High intensity interval training (or HIIT, for short) sounds simple enough: go as hard as you can for a short period, rest, repeat, and reap the benefits. HIIT is great, for sure, except what most people consider HIIT isn’t actually HIIT. Your all-out effort makes all the difference.
One thing that isn’t always clear is that unfit people should not try HIIT. Without a decent fitness base HIIT is counterproductive and more likely to cause injury. Beginners should stick to longer and less intense intervals to build up fitness and form as preparation for the demands of HIIT.