The biomechanics of cross country skiing
I've been working a bit lately on my cross-country skiing and started thinking about the biomechanics of skiing. The biomechanics of cross-country skiing are markedly different than that of downhill (Alpine) and even Rondenee (also called Alpine touring.) Cross-country skiing requires form that is centered over the ski with virtually no reliance on your bindings. Alpine and Rondonee bindings differ in that they are rigidly fixed to the ski (Alpine) or semi-rigidly/rigidly tethered as found in Rondonee bindings. The beauty of a cross country set-up is the simplicity of a toe clip. But that simplicity also means that the act of cross-country skiing requires a very calculated, centered load-bearing and swing phase to keep the skis in alignment.
To enable walking and running, the leg, ankle, and foot work as a lever delivering force from the calf to the ball of the foot. This kinetic chain of events enables forward propulsion. Your body strives for efficiency to reduce energy consumption. The single best way it achieves this is to keep your body moving forward in the sagital plane, not deviating in other directions. For instance, moving up and down or side to side are deviations that require additional energy input by your body.
As a foot doc, my thinking is about what happens to the foot during cross-country skiing. If we think of the leg ankle and foot as a lever, what can make this lever more effective and what can make it less effective. There are three things that come to mind. First, alignment of the feet out of the normal base of gait. Second, control of hip rotation during gait. And third, rotation of the foot in the frontal plane. Let me explain.
If you're standing barefoot or in street shoe, your normal base of gait (medical definition) is approximately 30 degrees of abduction form the center line. What's that mean? If you looked down at your feet and had a center line for reference, they'd look like this \ | /. Follow? Now with cross country skiing, you need to align like this | | |. And without the additional support of a fixed binding, this takes some practice.
When I was in school playing football, I played defensive end and linebacker. My coach taught me that the most reliable way to track a running back was to watch their belt buckle. And he was right. You can be fooled by the feet and fooled by the shoulders, but the belt buckle doesn't lie in terms of what direction the runner is headed. This is because in gait (both running and walking) your body will maximize efficiency by preventing the center of gravity from deviating from the direction in which you are headed. But there's always going to be a certain degree of deviation in normal gait. Arm swing will result in rotation of the torso. Flexion of the hip in swing phase and extension of the hip in weight bearing to toe off phases of gait certainly will contribute to deviation of your center of gravity. With Alpine and Rondonee skis, you can use these motions to manuever your skis. But in cross country skiing you don't have the advantage of fixed bindings. In cross-country skiing, it becomes a constant focus to keep your center of gravity centered. Just like coach taught me.
Rotation of the foot in the frontal plane is the final consideration in cross-country skiing. To define the frontal plane, pretend you are standing at a sliding glass door facing out. The glass door represents the frontal plane. In normal gait, the foot will roll out (supinate) and roll in (pronate). Supination and pronation in cross country skiing can contribute to significant deviation from forward motion in the sagital plane.
Limiting each of these three ways your body deviates from the sagital plane maximizes the efficiency of your cross-country ski experience. Keeping your feet in alignment and minimizing rotation of the shoulders and hips will keep you in the sagital plane and on-track.
When treating cross country skiers for foot issues, I use a lot of in-shoe devices to limit supination and pronation (motion in the frontal plane). In every case, a barefoot gait exam is used to determine the degree of supination and pronation in gait. Depending on the foot type, different in-shoe products may be recommended. First and foremost is management of pronation with a semi-rigid carbon graphite orthotic. Carbon graphite has the strength to limit pronation but enough flex to promote sagital plane gait. To limit supination, I use lateral sole wedges and heel wedges to control motion.
Keeping it centered. That's the key to a good ski.
Jeffrey A. Oster, DPM