Even though most ski manufacturers haven’t released their winter 20/21 ski lineups, it’s never a bad time to learn more about ski design and performance. It seems like nearly every skier is opting to ride skis that are wide underfoot, and indeed, the market reflects this trend. From 2012 to 2017, there has been a 25% increase in wide ski sales. However, studies have shown that bigger isn’t always better, at least not when it comes to ski width.
Before diving into the studies, let’s begin by clarifying some terms. All skis come in different dimensions but are measured essentially the same way. First, the length of the ski is measured from tip to tail and is typically given in centimeters. For example, a ski might come in a 166-centimeter length or a 187-centimeter length.
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Next, all skis have varying sidecuts. This is expressed by three numbers. For example, 135/98/119. The first number, 135, means that at its widest point at the tip, the ski measures 135 millimeters. The middle number, 98, refers to the ski’s width underfoot, or the width of the ski at its narrowest point. This middle number defines the width of the ski. The last number is the width of the ski at its widest point at the tail. The sidecut of the ski determines the ski’s turn radius, which can range from a 12-meter radius all the way up to a 45-meter radius, though most recreational skis fall in the 15-25 meter radius range.
There is no agreed-upon industry standard width for a narrow, medium, or wide ski. Backcountry.com classifies ski widths by intended use, so a groomer ski is anything 85mm underfoot or narrower, while an all-mountain ski can range from 85-105mm, and a powder ski is 98-125mm underfoot. Skis.com claims that skis 85-95mm “ are primarily used for on-trail skiing but have the ability to spend some time off the groomers in the right snow conditions,” while 96-110mm is the “ideal all-mountain width.”
The scientific community, on the other hand, defines a narrow ski as being 80mm or less, a medium width ski as 80-90mm, and any ski above 90mm is a wide ski. REI.com claims that the central difference between a skinny ski versus a wide ski is that “skinnier skis are better for carving turns on groomed runs, while wider skis provide better flotation in deep snow.” In fact, wide skis are often touted as being ideal for powder days, and who doesn’t want to have the most fun on a pow day?
The problem is that the purported advantages of a wider ski only occur on truly deep days with at least a foot of fresh snow. With less than a foot of fresh, the skier will be skiing on the bottom of the fresh snow or the top of the firmer, older snow, and they will be affected by ground reaction force, or GRF. GRF is a measure of how much force is exerted by the ground on a body in contact with it. Simply, it is a measure of how much the ground is pushing back against the skier.
The effects of GRF change as ski width increases. With a narrow ski, GRF pushes up from the edge of the boot; however, with a wider ski, the GRF pushes up from the edge of the ski which, because of the ski’s width, is wider than the boot. This forces the knees into something called the valgus position–in layman’s terms, the skier becomes knock-kneed.
Furthermore, skiers on wide skis tend to ski in a more extended stance, possibly to reduce force. This reduces their ability to use their muscles to stabilize their knee joints. Wide skis also create a mismatch in rotation of the femur and the tibia. With narrow skis less than 80mm underfoot, the tibia and the femur both rotate the same amount, around 12 degrees.
On wide skis, the femur only rotates 8 degrees. The effects of this mismatch are somewhat reduced by the skier’s extended stance; however, this mismatch still stretches and stresses the ACL and the MCL. There have been many anecdotal reports of knee pain after even just one day on wide skis.
But, again, if the skier is skiing on a deep day of a foot or more of fresh, they experience a dramatic reduction in GRF, which essentially eliminates the negative consequences of skiing on a wide ski. So it seems that wide skis are acceptable in deep snow.
In a study, Does Ski Width Influence Muscle Action in an Elite Skier?, Seifert et al studied muscle activation on wide versus narrow skis in both deep powder snow and on groomers, and found that the overall difference in muscle activation was not significantly different between the narrow ski and the wide ski on the different snow conditions. He also found that skiers on wide skis in powder used fewer muscles during the steering phase of their turn, which supports the claim that wide skis are easier to turn in powder. However, on wide skis in powder, skiers use quite a bit of muscle in the beginning and end of their turns.
The problem is that ski runs don’t stay untracked for long, and soon skiers find themselves no longer effortlessly floating through fresh, but fighting their way through the chop and finding the bottom of the new snow. When that happens, the negative effects of GRF on a wide ski come back into play: a knock-kneed position, stress on the knee joint, and the surrounding muscles’ reduced ability to stabilize the knee due to a more upright stance.
While Skis.com claims that skis 85-95mm wide are primarily groomer skis but “can be used off the groomers in the right snow conditions,” Seifert’s study shows that the overall muscle activation between a narrow ski (less than 80mm underfoot) and a wide ski (over 90mm underfoot) in deep powder snow conditions is not significantly different. Retailers claim that it is difficult to ski a narrow ski in powder, but the data does not support this claim.
For those of us looking for a new pair of skis this winter, it is worth considering a narrower ski. Your knees will thank you.