Kris Stallenberg 75-min Z3/4 (Sweet Spot) Easy #03 This workout, 'The Grinder,' is all about sustained effort and mental toughness. Two long, 20-minute sweet spot intervals build incredible muscular endurance and aerobic efficiency. This session directly translates to better performance on long, steady climbs and extended time trial efforts, teaching you to stay strong when the effort is prolonged. bike