Glide and Explosiveness in Classic striding -- Cross Country Skiing -- Roberts

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Quick summary
Is glide good?

- What kinds of glide?
- Is glide necessary?
- Is more glide good?

Racer's ideas about glide -- which ones work?

- ? long glide is more efficient
- ? elite World Cup racers glide longer
- ? glide long on a flat ski
- ? a ski that is gliding is a ski that is slowing down
- ? offset pole timing extends the glide
- ? long glide for long endurance

Is Classic stride explosive?

- Should I enhance the explosiveness? or fight it?

Quick Summary

Glide

Long glide is fun to do.
The ability to glide long in classic stride is a fine achievement -- requiring balance, coordination, and power.
More glide does not cause greater efficiency, speed, or endurance. Rather the glide phase is a result of speed. It's a "side effect".
The right way to increase glide is to improve speed and power.
Trying to increase glide the "other" way -- by putting in a longer pause between leg-pushes -- typically results in a decrease in efficiency, speed, and endurance.

Explosiveness

The classic stride is inherently more "explosive" than some other motions like bicycling and skating. But not much more "explosive" than dryland running.
It can be fun to feel the power of explosive kick.
But physics shows that efficiency, speed, and endurance performance are typically improved by smoothing the force -- by fighting against the necessary explosiveness.
Nevertheless, some racers might benefit from emphasizing explosiveness in some of their training exercises -- "plyometrics".

Is glide good?

Yes, surely glide is good.

It's the main thing that makes the movements of skiing so special.
It's what makes cross country skiing easier on our joints and bones than running.
It's what makes true classic striding different from shuffling on the snow.
It's fun -- the major separate sport of alpine downhill skiing has formed just to focus on the glide aspects of skiing.
and it's not just fun for humans -- many animals enjoy gliding on water, snow, and air. It seems that most of us are genetically "wired" to feel the goodness of gliding.
Glide is the "payment in kind" for our muscular effort. It's a special time when we get a body-motion reward in return for our body-motion work.

What kinds of glide?

"Glide" is when the ski is moving on the snow surface and the leg attached to that ski is not pushing.

In Classic skiing technique, the leg can help push the skier forward only if the ski is temporarily stopped on the surface of the snow. So there must be a clear distinction between the glide phase of the stroke cycle and the leg-push or "kick" phase.

It is helpful to distinguish two kinds of glide:

Active glide is when the ski is gliding on the snow surface and the skier is also applying a pole-push to help move forward (but of course there is no leg-push at this time). Active glide is pole-assisted glide.

Passive glide is when the ski is gliding on the snow and there is no pole-push (or leg-push) being applied. Passive glide is unassisted glide. Except on a downhill slope or with a tail-wind, a skier passively gliding is skier slowing down.

There is also "air-time": if the previous leg-push is finished and the next leg-push has not yet started, but the ski for the next leg-push is not in contact with the snow surface. Usually there is no need to distinguish between "active" and "passive" air-time.

Is glide necessary?

Yes, and No, and It Depends.

Yes

Indeed glide is pretty difficult to avoid on skis on the downhill sections.
On flat and moderate uphill terrain, it's the special step that makes classic striding different from beginner shuffling. It couldn't be "kick and glide" if it didn't have a glide phase.

No

Elite ski racers successfully go up steep hills on snow using hill bounds with no glide phase (though there is an air-borne phase).
Beginning skiers move successfully on the flats with little or no glide -- by shuffling.
Bicycle racers rarely use any glide on flat and uphill sections, yet they go significantly faster than skiers in both kinds of terrain.
Runners never glide, yet they can go fast -- and on steep uphills, faster than skiers.

It Depends

Whether or how much of each kind of glide is necessary depends on three things:

Timing of the pole-push
Skier's speed and cadence
How much extra down-force is applied in the leg-push

Let's analyze each of these . . .

Active glide depends on Pole Timing

Whether there is any active (pole-assisted) glide depends simply on the timing of the pole-push versus the leg-push. See the Offset Timing of Pole-Push "secret" for a description of the three main options for pole timing.

If pole-push is synchronized with leg-push, then there is no active glide.

If "offset and overlapping" pole-push timing is used, then there must be an active glide phase -- after the start of the pole-push and before the start of the leg-push.

