The benefits of low intensity cardio mixed with high intensity bursts of speed (like fartlek training) is that you not only increase your endurance, but at the same time you are also increasing your speed and your strength. (fitnessandpower.com)
While we all want to burn more calories and look great, HIIT Training is also a form of functional training for athletes who participate in a sport where their bodies experience high intensity bursts of activities. (weckmethod.com)
Interval training uses short, high intensity bursts alternated with with slower less intense work or rest periods, recovery phases throughout a single workout. (fitnessdoctrine.com)