Objective To compare the acute effect of high-intensity interval training(HIIT)and moder-ate-intensity continuous training(MICT)on ectopic lipid levels in overweight and obese youth.Methods Twenty obese or overweight subjects,aged 24.15±1.98 years,participated in two crossover random-ized own-control trials of HIIT and MICT.In HIIT,participants performed high-intensity cycling at 85%VO2max for 9 sets of 2 minutes,interspersed with low-intensity cycling at 25%VO2max for 10 sets of 2 minutes each and the session started and ended with low-intensity cycling at 25%VO2max.Howev-er,in MICT,all participants cycled continuously at 50%VO2max for 60 minutes,maintaining a pedal-ing speed of 50-55 rpm,with a 7-day interval between the two interventions.Before,as well as im-mediately,60 and 120 minutes after exercise intervention,all subjects underwent magnetic resonance imaging(MRI)scans to observe the fat fraction(FF)and spin relaxation rate R2* of the rectus femo-ris,biceps femoris and liver.Results Rectus femoris FF decreased significantly immediately after HIIT and MICT(P<0.05),without significant differences.Moreover,sixty minutes after MICT,rectus femoris FF returned to pre-exercise levels,while 120 minutes after HIIT,the values restored to the pre-exer-cise levels.However,no significant changes were found in the biceps femoris and liver FF before and after the two exercise interventions(P>0.05).Meanwhile,the rectus femoris R2* was significantly lower in both the HIIT and MICT groups immediately after exercise(P<0.05)and remained significantly low-er in both groups 120 minutes after the exercise(P<0.05),with no significant differences between the two groups.Biceps femoris and liver R2* were significantly higher in both groups immediately after the exercise intervention(P<0.05).Liver R2* returned to pre-exercise levels 120 minutes after HIIT group,but remained significantly lower than pre-intervention levels after MICT(P<0.05).Conclusion Both acute HIIT and MICT are effective in reducing intramuscular fat in the working muscles of over-weight and obese adults but have no significant effect on liver fat.Acute HIIT and MICT show similar fat-burning effects,but a single bout of exercise proves more effective for fat loss in the active mus-cles compared to the antagonist muscles.