They May Be Great in Powder, but Increasing Evidence Shows That Fat Skis Put More Force on the Knee Joint
Say you ski only a few weeks out of the year, and you’re always holding out hope for a powder day. You’d rather have powder skis for the odd powder day because they perform fine on the groomers and are heroic in deep snow. Plus, they feel stable and fast; having 115 millimeters underfoot to rip around on sure beats the skinny 210s you grew up skiing on 30 years ago.
So what’s the issue here? Chances are you’ve fallen into a marketing trap that has been gobbling up skiers for the last 15 years. The early 2000s marked a trend of skis getting wider as the industry pivoted to the powder prowess that had enabled snowboarding companies to dig into their market share. Then, they grew wider. And wider still, until manufacturers were pumping out models with 140 millimeters underfoot. As the industry looked to continually pivot to new styles to continue selling skis year after year, skiers themselves may have begun suffering.
Recent research, including groundbreaking studies led by Martin Zorko and John Seifert, has offered some compelling evidence and explanations as to why fat skis may not be skiing’s 21st-century savoir, which is how gear companies have portrayed them. Let’s dive into why wide skis are NOT the right tools for most skiers on most days.
Defining a Wide Ski
At this point, you may be wondering, “Just how wide is too wide?” Unfortunately, ski width is not an exact science, and there is no industry standard for what constitutes a “wide” or “narrow” ski. However, most retailers define “powder” skis as those with a width above 105 millimeters underfoot. You can add a few millimeters for the average man and subtract a few for the average woman.
However, John Seifert defines wide skis as those over 90 millimeters underfoot, which significantly changes the parameters of our focus. Depending on where you usually ski, almost everyone is already using skis well above that 90 mm threshold. The East Coast of the U.S. is one of the few places where skiers haven’t veered toward fatter skis due to icy conditions and a lack of off-piste terrain. Elsewhere, the majority of skiers are using skis over 90 millimeters underfoot, even if they mostly ski on-piste.
Moreover, many skiers, especially younger ones, are gravitating toward skis that are 110 or even 120 millimeters underfoot. Other innovations like rocker and reverse camber have created skis that are heavenly to ski in powder but entirely unsuited to hard snow.
Ground Reaction Force
Ground Reaction Force, shortened as GRF, is the force channeled from the ski edges’ contact with the snow through the skier’s body. Many skiers know this as “torque.” The physics can get a bit more complicated here, but the basic idea is that a narrow ski focuses GRF directly from the center of the foot toward the knee joint. Meanwhile, a wide ski moves GRF in a mediolateral direction against the knee. As opposed to pushing up into the bottom of the knee, the ski torques the knee from a slightly more sideways angle.
In his seminal paper, “The Waist Width of Skis Influences the Kinematics of the Knee Joint in Alpine Skiing,” Martin Zorko et al. were one of the first to propose that this shift in GRF has a degenerative effect on the knee joint. Moreover, he hypothesizes that changes in the biomechanical conditions also make acute injury likely by forcing the knee joint to the limits of its range of motion.
Colloquially, I’ve heard many skiers talk about how their knees get sore after even a day or two of skiing fat skis. My home ski mountain is entirely off-piste and subject to both enormous powder days and weeks on end of hard-pack snow. At this point, most skiers have trended back toward skis around the 100-millimeter mark for most days on the mountain. Not because they’ve read the literature but because their knees have mandated it. It doesn’t take a degree in physics to understand that these skis are easier on your body.
Stance
The second primary way fat skis negatively affect skiers on hard snow is through a fundamental shift in their stance and mechanics. Skiers on wider skis naturally assume a more upright stance, which is their attempt to compensate for the added force during ankle articulation. Two significant negative consequences result from this.
First, skiers who are not already experts will likely develop poor habits. They won’t articulate their ankles because the GRF is too strong when angling a turn—less angulation results in more of a slipping and slashing motion than carving.
Second, the upright position exposes the knee to acute injury much more than a bent, athletic stance. Simply put, the knee is more vulnerable to connective tissue injuries like ACL and MCL tears when the leg is upright because the leg muscles are not engaged to provide support. Most connective tissue ruptures happen when the leg is fully extended, whether during a fall or using poor mechanics while skiing.
When to Use Wide Skis
In his paper entitled “Does Ski Width Influence Muscle Action in an Elite Skier? A Case Study,” John Seifert of Montana State University includes a discussion of when fat skis might actually benefit skiers. You may find the results surprising.
Seifert makes the case that it takes about 30 cm of fresh snow to eliminate the effects of mediolateral GRF with fat skis. How many days do you actually get like this? At most ski resorts, it’s just a handful each season. Moreover, once the snow becomes tracked up and choppy, GRF is present. At ski resorts, most terrain is usually tracked up in less than an hour, especially on big powder days.
Another conclusion of Seifert’s paper is that narrow skis don’t require significantly more muscle action than wide skis in powder. I found this to be the most controversial point made in the paper. While it makes sense that fat skis would be detrimental to the knee joint in hard snow, could it be that there is no difference between fat and narrow skis in powder? After all, retailers will tell you that you need a quiver of skis to satisfy all the different conditions on the mountain. You’ll stand out in the lift line with anything less than 100 millimeters underfoot on a powder day.
I’ve skied both narrow and fat skis on powder days, and I find the experience of fat skis to be more thrilling. You can toss snow around, get massive face shots, easily jump off rocks and cliffs, point your skis straight, and reach epic speeds. In my experience, fat skis open the world of powder skiing to a broader range of skill levels. Even intermediates can ski deep powder with a good pair of fat skis. Meanwhile, expert skiers can ski much faster and more aggressively than they would otherwise.
Meanwhile, narrow skis have their own set of advantages. While you can’t go as fast, you can dive deeper into the fresh snow and make fun dolphin turns. An important distinction is that narrow skis require much more technique to navigate in powder because you sink into rather than float atop the snowpack.
Conclusion
In conclusion, Seifert and Zorko have provided scientific evidence on why fat skis are bad for your knees in nearly all snow conditions except deep powder. The evidence supports the opinions of many skiers I’ve spoken to over the last ten years. There’s a reason why you rarely see good, older skiers skiing fat skis. Their knees are simply too worn out to handle the torque.
Skiing is bad for your knees, no matter what. Switching back to narrow skis is not going to provide some kind of miracle cure for your knee aches. However, increasing evidence suggests that, over time, a narrower ski can be less degenerative on the knee joint than a wide ski. Narrower skis are easier to angulate and carve, which is the most important technique for any skier. Moreover, the improvement in technique may offer a buffer against acute connective tissue injuries like ACL tears.
I’ve personally put my money where my mouth is and switched to a Volkl Mantra M6 at 96 millimeters underfoot (no partnership exists here, paid or otherwise). This is after years of skiing on more big mountain freeride-oriented skis over 110 millimeters at the waist. I ski 100 percent off-piste terrain in a variety of conditions. The Mantra has performed great in nearly all conditions, from hard pack to powder.
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