Walking on snow is tough, so for centuries people have attached planks to their feet to improve movement across winter landscapes. More recently, cross-country skiing has become a popular sport, with sophisticated equipment that is painstakingly prepared before major competitions. Biomechanists Barbara Pellegrini and Chiara Zoppirolli from the University of Verona, Italy, work with top international athletes to assess and improve their performance. So, when cross-country skiers began coming to their lab at the Centre for Research in Mountain Sport and Health to optimise their performance, their colleague Federico Schena realised that they needed to learn more about the basic biomechanics of the different cross-country ski techniques. Having already shown why double poling (where only the arms are used for propulsion) is the most efficient method on flat surfaces, while diagonal stride (where skiers use both their arms and legs, pushing off at the same time with the alternate arm and leg) is most effective up hill and double poling with kick (where the skiers push off intermittently with one leg while double poling) is used to transition between the two techniques, Pellegrini, Paola Zamparo and colleagues have now turned their attention to how the different techniques relate to more conventional forms of motion, such as walking and running (p. 3910).
Fortunately, Pellegrini and her colleagues did not have to head out into icy winter conditions to assess the skiers' performance. Using roller skis and instrumented ski poles that had been designed and built by their colleague Lorenzo Bortolan, Pellegrini and Zoppirolli could measure the performance of skiers on a treadmill inclined at 2 deg and moving at 14 km h−1 as they used each technique.
The team also had access to some of the fittest cross-country skiers in Italy: ‘They are professional athletes from the military’, says Pellegrini, adding that the athletes were happy to help the scientists with their research by skiing on roller skis in the lab, as they often use them during the summer as part of their out of season training. Pellegrini and Zoppirolli measured the forces exerted through the soles of the skiers' feet and ski poles, and also filmed the athletes with a six-camera motion-capture system to accurately reconstruct the movement of each body segment.
Analysing the foot force traces and body segment motions, the team calculated how the energy of the body's centre of mass cycled between potential and kinetic energy while the skiers moved. They realised that when the skiers were double poling, the energy cycle closely resembled that of walkers. And when they calculated the kinetic and potential energy cycles for the skiers as they used the diagonal stride technique, the team realised that the pattern was most similar to running; however, instead of bouncing off the ground, the skiers slid. The team also investigated how much work the skiers had to do to overcome the effects of friction, and discovered that the roller skiing athletes were using 17% of their energy when diagonal striding and 24% when double poling with kick. However, double poling (without kicking) was the most costly technique of all: those skiers used 32% of their energy to overcome friction, and Pellegrini estimates that this could rocket to as much as 81% in bad snow conditions.
‘The quantification of the work required against friction for the different techniques will allow us to establish whether double poling is abandoned when friction increases and the skier makes the transition to double pole with kick or diagonal stride’, says Pellegrini. She is also optimistic that understanding that diagonal sliding is more similar to running than to walking could help novices to learn faster.
