*1.1. Problem Definition*

Cross-country (XC) skiing is a whole-body exercise endurance sport, which requires prolonged complex cyclical motions performed using skis and poles on the snow [1]. There are two main styles in XC-skiing: the classical and the skating style. The classical style can be performed both on prepared trails with pairs of parallel grooves cut into the snow or on natural undisturbed snow whereas the skating style is generally performed on firm and smooth snow surfaces. Each of the classical and skating styles have four techniques or gears. These are diagonal stride (DS), double poling (DP), push-off (P-Off), and kick-double poling (KDP) for the classical style, and V2 skate (V2), V2A skate (V2A), V1 skate (V1), and free skate (FS) for the skating style, respectively. For ease of understanding

and reference, we will henceforth refer to these techniques using the abbreviations mentioned after the name of the technique.

In XC-skiing, the performance of the skiers depends on the biomechanical and the physiological aspects of the motions of the body parts and the sequence in which the skiing techniques are performed on the uphill and downhill tracks (commonly known as a natural course) and flat tracks (flat course). As the results of the skiing races can be determined by time steps as small as a few milliseconds, it becomes imperative for the professional coaches to understand both these aspects of XC-skiing to recommend an improved set of techniques for optimizing the performance of the skiers. Traditionally, these analyses have been performed using video-based systems [2–6] and/or force measurement systems [7–9]. However, the utilization of equipment in both these systems interferes with the natural movements of the body parts and the heavy cost involved limits their practical usage to only a few researchers across the world [10].

Body worn sensors, particularly the inertial sensors have recently emerged as a convenient substitute for such systems due to their small size, light weight, and low cost. Inertial sensors are sensors based on inertia and relevant measuring principles. In general, inertial sensors include gyroscopes used for measurements of the sensor's angular velocity and accelerometers for measurements of linear acceleration. These sensors can sample at high frequencies and are easily attached to the skier's body without interfering with the natural motion during skiing. This ease of use has made it possible to carry out experiments that require sensor data outside the controlled environment of the laboratory and provide a more realistic analysis of the task at hand. Marshland et al. [11] were the first to demonstrate this potential of body worn microsensors in the identification of XC-skiing techniques by plotting acceleration and angular velocity curves for eight athletes for both the classical and skating techniques. By visual inspection of the cyclical patterns in these plots, they concluded that all the classical and skating techniques can be clearly identified for each skier, with certain variations unique to each skier.
