“High intensity interval training (HIIT) involves repeatedly exercising at a high intensity for 30 seconds to several minutes, separated by 1-5 minutes of recovery (either no or low intensity exercise).” (Gibala & McGee, 2008, p.58).

“HIIT generally involves alternating bouts of higher-intensity exercise (20 seconds to 5 minutes) sessions with either true rest or light- to low-intensity recovery workloads throughout an exercise routine and has traditionally been used to train athletes who require high levels of both aerobic and anaerobic fitness (e.g., track, team sport athletes).” (Porcari, Bryant & Comana, 2015, p.92).

Individuals work at 90% or more work rate (high-intensity) for a set time, followed by recovery (low-intensity) for a set time. The work-to-rest (WR Ratio) is usually unequal, with the work rate lasting seconds or minutes. This is repeated approximately 2-6 times, with sessions lasting between 10 and 30 minutes – although the exact structure is sport- and person-specific.

Rest periods are intended to be too short to provide complete recovery, and completing subsequent intervals in a partially recovered state is a key part of what makes these efforts effective.

HIIT training, also sometimes known as High-intensity Intermittent Exercise (HIIE), attempts to decrease the overall volume of training by increasing the effort expended during the high-intensity intervals – which are brief but challenging.

Alternative names and variations of HIIT include:

• Power Interval Training (PIT; or Power Intervals).
• Sprint Interval Training (SIT; or Short Sprint Interval Training):
  • Functional Threshold Power (FTP) Intervals, which explicitly considers lactate threshold (LT), and (All Out) Miracle Intervals. Both are typically used in cycling.
  • A commonly studied SIT model is repeated Wingate Tests.
  • “Training adaptations are highly specific to the type of activity and to the volume and intensity of the exercise performed. […] Similarly, the marathon runner would not concentrate on sprint-type interval training.” (Kenney, Wilmore & Costill, 2012, p.212-213).
• VO2Max Intervals.
• Steady State Intervals.
• Short Work-to-Rest Ratio Intervals, performed in series.
• HIIT Aqua (also known as Aqua HIIT, HIIT Splash or Water-based HIIT). HIIT training in the water generates cardiovascular and metabolic benefits without high impact, and with additional benefits facilitated by the properties of water:
  • The pressure and resistive forces of water (hydrostatic pressure) aids venous return (blood flow back to the heart), which results in an increase in stroke volume.
  • This effect decreases heart rate even though the exercise intensity is at a peak level.
  • This decrease in heart rate and rapid recovery time makes water-based HIIT suitable for a range of participants.

HIIT, and its variants, is not for new or beginning exercisers; you need to have a basic level of cardiorespiratory fitness first.

The most common HIIT intervention used in studies is the Wingate Anaerobic Test developed in the 1970s (Bar-Or, Dotan & Inbar, 1977).

You find out more about interval training, its derivatives, variables and history here.