Why does the knee cave in on skate skis?

Refer to the Hips and Lower Back course for exercises that may help with this problem.


Maybe you’ve noticed that many skate skiers have trouble tracking their knees over their toes. It’s common to see the knee at an angle during the glide phase or for the knee to collapse inward during the kick.

I have first hand experience with this problem. In this shot you can see my right knee is bent inward, whereas in this shot, which was taken about a year later, my knee looks straighter.

In this video, I’ll share some of the ideas that are helping me work through this process. I want to stress this is not professional advice. My motivation for making this video is that this problem is widespread and it’s easy to lose years, if not decades, investing time and money in ineffective solutions.

So the questions I’m addressing in this video are, “Why does the knee collapse inward? and “What can you do about it?”

If you search online, you’ll find a number of suggestions for causes and fixes for knees that don’t track properly over the foot.

Once you rule out underlying structural issues, the idea you’ll most commonly encounter is that this problem is caused by various tight, weak and inactive muscles and can be fixed by corrective exercises that target those weak, tight and inactive muscles.

But what if this idea, that certain muscles cause the problem and exercises that target their deficiencies will fix the problem, is a logical fallacy? What if those weak, inactive and tight muscles aren’t the cause of the problem, but symptoms of something else?

What if the whole idea of targetting specific problem areas is keeping us from seeing something important in the big picture?

When we encounter problems in our bodies, such as pain, discomfort, loss of balance or performance issues, we naturally seek explanations. Those explanations bias our decision making and ultimately determine whether we successfully resolve the problems or not.

So the mental models we construct to explain our issues are one of the most important determinants of our success and we must avoid latching on to incorrect ideas because the closer to the truth we get, the better our chances of success.

In my opinion and experience, explaining this issue of the knee collapsing as a problem of weak and/or tight muscles is not close enough to the truth to be helpful. The weak and tight muscles are a red herring, and they distract us from a more central problem.

The central problem has more to do with the lines of force that act on our bodies. Those lines of force can be generated internally, by our muscles, or they can come from external sources, most especially from gravity.

The problem with knee tracking in skate skiing, has to do with an inability to apply forces evenly, down the length of the leg, through the foot, into the ski and into the ground.

Yes, there certainly will be weak and tight muscles, but solution to the problem has to go beyond targetting specific muscles and get to the root of the issue, which is learning how to generate straighter lines of force and direct those lines of force along the central axis of the leg.

Let’s unpack this idea by talking about forces and, since we’re skiers, let’s talk about forces in skiing.

In nordic skiing, we move forward by pushing down and back against the ground with our skis and poles. When analyzing video of skiers, we make inferences about those forces based on the skier’s position, body angles and movements.

We might draw big arrows to represent our best guesses about these pushing forces. This is a helpful way to represent what’s happening at a macro level. But if we could actually zoom in and look with a finer resolution, we’d see a more complicated picture.

This is a devise used to measure the forces across the bottom of the foot. There are many such devises on the market and they are used in research labs and clinics. Maybe you’ve used one at a therapist’s office.

Sensors in the mat detect the pressures across the base of the foot and then send the information to a computer, which outputs an image like this.

So this image is a representation of what the actual forces across the base of the foot look like. If your feet are on the ground right now, something like this is happening. The larger peaks and redder colours represent areas of high pressure and the lower peaks and bluer colours represent areas of less pressure.

Pressure is just force expressed in terms of area, so talking about pressure is similar to talking about forces.

You can also think about the forces acting across the bottom of the foot as a vector field. A vector field is simply a way to visualize and understand forces that are otherwise complex and abstract.

Even if you dropped out of high school science, you will be familiar with vector fields. You just might not know that’s what they’re called.

A magnetic field is an example of a vector field. It helps us see and understand magnetic forces, even though they’re invisible.

A weather map showing winds across continents is another example of a vector field. Vector fields simply make things that are abstract, complex and invisible, easier to conceptualize.

Getting back to our skier…there’s not a single line of force working into the ground, rather there’s a complex vector field of forces working across the bottom of the foot.

And of course the forces change as the skier works through each stride cycle. In fact, the forces change even if you simply stand still in one place.

Now, at this point, you might be wondering, “Why on earth does this matter?” and thinking “This is way too much detail!”

I promise you, it’s not.

Imagine two skiers:

First a skier who can apply pressure evenly and optimally across the base of the foot. Then there’s a second skier. The force field field across the base of her foot looks more like a stormy weather system than a well-organized vector field.

It should be obvious that the second skier will have trouble with balance and won’t be able to apply force into her ski as effectively as the first skier. This will affect her skiing efficiency and economy.

