-push force for a given level of
down-force, the only way I can get more impulse is to increase the length
of time I'm pushing it.
It is possible to "raise" the limit of
grip friction -- by using the big leg muscles to apply explosive down-force to increase
the grip friction needed to prevent the explosive forward-force from
slipping back. So the classic kick-and-glide becomes a
"stomp and glide".
But
more down-force requires more up-and-down motion of my body, and that
has a cost in wasted muscular energy -- wasted because it is
not driving my body forward. Actually it has a double cost,
in both the "launching" and the "landing" of the
up-and-down motion. For lots more on this see the
Double
Cost of Extra Down-Force page.
Therefore the more energy-efficient technique for me is to spread
the force over more time -- make the leg-push more smooth and
relaxed. Start the next leg-push early, and minimize the
"dead" time in between leg-pushes.
Taking a longer rest pause (or dead spot) in between repetitions does
not fully compensate for the muscle-and-joint stress of that
"explosive" peak intensity. So using stroke cycles with
higher peak intensity will result either in lower effective power output
or shorter endurance time before fatigue.
I see this all the time in my weight training sessions. If I
use very heavy weights, I can only do very few repetitions, and
slowly. If I use lighter weights, I can do more repetitions and
faster.
In bicycle racing, the pedaling mechanism allows a stroke cycle
with either less or more peak intensity. A key thing that
serious racers work on is to smooth out their stroke, fill in the dead
spots, and "pedal circles". Sharp push-force intensity
is often thought of as a beginner mistake.
Power and efficiency of bicycling technique has been extensively
and rigorously studied. The elite bicycle racers and coaches know the
results, their equipment gives them the choice -- and they choose
smoothness over explosive.
Conclusion:
For lots more on this, see the page on
Glide
and Explosiveness.
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-
On steep uphills, reach up and forward with the new ski.
[ more on this ]
-
Reach the foot and ankle joint out in front of the knee
joint on gentle terrain. Start the leg-push early, by
pulling with the hamstring and hip-extensor muscles.
I began thinking about these concepts from
analyzing slow-motion video of the great Elena Vaelbe in the
Norwegian Ski Federation video of the 1997 Trondheim 5K Classic
race. Then I found confirmation in an article by Steiner
Mundal in The Master Skier.
The earlier I start the next
leg-push, the shorter the "dead spot" gap, and the more I
can spread out the forward-push force. It also gives me longer
range-of-motion to engage my hamstring and hip-extensor muscles to
help power the leg-push. Getting the ankle in front of the knee also
gives more range-of-motion for using the shin muscle to help the
push a little, by flexing the ankle to move the knee joint forward.
Since my body weight is not yet over the grip zone
at this point -- to get grip for this "early initiation"
of the leg-push, it helps to press with my toe [more
on this].
Really what's starting with the ankle-joint
out in front might not be the propulsive push of the leg, but
the stopping of the grip zone of the ski against the ground to be ready with static friction to transmit the propulsive force of
the main push --
see lots
more discussion of this.
With this approach there's two phases of the
backward move of the foot: (1) accelerating the foot backward
relative the its hip joint, until the backward speed of the foot
relative to the hip is exactly equal to the forward speed of the hip
relative to the ground, at which point the foot is stopped relative
to the ground; then (2) keep applying leg-push backward to be
transmitted into the snow.
Anyway it's still valuable to get the ankle joint
out there forward before the leg's backward move starts, even if the
propulsive phase of the leg-push often does not start until the
ankle-joint gets underneath the knee-joint or hip-joint. [
see lots
more discussion of this ]
Using this forward-hip-rotation move, at the
moment the next ski is set down to grip for the next leg-push, the
next-pushing hip is ahead, and the hip of the leg which just
finished its push is behind -- so the hips and pelvis are "facing"
somewhat away from the side of next push. Then as the forward-hip
rotation move is made during the leg-push, the pelvis and hips come
to face straight forward, then continue to rotate to face finally
somewhat toward the side of the leg-push which is completing.
