Every now and then you really feel like a "reset" HIIT session, the kind of session that will superheat your muscles, make you sweat hard and leave you feeling totally wiped out afterwards. There are good reasons for sessions like that and they have to do with levelling up. Bring your knees waist high during High Knees, sync your arms and legs, and try to get in as many reps as possible in each 20-second segment. Even done once a month this particular HIIT workout will deliver tangible benefits in overall physical performance.