Do you have trouble balancing on one ski? Do you roll onto the edge of your skis when you want them to run flat? Maybe you practice balance drills, but the problem persists. I know it sounds crazy, but at least part of the issue has to do with these force fields across the base of the foot.

But there’s an even bigger issue to think about. These pressure maps, or force fields, or vector fields – however you like to think about them – these are two-way mirrors.

Do you remember learning about Newton’s third law in highschool? That’s the one that states that for every force in nature, there is an equal and opposite force.

We work these forces into the ground to help move us forward, and the ground pushes back with an exact reflection of what we give it.

If the force field across the base of the foot is jumbled up, like a stormy weather system, then stormy, jumbled up lines of force work right back into our bodies.

This matters because force is what shapes our bones, muscles, ligaments, tendons, fascia and so on. When our feet work optimally and the forces we apply to the ground are even and balanced, then our bodies are better balanced.

But when the forces we apply to the ground are uneven and imbalanced, then uneven and imbalanced forces work across the tissues of our bodies. Over time, the physical structure of our bodies grows uneven and imbalanced as well.

So yes, if you have this problem, you will certainly detect tight muscles, weak and inactive muscles, strength imbalances and so on. These forces from the ground work all the way up your body. You have more than 600 muscles. The pelvis alone has more than 30 muscles attached to it.

Which of these muscles won’t be affected? Which of them won’t be imbalanced?

Now, maybe you’re thinking, those strength imbalances are the reason the forces across the base of the foot are imbalanced and so if we target the weak muscles, then we’ll correct the problem.

But are they the problem? Are the muscle imbalances causing you to apply uneven forces to the ground or is the application of uneven forces across the bottom of the foot causing you to develop muscle imbalances?

The answer is yes, of course! Both are happening. This is not a cause and effect scenario with a single driver that we can simply “fix”? This is a feedback loop.

This is a complex situation and a complex problem. You can’t wish the complexity away. These force fields or lines of force lie at the heart of the issue and they are as complex as we are.

Targeting tight or weak muscles won’t help unless you can get at the central issue which is how are you applying forces to the ground and how well do those lines of force match the central axis of the leg?

A metaphor I find helpful is to imagine the leg as column, the kind of roman column with fluted channels running up and down the pillar. You can think of lines of force running up and down those channels. When the forces are balanced around all aspects of the leg, the knee is fully supported, but when the forces become unbalanced, the integrity of the support system is compromised.

I’ve limited this discussion to the leg and framed it around the specific question of the knee collapsing in skate skiing, but the truth is, this is a whole body issue. The body works as a system and lines of force work through the whole body and not just on the knee jojnt, but on all the joints and tissues in our bodies.

The interesting thing about skate skiing is that it tends to shine a light on these problems because it puts our hips and legs into more challenging positions than we’re used to.

My knee didn’t collapse inward in classic skiing. I didn’t even have trouble with knee tracking in the gym with single leg exercises, probably because I was making tiny, unconscious compensations with my hips and shoulders. But the problem was still there, in the background and getting worse as the years went by.

There are many movement practices that can help you get a sense of these lines of force and help you re-train your ability to direct forces more evenly through your body. Yoga, pilates and martial arts all come to mind.

There’s a course in the XC Ski Nation biomechanics library about the hips and lower back and it includes some workouts that are very good for helping with this issue.

Think about what’s happening during this exercise in terms of lines of force: You’ve got a roll supporting the upper back edge of the pelvis. It’s anchoring the pelvis in better alignment and helping to keep the leg moving from the ball and socket joint in the hip and not from the lower back.

Then you’ve got a strap acting as a second anchor point. You could even insert a yoga block between the foot and the strap to get more feedback about the forces working across the bottom of the foot.

You can slowly activate the leg between these two anchor points, creating internal lines of force and making subtle adjustments to how those lines of force are working up and down the leg. I think of this as laying down a force field in the leg. You can even use your breath and imagine you are inflating the column of your leg. If you point and flex the foot in different ways you can also change up those lines of force.

Finding ways to get a better sense of how I use my feet and especially how I create lines of force in my body, or, as I like to think of them, internal force fields, has been instrumental for me.

So, we’re coming to the end of this video.

I seriously debated whether I should even record this. These aren’t original ideas that I’ve outlined, but, for whatever reason, they’re not widely understood or discussed.

Perhaps they’re too esoteric. Perhaps people don’t consider the possibility that we can take steps to change the ways internal and external forces shape our bodies.

This kind of work certainly takes a high degree of focus and mindful, creative movement. Not everyone has the patience for it or can see its value.

I hope you found the ideas in this video helpful and that you might use them to enrich your understanding of how your body works. As I said, our bodies and movements are complex systems. The more we deepen our understanding, the better our chances for remaining active and healthy for years to come.

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