If pole-push timing is "sequential non-overlapping" with leg-push, then there must be a large active glide phase -- throughout the entire pole-push.

Most glide depends on Speed and Cadence

For classic ski techniques on flat or gentle terrain, whether a glide phase is necessary depends on how fast the skier is moving.

At the speed that racers travel on flat terrain, a substantial glide phase is necessary at any reasonable cadence. Here's the analysis:

Arithmetic

maximum ground-distance of leg-push is determined by biomechanical geometry of the skier's body, with some modification by technique choices.

Here is how to calculate two key quantities:

maximum time-duration of single leg-push (kick only, no glide)

equals

maximum ground-distance of leg-push (based on the skier's body geometry and details of skier's leg-push technique)

divided by

skier's velocity

time-duration of single leg-stroke cycle (including both kick and glide)

equals

60 (seconds in a minute)

divided by [

2 (single leg-strokes per full cycle)

times

cadence (full two-leg stroke cycles per minute)

]

Condition for total glide on gentle terrain

There must be some glide phase -- active or passive or some of both . . .

If . . .

time-duration of single leg-stroke cycle (including both kick and glide)

is greater than

maximum time-duration of single leg-push (kick only, no glide)

Condition for passive glide

Let's assume that the pole-push timing versus the leg-push is "offset and overlapping" or "synchronized". The biomechanical geometry of the skier's body and the pole make it so that the ground-distance of the pole-push is longer than the ground-distance of the leg-push. Therefore . . .

There must be a passive glide phase . . .

If . . .

time-duration of single leg-stroke cycle (including both kick and glide)

is greater than

maximum time-duration of single pole-push

(If the pole-push timing is "separate and sequential", then the condition is very different, and much higher speeds can be handled without significant passive glide -- though there will be a rather long active glide phase.)

Sample numbers for gentle terrain

Suppose the skier's body geometry and stroke technique makes it so that

maximum ground-distance of leg-push = 0.5 meter (19.7 inches)

maximum ground-distance of pole-push = 1.0 meter (39.4 inches)

Suppose also that the skier's current speed and cadence are

speed = 5 meters per second (about 11 mph)

cadence = 40 complete two-leg stroke cycles per minute.

Then:

maximum time-duration of single leg-push = 0.1 second

maximum time-duration of single pole-push = 0.2 second

time-duration of single-leg stroke cycle (including both kick and glide) = 0.75 second

Therefore if the skier chooses to use the full maximum ground-distances for leg-push and pole-push, we get these results for glide:

active glide phase time = 0.1 second

passive glide phase time = 0.55 second

This means that

leg-push "kick" phase time is less than 14% of the stroke cycle

passive glide time is over 73% of the stroke cycle.

Sample numbers for climbing up a steep hill

Suppose instead the skier is climbing up a steep hill with much lower speed, but a higher cadence:

speed = 2 meters per second (about 4.5 mph)

cadence = 60 complete two-leg stroke cycles per minute.

Now we get

leg-push "kick" time = 0.25 second

total glide phase time = 0.25 second

which means that the proportion of kick versus glide time is very different:

leg-push "kick" phase time is now 50% of the stroke cycle.

It can depend on something else

If I apply extra down-force -- beyond just my body weight -- to the ski's grip zone during my leg push, then there is another reason for a time gap before the start of the next leg push.

Applying this "extra" down-force during the kick phase improves my grip -- but it also has a side effect:

It makes my body's center-of-mass move upward.

(for more on this, see the Double Cost of Extra Down-Force page.)

But once my leg-push ends this upward momentum of my body would reduce the down-force that would be available to support grip, if I were to start my next leg push right away. Assuming that I needed extra down-force on my previous leg-push in order to avoid slipping, then with this reduced down-force, I must not have sufficient grip for the next leg-push at this point in the stroke cycle.

Therefore I must wait to start the next leg-push -- until gravity slows and stops my body's rise, and makes my body sink start to sink back down again.

If extra down-force is used, this "wait" is a necessary time gap between leg-pushes. This gap must be filled with active glide or passive glide (or air-time).

The more down-force used, the larger the time gap. And there is no other way to influence length of the gap -- because the only force available to stop my body's rise is gravity.

What about air-time?

If a large enough extra down-force is applied, then the reactive force can launch my body up into the air. I've seen videos of elite racers (and of myself) with both skis up in the air simultaneously.

