Group 1 worked out 5 hours a week with cardio and strength training but no high intensity interval training. Group 2's routine included 2 sessions of 30 minute LES MILLS GRITTM SERIES high intensity interval training in addition to cardio and strength training. After 6 weeks, Group 2 saw significantly greater improvements in triglyceride levels, oxygen consumption, body fat percentage, waist circumference, and back and leg strength compared to Group 1. The study demonstrates that including high intensity interval training with LES MILLS GRITTM SERIES can more effectively improve health and fitness levels in already active adults.