This hip-rotation move engages the "horizontal
pelvic rotator" muscles to help power the leg-push -- a clever way
either to take load off the main leg muscles, or to add to total
forward-propulsion power and speed.
But there's no point in reaching further forward
than my muscles have been developed to push effectively (and
sustainably). My hip might not get as low as my favorite eliter
racer video, because my muscles and joints are not strong enough to
push effectively without undue strain from such a low position.
And I'm thinking there's an inherent problem with
getting the hip too low at the start of the leg push --
because there must be enough space underneath the hip for the knee
and ankle to fit during the middle phase of the leg-push. The more
the ankle flexes during the middle phase, the lower the hip joint
can be at the start. (Dropping the hip lower toward the end of
the leg-push avoids this problem -- see below).
Several people have said they find this tip
very helpful. [see
Discussion]
I haven't gotten to "working this through" on snow yet. For now, I think this
tip is valuable at least as a mental image:
to
help make the leg-push stroke longer, by delaying the final push-off with the
toe.
Perhaps this mental image can also make the kick
stronger, by helping to engage some big leg muscles such as the
gluteus.
Some people go further and say it's more than just
a matter of "keeping" or "holding" the heel down
in position: They want to think of "driving the heel down
and back" -- with weight and pressure on the heel.
[see
Discussion]
First, I do not see any
contradiction between the "focus pressure on the toe"
secret and the "keeping the heel in a down position"
version of this tip. I do not find it difficult to press my
toe without visibly lifting my heel.
As for the stronger "drive the
heel down" version of this tip:
My suspicion is that it's
mostly a difference between what's
happening in the objective physics of the foot on the ski, and what helps some people
with mental visualization.
It's pretty difficult to avoid
keeping significant pressure on the toe once your heel is out there
behind you. And it seems to me that the obvious interpretation of
the two-person dry land drill with a finger under the
"skier's" heel [see Discussion ]
is that there is little actual pressure through the heel -- even by
believers in the stronger "drive" version. Otherwise
the "non-skier" partner in the drill would be in
pain.
Or perhaps it's a matter of different phases
of the leg-push stroke: I find I use the "press the
toe" mental image mostly at the initiation of the kick -- and
I've done some practice on dry land with a sequence of mental
images: Press the toe to start the kick, then keep the heel
down to the finish.
[see Discussion]
Clarification: There's no doubt that objective
pressure focused thru the heel helps the glide phase -- by reducing
sliding friction in the grip zone. The question here is different:
about improving the leg-push or "kick" phase.
Seems to me it would have to be in a situation
where I've got lots of extra grip that I don't need. So let's assume
that focusing pressure thru the heel does not cause my ski to slip
back.
The first idea is that it's just easier to
allow the push of the leg-push to go through only the heel instead
of requiring that the ankle joint transmit it forward to the
toe-ball of the foot. Easier in the sense that it requires less
tension in my ankle-extension muscles (e.g. calf muscle) and tendons
and less strain on my ankle joint. So if I learn to perceive
situations where I can get away with using more heel-pressure and
less toe-ball pressure, I can relax my ankle-extension muscles more
-- so I can save more of my ankle-extension strength and endurance
for other situations where I really need it.
The second idea is that I could use an ankle-flexion move to apply force to directly push my foot
backward relative to the rest of my body going more forward -- see
discussion below under Using ankle-flexion to add direct propulsive work.
The closer my hip joint gets toward the ground,
the longer I can keep my foot pressing down and back into the snow
out behind me.
Added propulsive work from gravitational
potential energy is also delivered if the hip drops during the leg-push.
The hip is raised again during the passive glide phase, using the big
knee-extension muscles (e.g. quadriceps) to deliver propulsive work by
building gravitational potential energy. (Otherwise there is little way
these big muscles be used to add propulsive work to classic striding).
But dropping the hip during the leg-push does have
an undesirable result: It generates a reactive up-force which
reduces the down-force for maintaining grip -- see explanation on
the Back-Lift more "secret".
