Smooth Classic Striding -- XC ski Roberts

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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".

- - Explosive versus Smoother

What to Do

- - Making the stride longer : in front | behind | heel-drive? | knee-drive

- - Making the stride smoother

My story

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.

What to Know

Explosive versus Smoother

Advantages of Explosive

The leg-push in Classic stride is unavoidably explosive, by objective physics.

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.

If you like the feeling of using intense muscle power, an explosive kick feels good -- and it is then followed by the further reward of the fun feeling of a longer (non-pushing) glide.

Disadvantages of Explosive

Grip friction is a key limit for Classic striding.

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.

Raising the grip limit by "extra" leg-push down-force has undesirable side-effects.

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.

Using more intense peak force puts more stress on the muscles (and tendons and joints).

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:

The objective physics of Classic striding is inherently "explosive", but it's more energy-efficient and easier on muscles to fight that objective reality: Try to make the stride longer and smoother.

For lots more on this, see the page on Glide and Explosiveness.

What to Do

Making the stride longer

longer in front

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 ]

Rotate the hips and pelvis horizontally, so that the non-pushing hip goes forward to prepare for the next leg-push.

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.

The lower the hip joint, the further forward it is possible to reach with the foot and ankle.

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).

longer out behind

Keep the heel down during the remainder of the leg-push stroke.

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]

Is there a conflict between this "keeping the heel down" tip and the press-the-toe "secret"?

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]

Is there a situation where downward pressure focused thru the heel instead of the ball -- as objective physics, not mental image -- could help add propulsive work to the leg-push phase?

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.

Allow the hip joint to drop during the leg-push

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.

Visualize bending the ankle more deeply.

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.

Try driving the knee down and forward during the second half of 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 ...

Can a forward-ankle-flexion move with the shin muscle add direct propulsive work?

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.

Pushing back with the ball and toe of the foot near the end of the stroke, using the calf muscle also lengthens the leg-push.

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.

Making the stride smoother

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 ?

Not necessarily the highest power for classic striding by making the pole-push as long as possible.

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.