**3.2 Cross-country skiing in SO**

In SO cross-country skiing athletes with ID propel themselves across snowcovered terrain using skis and poles, and the distance differs from 50 m XC skiing race classical technique up to 10 km XC skiing race according to each athlete's skills [27]. Fifty and 100 m events are held on a flat terrain; 500 m to 10 km race respects the terrain plasticity (uphill, downhill, flat, and curves) [28].

During XC skiing HR values were described to depend on the terrain (uphill, downhill, flat section) [29], and during the race, the average HR of professional XC skiers is approximately 90% of the maximal HR [20].
