High Intensity Interval Training (HIIT) is exactly that, high intensity training. This type of workout typically consist of short bouts of intense cardio followed by periods of rest. The two are alternated until exhaustion. For example, 45 second sprints followed by 45 second jogs. Research shows that this type of exercise significantly boost the metabolism while also building thigh muscle. In fact, in a recent study, women gained 25% more muscle in their thighs while burning fat(Mourier et al.). But these fitness goodies don't come without a cost: injury. HIIT requires the athlete to push themselves hard and without proper coaching athletes run the risk of injury. The most common being the knee.