This is another one of the necessary compromises in the physics of
classic striding.
Usually the elite racers allow the hip to drop
during the later phase of the leg-push. They compensate partly by
doing the back-lift move to
generate some reactive down-force.
So if my grip wax is not working well at the
moment, it might be better for me to keep my hip joint level through
my leg-stroke. Or perhaps my unconscious muscle-control patterns
could learn to compensate with a stronger final toe-push downward.
There's two main approaches to drop the hip joint:
(a) bend the knee; or (b) bend both the ankle joint and the knee.
For some reason most skiers instinctively focus on knee bend and
forget the ankle. So it's worth giving conscious attention to the
ankle -- see under knee drive move.
This idea seems to come from advanced skating, but
I've seen a couple of really good and technique-savvy skiers have said that they also find it
helpful for classic striding.
Taken strictly literally it does not objectively make
sense for classic striding, because the degree of flex in the ankle
varies widely through most of its range of motion during a long classic leg
push. And it would contradict the principle of starting the leg push
early out in front (see above), which is plainly visible in videos of
elite racers.
What I suspect this idea means in objective physics
is that the ankle is more deeply flexed at the point when the ankle
passes underneath and then behind the hip joint.
Why this change can be effective in objective
physics is that (a) it lowers the hip joint, and by the geometry of
our bones and joints, if we are closer to the ground we can keep our
foot in a longer distance of effective contact with the snow; and (b)
it adds range-of-motion to the final ankle-extension "toe push"
move, which results in additional propulsive work.
So the sequence for the ankle joint is
complicated:
(1) It starts somewhat extended at set-down (or
soon after) -- just enough to keep adequate down-pressure thru the
toe-ball of the foot.
(2) As it passes underneath the hip, it flexes
forward deeply (mainly from the weight of the skier's body pressing
down on the knee joint, not from specific muscular force).
(3) Near the end of the push, the ankle extends
very quickly and strongly, sometimes called the "toe-off" move, or
the "final toe push" (though actually the force is more through the
ball of the foot). This is when ankle-extension actually adds
propulsive work to the leg-push.
This has the effect of bending the ankle forward -- but as
an active move, instead of a position. The knee going down
can help with lowering the hip -- discussed elsewhere as a key factor for lengthening the stride, and adding
range-of-motion to the "final toe push" move.
Whether moving the knee down and forward in
itself adds propulsive work (as opposed to preparing other
muscle moves to add work) is a trickier question ...
There's no doubt that flexing the ankle forward in
the earlier parts of the leg-push helps get the leg into a
configuration better prepared to deliver propulsive force thru a
longer effective range-of-motion during the later parts of the
leg-push stroke. The question here is whether this ankle-flextion
move in itself adds propulsive work.
My answer: Not necessarily.
Because the easiest way to get the ankle joint to
flex forward is simply not to resist the gravitational force of
body-weight coming down on the knee. That works when the knee joint
is further forward than the ankle joint. If the knee joint is
currently vertically above the ankle joint, or behind the ankle
joint, I can first do a knee-flexion move, using the hamstring
muscles to increase the bend of my knee, which will move my knee
forward relative to my ankle.
So relying mainly on gravity from body-weight will
work to get the ankle flexed, and I think that's how I and most
skiers do it. Even if we think that by using the "forward knee
drive" image we're engaging our shin muscle, really its mostly
knee-flexion followed by gravity that's making it happen.
Shin muscle -- This muscle on the front of the
lower leg is key for this knee-drive move. One obstacle to knee
drive and ankle flex is that the skier's shin muscles do not yet
have enough strength and endurance to keep delivering what the
skier's mind is asking for. Developing this specific endurance and
strength for most of us takes well-planned training exercises -- and
weeks and months.
Using the shin muscle to flex the ankle forward
requires something else: Pulling up with the front of the
foot. This requires pressing the top of the front of the foot upward
against the upper of the ski boot -- not a move or feeling you'd be
likely to encounter while focusing your mind somewhere else, say
like on "driving the knee" or "driving the heel".