What's the difference between air-time and glide?

Not much.

In the first place it's just a result of how much I bend my knee during the time between leg-pushes. If I extend my knee so my leg is nearly straight, then I might be able to maintain contact with the snow even though my body's center-of-mass has been substantially raised. On the other hand, even if my body's center-of-mass is quiet and stable, I could temporarily take my ski off the snow with a very sharp knee-bend motion -- you can try this on dry land.

The important thing is not whether my ski is touching the snow or not. The important thing is whether my center-of-mass is still moving upward or starting to move downward -- whether the upward or downward momentum of my body at this instant can hurt my grip or help it.

Stride versus hill bound

The key criterion for the transition from normal classic stride to hill bound is when:

the necessary time gap from extra down-force becomes similar to the necessary time gap from speed and cadence.

On flat terrain, the speed of a racer is so fast that its necessary time gap is much larger than any gap from down-force.

But when the skier gets on an uphill slope, as the slope gets steeper the skier's speed drops, and the need emerges for extra down-force to prevent the grip from slipping. If the skier increases cadence and the extra down-force, then the time gap from speed and cadence decreases and the time gap from extra down-force increases -- then the cross-over criterion is reached.

Then it is more effective for the skier to switch from normal classic stride to hill bound, and make several other adjustments that go along with that -- see the Climbing up a Steep Hill is Different "secret".

Is more glide good?

For fun, Yes. If you enjoy getting that glide-feeling in every stroke. I do.

Is more glide good for racing?

Bicyclist racers do fine without it. They could easily have a glide phase, but they choose not to.
Hill Runners do fine without it. They're faster than skiers up steep hills.
Elite Classic ski racers could use a longer glide than they do -- by substituting pole Timing Option (b) "Separate and Sequential" for (c) "Offset and Overlapping" -- but they choose not to.

Racer's ideas about glide -- which ones work?

? long glide is more efficient ?

No -- if this is a question about covering the same distance in the same amount of time.

This requires slightly more energy if the passive glide phase of each stroke is longer and the leg-push phase shorter.

This is because the skier is slowing down more during the longer passive glide phase, which must be compensated for with higher speed in the rest of the stroke in order to maintain the same average speed -- so it must have larger variations in speed. Since air resistance is non-linear with speed, the extra air resistance in the higher-speed phase outweighs the lower resistance in the lower-speed phase. So the total energy expended is higher.

Or it could require significantly more energy beyond that -- if the larger force required to provide the similar kinetic energy during a shorter leg-push phase requires extra down-force to maintain grip and avoid slipping. That's because applying that down-force requires real work which does not go into forward motion. Worse yet, when there is long glide, a substantial percentage of the down-force work does not go into improving grip -- because the "landing" down-force falls in the glide phase -- see Double Cost of Extra Down-Force.

What about covering the same distance in a longer time (and lower average speed)?

This gets tricky -- because in general, skiing the same distance at a lower speed takes less energy because the air resistance is lower.

passive glide is not a free lunch

According to basic physics, the energy that is used to keep the skier moving forward despite air resistance and sliding friction must be re-supplied in the next leg-push and/or pole-push. Passive glide saves energy only in the sense that "skiing slower" saves energy.

? elite World Cup racers glide longer ?

some say: "The main difference between elite racers versus slower skiers is not the cadence of strides (frequency per minute), but the length of the glide on each stride."

I've never checked that myself, but I've heard it from credible sources.

My reply

But this does not imply that if I focus on longer glide, I will become faster like an elite racer.

As the analysis under "Is glide necessary" shows, the observation of longer glide is an inevitable result of moving at a higher velocity with a similar cadence and similar biomechanical geometry. Not the cause.

Elite racers get that longer glide not by inserting some artificial pause into their stroke cycle, and not by slowing down their cadence or turnover. They get it by generating more force impulse in their leg-push and their arm-push.

? glide long on a flat ski ?

It is true that a ski riding flat on the snow surface has less sliding friction than if it has pressure on one edge.

Therefore if I choose to glide long, it is better to do that gliding on a flat ski -- other things being equal.

But other things are not equal.

Sustaining glide on a flat ski requires complete sideways transfer of my body's center of mass to directly over the gliding ski. But that takes extra work.

