I'd argue that to be considered "high intensity" the workout should be higher than Lactate Threshold, and really, I'd consider it to be at least 100% of the effort at V̇O 2max. There are several variables that shape a HIIT workout. There is some evidence that even very low doses of HIIT (1-2 intervals) can improve fitness and insulin sensitivity in an untrained population.There is some evidence that HIIT improves race performance in short events (There is some evidence that HIIT results in greater body fat reductions than continuous moderate exercise in untrained or moderately trained subjects.The ideal intensity for HIIT is unclear, but generally believed to be above 90% V̇O 2max.The preponderance of evidence shows that HIIT provides greater improvements in V̇O 2max than continuous moderate exercise.Here are the high-level conclusions from the available research. 9.3 High-intensity interval exercise induces 24-h energy expenditure similar to traditional endurance exercise despite reduced time commitment (SkellyAndrews2014).9.2 Impact of exercise intensity on body fatness and skeletal muscle metabolism (Tremblay-1994).9.1 The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women (Trapp-2008).8.6 Acute high-intensity interval training improves Tvent and peak power output in highly trained males (Laursen-2-2002).8.5 Interval training program optimization in highly trained endurance cyclists (Laursen-2002).8.4 Optimising high-intensity treadmill training using the running speed at maximal O(2) uptake and the time for which this can be maintained (Smith-2003).8.3 Effects of 4-wk training using Vmax/Tlim on VO2max and performance in athletes (Smith-1999).8.2 Metabolic and performance adaptations to interval training in endurance-trained cyclists (Westgarth-Taylor-1997).8.1 Improved athletic performance in highly trained cyclists after interval training (Lindsay-1996).7.2 Linear increase in aerobic power induced by a strenuous program of endurance exercise (Hickson-1977).7.1 Six sessions of sprint interval training increases muscle oxidative potential and cycle endurance capacity in humans (Burgomaster-2005).7 HIIT Studies on Untrained or Moderately Active Subjects without Controls.6.10 A comparison of the health benefits of reduced-exertion high-intensity interval training (REHIT) and moderate-intensity walking in type 2 diabetes patients (RuffinoSongsorn2017).6.9 Towards the minimal amount of exercise for improving metabolic health (MetcalfeBabraj2011).6.8 Low- and High-Volume of Intensive Endurance Training Significantly Improves Maximal Oxygen Uptake after 10-Weeks of Training in Healthy Men (EarnestTjønna2013).6.7 Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans (BurgomasterHowarth2007).6.6 Improved running economy following intensified training correlates with reduced ventilatory demands (Franch-1998).6.5 Uniqueness of interval and continuous training at the same maintained exercise intensity (Gorostiaga-1991).6.4 Short-term sprint interval versus traditional endurance training (Gibala-2006).6.3 Aerobic Interval Training Versus Continuous Moderate Exercise as a Treatment for the Metabolic Syndrome (TjonnaLee2008).6.2 Aerobic High-Intensity Intervals Improve VO2Max More Than Moderate Training (Helgerud-2007).6.1 Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max (Tabata-1996).6 HIIT Comparisons on Untrained or Moderately Active Subjects.3.1 Effects of different interval-training programs on cycling time-trial performance (Stepto-1999).