Using this move to deliver significant force is
very strange for most of us.
<> If you don't feel strange and strong pressure
against the top of the front of your foot, then you're not engaging your shin muscle to add propulsive work.
<> If it feels like you can practice it while
walking or running (using images of "forward knee drive" or
"driving the heel down"), then you're not engaging your
shin muscle to add propulsive work.
Pros + Cons:
<> Pro: If you're ever in the special waxing
situation, and you had practiced the special neuro-muscular
coordination, and you had been keeping up a program of
special training of your shin muscles for strength and endurance --
then you really would go a little faster in that special situation.
<> For most of us, the shin muscle doesn't have
much strength or endurance compared with lots of other muscles we
normally use for propulsive work. So modifying our technique and
ski-preparation to enable it will not produce much increase in
speed.
<> Con: Delivering serious propulsive force by the
shin muscle results in maximum down-pressure thru the heel and
minimum thru the toe-ball: Which substantially reduces grip-friction
for most classic ski designs (see the
Exploiting the Wax Pocket
secret).
<> Applying grip wax to the ski base in a way that
offered sufficient extra grip so that you could get away with this
heel-focused pressure distribution would be slower for gliding.
Almost always the slow glide would outweigh the extra muscular work,
so you'd go slower. Therefore ...
<> The special situation for this move is when you
mis-waxed, or when waxing is very difficult or unusual, or snow
conditions vary widely on different sections of the trail.
<> Con: Most other motion activities and sports do
not use the shin muscle much for significant propulsive force (e.g.
running, walking, skating, non-expert bicycling). So it's not
convenient to train the shin muscle for propulsion -- and it you do
train it, then it's not useful for much else. Except ...
<> Pro: The shin-muscle can be used to add direct
propulsive work to pedaling a bicycle -- in the upward section of
the pedaling cycle. The problem is that most cyclists don't do it
that way -- instead they rely on the other stronger muscles to avoid
any need to learn to engage the shin muscle. Takes hours and months
of special practice to learn the neural control patterns to reliably
engage the shin muscle in general cycling -- one approach to help
focus on it is to practice on a stationary bicycle indoors: pedaling
with only one leg.
<> Pro: It should be possible to possible to
practice this move and train the shin muscle using long classic
rollerskis -- because most classic rollerskis (as of 2005) deliver
lots of grip friction even down-pressure is focused thru the heel.
(Normally this is thought of as the key flaw of training or
technique-learning on classic rollerskis).
<> Pro: If you had futuristic skis that used
electricity to turn strong gripping force on and off, then you might
have enough extra grip to support shin-muscle propulsion, without it
hurting your glide.
Conclusion: Using the shin muscle for
propulsion is one of the last things to work on in classic
striding technique.
But many good skiers have
found that it is counter-productive to think about this move. Instead they generally allow their unconscious
neural controller modules to take care of this.
But it might be to be aware of this "final
toe-off" move for designing training exercises for the specific
muscles used. And it's probably worth it for a serious racer to
check slow-motion video to verify that the move is actually
happening.
When I decide to apply conscious attention to this
move, my two concepts are:
(a) Hold back the start, then
(b) Make it very quick and explosive.
-
Focus more on complete weight transfer and balance to apply
down-force, instead of up-and-down motion. [
more
on this ]
-
Offset the start of the pole-push from the start of the leg-push, to help
fill in the "dead spot" time gap between leg-pushes.
[
more on this ]
? Arm-push longer ?
Might get higher total power by cutting off the
extension of the arms a bit, to increase the turnover frequency of
the overall stroke-cycle.
Because the length of the pole-push against the
snow is not the critical "bottleneck" for classic striding. The
length of the effective driving contact of the pole-tip down and
back into the snow surface is already greater than the leg-push
snow-contact length.
What's best for legs is not necessarily best for
the arms.
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see also
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