Ground Reaction Force is a major contributor to knee vulnerability on wide skis that also causes skiers to change how they navigate downhill. Prof. Seifert explains, "I would define ground reaction force as the force exerted by the snow in contact with it. GRF is always present, even in deep snow, as it's part of what makes a ski turn. GRF is influenced by skiing velocity and turn radius. On hard snow with wide skis, this is why drifting is so common or the turn radius is lengthened in order to minimize forces."
Here's the kicker: depending on snow density and water content, it takes 8 to 14 inches of snow to reduce the negative effects GRF. When you're romping around on 6 to 8 inches, GRF is with you all the way. The wider the ski, the more GRF is multiplied and the greater the strain on knee ligaments when trying to get the ski on edge.
Whether by instinctive reaction or conscious decision, a skier with a wide ski on hard snow defuses GRF simply by not edging. Drifting along on the heavily rockered baseline limits directional change to foot swiveling with relatively little loading in the belly of the turn (where pressure would build on a narrow ski on hard snow). While being in a constant power drift helps reduce knee strain to some degree, it also entails an inherent loss of control.
This isn't a supposition or a theory. The physics of GRF isn't imaginary. The study of knee kinematics isn't based on guesswork. There will always be a certain amount of fuzziness to the data because test conditions are so difficult to control; researchers don't get any more powder days than you do. But the trend in the data is factual and irrefutable. Wide skis on hard snow are a bad idea, period.
Prof. Seifert opens his remarks by enumerating all the unquantified variables that make research in the field so challenging. When it comes to skis and knees, perhaps the most basic question is, how wide is too wide?
In the absence of standard definitions, Prof. Seifert settles on 90mm underfoot and up as wide, 80-90mm as mid-width and 80mm and below as narrow. With all the other variables that can contribute to a change in outcomes - snow surface conditions, skier age, gender, mass and ability, to name a few - this is perhaps as precise a definition as we need.
While I have absolutely no data to support my suspicions, my theory is that a ski's width is essentially invisible to the skier's knee as long as the ski is no wider than the skier's tibia head, or the point of ligament attachment. When advising skiers on ski width, my homemade metric is most men can ski up to a 100mm-wide ski without feeling routine strain when edged, while for women this guideline is 90mm.
My thoughts on the subject can be seen in this brief video
One reason no one knows exactly how wide is too wide is that research in this field requires money, perseverance and patience to test any proposition about ski-skier interaction. Money for this purpose doesn't drop from the sky. When Prof. Seifert sought funding from the National Institute of Health, his request was denied on the grounds that skiing was an elitist sport. Ouch.
I hope that many of you will view Prof. Seifert's lecture, but for those of you who don't, here's the distillate:
- Physics doesn't lie. All the supporting numbers from various research projects may not be a complete match, but they come very close.
- The best protection against injury is good ski technique.
- Knee injuries are positional in nature. The tall stance induced by wide skis takes the muscles supporting the knee out of their ideal length-tension angle.
- The strain required to overcome Ground Reaction Force on hard snow causes skiers instinctively to drift instead of edge, thereby losing a measure of control.
- If you don't already possess strong technical skills, a wider ski will hinder your skills development.
- To increase your chances of passing your PSIA Level III exam, use a narrow ski.
- To mitigate the risk associated with their use, wide skis need to be in powder around a foot deep. Even if you find fresh snow this deep, how long does it last? Once powder turns into crud, GRF raises its intensity.
Thanks to research like that conducted by Prof. Seifert, the pied pipers of fat skis can no longer pretend that wide skis are just as good on hard snow as they are in powder.
More research is needed, and thankfully there are four new studies looking into leg extension on wide skis. New work is also being done to illuminate the perils of mixing AT boot soles with Alpine bindings and to better define the limits of tech binding performance in resort conditions. I hope those of you, my Dear Readers, who are able will contribute to Prof. Seifert's work by donating here