High Knees, High Knees and more High Knees.
Feel your quads burn as you get through 5 rounds of this, 10 if you are at an intermediate level and 15 if you are advanced.
Disclaimer: Do not perform these exercises if you have any injury, get rehabilitation first. If you get short of breath, feel fatigued or tired during any of these workouts, stop and take a break. Consult a doctor/trainer if necessary before starting any workout session. Children should do them under supervision of an adult.
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These workouts are taken from the Darebee 30 days of HIIT program.