Subjects ( n = 36 ) were assigned to one of three training groups : endurance training ( ET ; 60 min per session for weeks 1-2 , increasing to 75 min per session for weeks 3-4 ) , or sprint interval training consisting of either repeated 30 ( SIT 30 ) or 15 ( SIT 15 ) second all-out intervals ( starting with 4 bouts per session for weeks 1-2 , increasing to 6 intervals per session for weeks 3-4 ) .