The overall objectives are to:

1. Assess mechanisms underlying HIIT-induced improvements in insulin resistance at the whole-body, tissue, and cellular levels.
2. Systematically compare different HIIT regimens to help identify effective "doses" of HIIT that may be optimal for improving metabolic health in obese adults.
3. Assess the ability/willingness of obese subjects to adhere to a long-term HIIT program.

Findings from these studies will greatly expand knowledge about the effects of HIIT on metabolic health, and will provide valuable information for development of programs aimed at maximizing key metabolic benefits of exercise.

If subjects are eligible for the study, they will be randomized into one of four different exercise training groups. Before training (and twice after 3 months of training) subjects will participate in a "clinical study" in which they will be required to stay overnight in the hospital. During the clinical study, the research team will perform a series of metabolic tests. Before and after the 3 months of training, the research team will assess subjects "free-living" 24h blood sugar control using a Continuous Glucose Monitor (CGM). The research team will measure several other relevant clinical markers (e.g., whole body and regional body fat assessments [Dual Energy X-ray Absorptiometry (DEXA) and Magnetic Resonance Imager (MRI)], hepatic lipid accumulation [MRI analysis], blood lipid profile, resting metabolic rate, glycated hemoglobin [hemoglobin A1c]). After the first 3 months of training, adherence to the subjects assigned training program will be tracked for the next 9 months (1 year of training in total). One year after initiating a training program, subjects will be asked to return to the clinic for a quick follow-up visit where the research team will complete some standard clinical assessments (e.g., body weight, body composition, blood pressure, and a blood sample).

Inclusion Criteria:

• Age: 18-40
• Body Mass Index: 30-40 kg/m2
• Waist circumference: 88-98cm for women and 100-110cm for men
• Glucose intolerant (i.e., 2h glucose concentration during a screening oral glucose tolerance test (OGTT)= 140-199mg/dl
• No regularly planned exercise/physical activity
• Women must have regularly occurring menses and must be premenopausal

Exclusion Criteria:

• EKG abnormalities as assessed by the cardiologist on the research team
• Evidence/history of cardiovascular or metabolic disease
• Medications known to affect lipid or glucose metabolism
• Pregnant or lactating
• Tobacco or e-cigarette use