The appliance of science to Ski Cross

Ski Cross is one of the most thrilling sports under the International Ski and Snowboard Federation (FIS) umbrella with four skiers vying for position at speeds of up to 100km/h. 

Banked turns and steep jumps make for spectacular viewing, but the nature of the discipline makes falls and collisions inevitable. As one would expect, safety is of paramount concern for FIS and race organisers. 

Ski Cross courses are already subject to guidelines regarding length, width and vertical drop with stricter recommendations in place for take-off and landing areas for jumps, netting around turns and run-off areas for potential contact called ‘spill zones’. 

However, FIS is determined to keep making progress in the area of athlete safety. Last season, it collaborated with the Norwegian School of Sport Sciences (NIH) and the FIS Athlete Health Unit (AHU) for a Ski Cross Assessment Project to evaluate Ski Cross courses. 

The project was designed to ensure that courses are both safe and challenging for competitors. The assessment included detailed measurements and analysis of various course elements, with the ultimate goal of improving course safety and enhancing the overall appeal of Ski Cross races. 

Five venues - Alleghe, Innichen, Reiteralm, Veysonnaz and St. Moritz - were assessed with regard to course design and safety measures, and how these affect athlete performance and injury risk. 

Things to consider when preparing a Ski Cross course 

Among the requirements for a Ski Cross course are a length of 800m-1200m, a vertical drop of between 100m and 250m, and a minimum width of 30m. An average downhill gradient of 7°-11° is recommended with a flatter layout offering the course designer more options in terms of elements. Steeper slopes require greater width to allow for sufficient speed control and protection installations. 

As well as boasting elements which challenge the athlete, the course should have features which promote overtaking and yield a significant number of changes in position, known as rank shifts. 

FIS Ski Cross Race Director Klaus Waldner, a former Ski Cross racer himself, says, “We want to make Ski Cross racing as exciting as possible so we need a good course on a decent level with decent elements. The goal is to create as many rank shifts as possible and have a photo finish at the line to see who advances to the next round. It should all come together at the finish and we want to reach there as safely as possible. 

“We need technical elements at the start to separate them a little bit and they should close up again two times on the course, so we have to slow them down twice to bring the pack together. Then we almost start the race afresh and this is where we create overtaking manoeuvres and rank shifts most of the time. We need to look closely at these parts of the slope to see if they are safe, if we have large enough spill zones, and whether the elements are working.” 

The terrain plays a key part in the preparation of a Ski Cross course, according to Assistant Race Director Christian Cretier. “It all starts with the choice of the slope itself,” he observes. “We have to build the safest course according to the terrain, taking into consideration the space we have, and decide where to place the elements according to the width and steepness.” 

Michael Lasshofer of the AHU says, “Course design is essential for a successful competition. In this context, successful on the one hand means attractive to athletes and spectators, and on the other hand controlling modifiable risk factors to reduce the overall risk for injuries.” 

The reality of racing

Ski Cross is unlike the vast majority of ski disciplines in that athletes go up against each other at high speed. Crashes are inevitable and largely caused by a racer getting ‘out-of-balance’ or making contact with a rival. While one of those occurences is enough in many cases, a combination of the two further increases the risk of a fall. 

With changes in weather potentially altering course conditions within hours, the onus is on athletes to race hard but fair. Those who do not respect the rules or their rivals receive warnings with more serious infractions resulting in yellow and even red cards. 

“Every athlete is responsible for every other athlete,” believes Waldner. “Every coach is also responsible for their athlete. It could be that from one day to another the course could be six seconds faster, or six seconds slower if we get snowfall. 

“We can make small adjustments but it’s a coaching job. Or it needs athletes, smart athletes, to think, ‘OK, now I’m slow, I have to jump at this jump or I’m in the flat,’ or, ‘I have to press more because I am faster.’” 

In other disciplines, difficult elements such as sharp turns and big jumps may increase the risk of accidents. Training can also improve skiers’ safety as they become more familiar with the track they will face on raceday. But in Ski Cross, certainly for the men, the opposite is true. 

“The data is clear,” concludes Waldner. “The more elements a course has, the more the athletes have to concentrate on themselves instead of looking at others. This means less contact and getting out-of-balance.” 

Cretier adds, “We need to keep them busy and focused on their skiing instead of getting stupid ideas!” 

Waldner continues, “When it’s too easy for them, they risk more. If they’re thinking, ‘I go into a turn and it doesn’t matter how I come out because the next elements are easy,’ they will go for that risky manoeuvre. But if they have to concentrate on the element ahead, there’s less contact.” 

The difference between men and women in Ski Cross 

Analysis of data from the five courses last season showed a marked divergence between men and women in the cause of crashes. Around 60 percent of crashes in women’s races were down solely to the skier getting out-of-balance compared to less than 15 percent in men’s races. 

In almost 50 percent of men’s crashes, the cause was a combination of the athlete getting out-of-balance and making contact with a rival. The same scenario accounted for around 30 percent of women’s crashes. 

