Cross-country skiing is one of the world's finest aerobic fitness activities.
The science says simultaneous use of the arms and legs means the body is burning more calories than a long-distance runner or a swimmer.
As a result the sport requires great stamina, and cross-country events at previous Winter Olympics have been blighted by positive drugs tests.
In 2002, Spain's Johann Muehlegg was stripped of two gold medals after testing positive for NESP, a substance closely related to erythropoietin (EPO).
Russia's Larissa Lazutina, who was stripped of her gold from the 30km race, and Olga Danilova, also tested positive for the same substance.
HOW THE COMPETITION UNFOLDS
Each race takes place on a specially designed course with two ingrained tracks designed for the skis.
There are three styles used by cross-country skiers:
Classic, or traditional - The skis are kept parallel and do not deviate from the tracks in the snow.
Skiers use alternate steps on climbs with the "double push" on downhills, using the ski poles simultaneously to gain momentum and speed.
Freestyle - Anything goes in the freestyle method, but most skiers use a v-shaped motion. This style takes its influence from the world of ice skating. It's more energy-sapping, but it's faster.
Skis for the freestyle technique are generally about 15-20cm shorter than for the classic.
Classic/freestyle - The pursuit races see the competitors using both the classic and freestyle techniques.
The longer-distance events do not require qualifying heats, but the sprint events do.
Each skier must complete an individual time trial, with competitors starting at 15-second intervals. The fastest 16 then qualify for the quarter-finals, which comprise four races of four skiers each.
The two fastest skiers from each race move into the semi-finals, which follow the same format as the quarter-finals. The top two finishers from each race qualify for the final.
The winner is the skier that crosses the line in the fastest time.
Wax is applied to the skis to help them tackle the different types of terrain. There two types of wax - glide and kick.
Glide wax, as its name suggests, reduces friction between the ski and snow to help the skis glide faster.
Kick wax, which is only used in the classic technique, prevents the skis from slipping.
Cross-country skis are only attached to the toe of the boot to make it easier for the skier to climb or descend, depending on the undulations of the trail.
Poles help provide momentum and speed. Classic poles should reach the armpit when standing, while freestyle may reach as far as the mouth.
WANT TO GET INVOLVED?
Because of its fitness properties, cross-country is one of the most popular forms of skiing in the UK. However, because of the lack of snow, it is mainly done on roller skis.
Cross-country skiing, like biathlon, has been traditionally associated with the military.
These websites contain all the information you need to get started: