In a double-blinded fashion , 17 competitive cross-country skiers performed six sessions of repeated sprints ( each consisting of four sets of five 10-s sprints , with 20-s intervals of recovery ) either in normoxia ( RSN , 300 m ; FiO2 , 20.9 % ; n = 8 ) or normobaric hypoxia ( RSH , 3000 m ; FiO2 , 13.8 % ; n = 9 ) .