Friction -- or the lack of it -- in cross-country skiing events at the Winter Olympic games in Vancouver is a decisive factor in who wins the gold. Researchers at the Norwegian University of Science and Technology (NTNU) explain the physics behind what makes the best glide.

Fully seven of Norway's 11 Olympic medals to date have been won by residents of the small counties of Nord and Sør-Trondelag, which is also home to Norway's main science and engineering university, NTNU. Among the university's researchers are experts on the physical demands of cross country skiing, the physics of ski glide, physical training and the aerodynamics of ski jumping.

Felix Breitschädel, a PhD candidate at the Norwegian University of Science and Technology, has studied the interplay between the choice of skis and wax that makes a winning combination for skiers.

Cross-country skiing takes enormous physical skill and endurance -- but it also takes the right skis and the right wax to bring home the gold, as Norway's elite athletes have learned during the Vancouver Olympic Games.

The wrong wax, wrong skis or mistakes in preparation of the base of the ski, "might lead to a change for the worse by up to 3 per cent," he says.

Cross-country skiers are able to kick and glide because of the way the wax and the physical structure of the ski and its base interact with the snow. When the skier presses down on one ski during a kick, the wax and ski base grip the snow, enabling the skier to push off and glide on the other ski.

Breitschädel, who is in Vancouver with the Norwegian national team, says ski preparation specialists that travel with racing teams have developed a four-step process that helps them decide how the skis should be prepared and what will work best. The steps are:

1) Different skis are tested on the track the day of the race to see what works best.

2) Once a ski itself has been chosen, the prep specialists go to work to create a micro structure on the ski base that will work in specific snow conditions. This structure is tested prior to the race.

3) Just a few hours before the race, the prep specialists have to test different waxes and wax combinations and wax the skis, which are then tested.

4) Just minutes before the race, the base of the ski is fine-tuned.

Breitschädel reports that the weather and track conditions at the Whistler Olympic Park in the Callaghan Valley are very special. "The arena is located 10 km west from Whistler, and about 200 km from the Pacific Ocean, and the area gets an average snow fall of 10 m in the surrounding mountains," he says. "Currently, the average snow depth is 1.2 m at the Nordic area."

Even though the site is not directly on the coast, it is still affected by coastal weather he says. The average temperature in February has been + 0.6°C, far warmer than the -1.4°C that has been the 4 year February average temperature.

But as long as there is enough snow, why does snow temperature matter to skiers? Breitschädel, says the mild temperatures in combination with regular showers increase the speed at which the snow changes structure, transforming pointy freshly fallen snowflakes into rounded snow grains. Regular freeze-thaw cycles further increase the snow grain size. Clusters of wet and round bonded snow crystals are the consequence.

Because the ski slides on the snow, the actual amount of surface area on the ski base is one important factor that determines how much friction there is.

If there is too much real surface contact area, the skier will actually experience some suction under wet conditions, but if it is colder, lots of surface area generates enough frictional heat to generate a thin water film for the ski to glide on.

"The ski base structure has to fit to the given snow grain condition," Breitschädel says. "New snow, with its complex snow crystals, requires a different ski base structure than old transformed snow grains." That means cold conditions call for fine grinds while coarse grinds are best for wet snow.

But what of the disappointing results for the Norwegian men's team in the 15 km freestyle race during the first week of the Winter Olympics? After race favourite Petter Northug turned in a disappointing finish, Norwegian media speculated that the wax might have been wrong. Breitschädel says that's an overly simplistic assessment.

"Waxing is one out of four parameters which affect the total performance of a ski. In addition to the ski characteristic, structure and track conditions, the waxing and the final ski tuning with a manual rilling tool are all important," he says. Each team carefully guards its wax and ski structuring secrets, but mistakes happen. The 3 per cent decrease in performance wouldn't make much of a different for the average skier, he says, but "at such a high level they are crucial and can make the difference whether an athletes wins a medal or not."

See also: Vancouver Olympians Prepared For High And Low Altitudes and Aerobic Efficiency Is Key To Olympic Gold For Cross-Country Skiers

Source: The Norwegian University of Science and Technology (NTNU).

Cross-country skiing is one of the most demanding of all Olympic sports, with skiers propelling themselves at speeds that exceed 20-25 km per hour over distances as long as 50 km. Yet the difference between winners and losers in these grueling races can be decided by just the tip of a ski, as a glance at any recent world-class competition will show. So just what gives top racers the advantage?

In an article to be published in the European Journal of Applied Physiology, Øyvind Sandbakk, a PhD candidate in the Norwegian University of Science and Technology's Human Movement Science Programme, reports with his colleagues on the metabolic rates and efficiencies of world-class skiers. Sandbakk's research offers a unique window on what separates the best from the rest in the world of elite cross-country racers.

"Skiers need high aerobic and anaerobic energy delivery, muscular strength, efficient techniques and the ability to resist fatigue to reach and maintain top speeds races," Sandbakk says. Those physical attributes may not be so very different from other world-class athletes, except that cross-country skiers also need to have mastered a variety of techniques and tempos, depending upon the course terrain, Sandbakk notes.

These challenges mean that the importance of the athlete's different physical capacities will differ in different sections of races, and between different types of competitions. For example, during the 10- and 15-km freestyle (skate) races in the Vancouver Olympics (the first of which are scheduled for February 15, with a 10km women's race and a 15 km men's race), skiers with high aerobic power (often referred to as maximal oxygen uptake per kilo body mass) will have an advantage in maintaining high speeds during the race, especially in the uphill terrain, Sandbakk says.

He says it is the uphill terrain that normally separates skiers the most during freestyle races. However, the 10- and 15-km courses also contain a great deal of level terrain, where an athlete with higher muscle mass and anaerobic power may have the edge needed to win.

Cross-country skiing also challenges skiers to master a great range of techniques for different speeds and slopes. Sandbakk predicts this factor will be crucial in the technically difficult Vancouver competition tracks. In skating races, skiers have as many as seven different skiing techniques (much like the gears on a bicycle) at their disposal, and they constantly shift between these different techniques during a single race.

"Skiers even adapt these seven techniques depending on the speed and slope," Sandbakk says. "The best skiers tend to ski with longer cycle lengths (the number of metres a skier moves his centre of mass per cycle), but with a similar cycle frequency," he says. "But during the last part of the race, the cycle frequency seems to be higher in the better skiers."

Another crucial aspect of technique is when the skier pushes off with his or her skate ski, and the skier's ability to recover quickly from the tremendous physical demand of providing a forceful push. "The ability to resist fatigue seems tightly coupled to the ability to maintain technique and keep up the cycle lengths and frequencies during a race," Sandbakk says. "In two skiers of otherwise equal fitness, this may be the deciding factor during the last part of the race in determining who wins the gold."

See also: The Physiology Of Speed and For Rock Climbers, Endurance Is Key To Performance

Source: The Norwegian University of Science and Technology (NTNU) and Metabolic rate and gross efficiency at high work rates in world class and national level sprint skiers. European Journal of Applied Physiology