NIH Associate Professor Matthias Gilgien said, “What’s maybe most surprising about the data are the clear differences between men and women. This illustrates the challenge of using the same courses. If they are more accommodating for the women, they may become too simple for the men who would fight even more. We need to find a balance with a course that is not too difficult for the women but still challenging enough for the men.” 

To try and prolong the novelty of the test, the number of training runs for the men has been halved from four to two. And Waldner is keen that athletes do not become too familiar with a course over time. 

“It’s also important that we change the line or the elements every second year on a course,” he says. “After one weekend of racing they know exactly where to overtake and which is the fastest line. They think, ‘I have to overtake on this turn because - if no one in front makes a mistake - I can’t overtake on the next two turns.’ As a result, they take greater risks. 

There were also big disparities between the genders when it came to rank shifts. Men’s races generally saw overtaking at a number of places during contests, while women’s races were often decided at the start with minimal position changes after the first couple of elements. 

“Projects like this provide a huge amount of measured data and systematic analysis,” says the AHU’s Michael Lasshofer. “This analysis can help to better understand the competition characteristics, e.g. overtaking, but also give deeper insights in crash/fall situations to optimise safety installations. With this deeper understanding, it is also possible to learn more about gender-specific aspects.” 

“Can we do something to the course that allows women to overtake more and make the race more exciting?” asks Gilgien. “The hypothesis from the race directors is that rank shift is related to speed  - a high-speed section will see little overtaking - and that’s borne out by the data. But apart from the speed, is it the obstacle - jump, roller, turn - that can lead to overtaking?” 

With detailed measurements to be taken from another five races this season, he admits, “We want to do more analysis - with flat speed, type of obstacle - to figure out what is causing those rank shifts. Is the difficulty of the obstacle prompting mistakes which lead to overtaking? That’s the analysis we’ll be running now.” 

Risk mitigation through data analysis

Like most forms of racing, and despite the best efforts of those in authority, falls come with the territory. As in alpine skiing, among the most frequent Ski Cross injuries are anterior cruciate knee ligament (ACL) tears which are hard to prevent due to multi-causal risk factors. 

Severe injuries are defined as those which render an athlete unable to compete or train for at least 28 days. As well as ACL tears and lower-leg fractures, concussions and shoulder injuries are also common in Ski Cross. However, courses can be designed so that crashes - when they occur - are less likely to cause serious injury. 

“The best scenario is no crashes, but crashes are part of Ski Cross,” said Waldner. ”If they have enough space and the spill zone is large enough, normally the crashes are under control. But one type of crash we want to avoid is before a jump take-off. If there is contact before take-off, athletes don’t have a chance to react. 

“We can have more technical elements on the snow surface and avoid too much contact and out-of-balance before a take-off. This is one of the key elements, because it’s unpredictable before take-off.” 

The positioning of B-nets - the most common nets used in races which actively reduce the speed and energy of impact - relative to elements is vital for safety especially around trees or TV towers near to the course. 

Using 3D modelling, Gilgien and his team run simulations of jumps and turns which calculate the angle and speed of impact with netting in crash scenarios. He explains, “This sort of information helps point out to organisers where, if there is a crash, they need really good protection.” 

For Waldner, the data helps give legitimacy to decisions and potential requests to host venues. “if there are two trees next to the course, it can be hard to get permission to cut them down,” he admits. “With this scientific report, you can go to the local government or whoever and say, ‘We have a big problem with these two trees. Can we do something?’ It’s important for us that the organisers have something on paper to show responsible parties or those in charge.”

Making Ski Cross courses safer - the next steps 

After five venues were assessed last season, five different courses will undergo the same treatment this term with Cretier keen to have “the full scope of different courses from short to long, difficult ones”. 

In the future, it is hoped that NIH’s 3D modelling and simulations will assist viability studies of new courses as well as give guidance as to which features can and cannot be incorporated. 

Cretier adds, “Our experience tells us certain things but this helps provide knowledge to organisers and people who don’t have that real experience of courses and terrain. We can point to the data saying, ‘This isn’t going to work,’ or, ‘We need more space,’ or, ‘We need a wider course at these points or it’s not going to be feasible.’” 

However, it is almost impossible to be definitive about the safety of elements. Gilgien revealed, “We cannot say, for instance, that rollers or jumps are more dangerous than turns. It depends on many things, not least the way they’re built. 

“In one race, you may find that most of the crashes or out-of-balances happened on a particular element. But then you go to another race and the same element does not have the same effect. Things need to fit together.” 

For Waldner, the importance of this analysis goes beyond courses for World Cup competition: “Maybe in the future we can say to a new venue organiser, ‘Send us the 3D model from your race venue and we can tell you already from the data whether it looks like a good spot.’ 

“This transfer of knowledge is really important because we also need really good safe courses at European Cup and FIS level. We don’t want to lose athletes through poor course builds. That’s one big goal of the study.” 

The athletes, Race Directors, AHU, plus Gilgien and his team, will get to work again at the World Cup season-opener in Val Thorens in France on 10 December.