Sideways weight transfer requires that the mass of my body first be accelerated sideways toward the new glide ski. That takes real work. Then the mass of my body must be decelerated over the new ski, so I do not fall over to the outside. That's more work.

At a cadence of 60 full stroke cycles per minute, that's 4 "work" pushes per second, or 240 "work" pushes per minute. Even if each push is small, it adds up to something significant.

Temporary glide on a flat ski does not require complete sideways weight transfer -- because in that case I do have to balance over the gliding ski.

And there's more . . .

? a ski that is gliding is a ski that is slowing down ?

If I am gliding, then I am not pushing with my leg, but the forces of sliding friction and air resistance are still slowing my body down, other things being equal.

And since the idea of a race is to move along at some velocity, then I have to follow this glide with some extra work to accelerate my body back up to speed.

So there's no "free lunch" from this glide. Every bit of "free ride" glide energy must be fully paid back with kick energy.

But it's worse . . .

More energy is wasted on air resistance if my speed goes up and down.

This is because the air resistance force is "non-linear" with my speed -- roughly the "square" of my speed. So with air resistance the cost of high speed is greater than the benefit of low speed. Therefore the best way to minimize energy lost to air resistance is to maintain a constant speed. So less glide is better for that, other things being equal.

But perhaps this is all a misunderstanding, because . . .

Offset pole timing can keep the ski from slowing down

Competent racers use offset pole timing -- so the arm can be pushing for much of the time when the leg is not.

? offset pole-push timing extends the glide ?

Some say that offset pole timing is good because it extends the glide.

But the logical conclusion of this reasoning would be to use "(b) sequential non-overlapping" timing of pole versus leg -- not the recommended "(c) offset and overlapping" timing used by elite racers. See the Timing Options on the Offset Pole Timing page.

If you desire to extend your glide phase for other reasons -- like because it's just fun -- then offset pole-push timing is normally an excellent way to help accomplish that.
Offset pole timing is good for several important reasons that have nothing to do with extending the glide phase.

See the discussion on the Offset Pole Timing page.

? long glide for long endurance ?

Adding an extra moment to each glide phase gives an extra moment of rest for the leg muscles in between pushes. This should enable the leg muscles to keep pushing for a longer distance -- although the speed is slower.

But the lesson I get from elite bicycle racing is that a smoother longer stroke is better than a sharper more intense push combined with a rest pause.

I say this because the bicycle mechanism makes it possible to stroke with or without rest. And the speed, efficiency, and endurance of different pedaling options has been carefully studied.

Beginner bicyclists often have a rest moment for each foot. But elite racers deliberately practice applying force with each individual pedal throughout its full circle of motion. They and their coaches know the results of the pedaling endurance studies. They could choose to include a rest moment for each leg, but instead they put in a lot of practice to avoid that.

a better way?

Perhaps there is a better way to increase endurance performance in skiing -- if slower speed is OK. Here's an idea:

Leave the glide phase alone -- instead reduce the intensity of the force during the leg-push phase.

Then the leg muscles will not need a longer rest in order to deliver better endurance performance -- since they are not stressed so much in each stroke to begin with.

Is Classic stride explosive?

Yes -- in the sense that at racing speeds on gentle terrain, all the leg-push force is applied during only a small percentage of the total stroke cycle time. See above under "Sample numbers for gliding on gentle terrain".
Yes -- in the sense that at racing speeds on gentle terrain, all the forward-push force from both leg-push and pole-push is applied during less than half the time of the total stroke cycle. See above under "Sample numbers for gliding on gentle terrain".

No -- The percentage of time in the leg-push phase for Classic striding on snow is not significantly lower than it is for dryland running at comparable speeds.
Yes -- The percentage of time in the leg-push phase for Classic striding on snow is significantly lower than it is in skating or bicycling.

For more analysis of explosiveness, see Is more glide good?

Should I enhance the explosiveness? or fight it?

I fight against explosiveness in performance.

I practice several of techniques to smooth and lengthen my leg-push motion -- see the Smooth Classic Striding "secret".

But despite all that smoothing, in the objective physics my leg-push time will still be a minority percentage of the total stroke cycle time on gentle terrain -- it will still objectively remain "explosive".

I've heard that some racers prepare for explosiveness in practice sessions -- with muscle-training and neuro-muscular-training practice exercises.

For more analysis of explosiveness, see

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