Participants received 15 , 90-second hypoxic exposures ( dAIH , fraction of inspired oxygen [ Fio2 ] = 0.09 ) or daily normoxia ( dSHAM , Fio2 = 0.21 ) at 60-second normoxic intervals on 5 consecutive days ; dAIH was given alone or combined with 30 minutes of overground walking 1 hour later .