HIIT (High Intensity Interval Training)

“I don’t have time!” is one of the top reasons for not exercising, as many traditional workouts push a commitment of about an hour. High intensity interval training, or HIIT, challenges this barrier by incorporating an effective workout in half that time. In our time-pressured culture, HIIT has claimed a spot in the top 10 fitness trends since 2014 as surveyed by the American College of Sports Medicine. [1] In about 30 minutes, it is considered a complete workout that combines both aerobic and strength (resistance) training. In order to achieve similar benefits to a longer workout, the intensity is vigorous.

HIIT is a type of interval training exercise. It incorporates several rounds that alternate between several minutes of high intensity movements to significantly increase the heart rate to at least 80% of one’s maximum heart rate, followed by short periods of lower intensity movements. Interval training was first introduced in the 1950s as a higher intensity form called sprint interval training, which reached 100% maximum heart rate and was used to improve the performance of elite Olympic athletes.

Body weight can be used as the main form of resistance so that additional equipment is not needed. HIIT workouts also generally do not require a large amount of space, making the format ideal for a home workout. HIIT workouts can be integrated into various exercise formats, such as running (outdoors or on a treadmill), dancing, rowing machines, stationary bicycles, or stair climbers. [2] The interval durations can be timed by using one to five-minute music tracks.

Other terms that are used interchangeably with HIIT are Tabata and circuit training. Tabata is a form of HIIT that was created by Professor Izumi Tabata in 1996 involving Olympic speedskaters. His exercise intervals were extremely high intensity, followed by very brief rest periods. Fitness centers and gyms that offer Tabata classes are typically 20-30 minutes and encourage participants to reach their highest intensity ability, but they can self-regulate their workouts. Circuit training involves 8-12 exercise stations that target different muscle groups. Participants rotate through each station, completing one exercise that lasts several minutes. The difference with circuit training is that the intensity is variable, whereas HIIT encourages maximum effort by reaching 80-90% maximum heart rate.

HIIT can help to decrease body fat, increase strength and endurance, and improve health outcomes, but it is not necessarily better than other exercise formats. Its main appeal is that it can achieve similar fitness and health benefits in a shorter duration, and that it includes periods of rest.

Safety

People who are deconditioned, recovering from injury, elderly, have overweight, or have medical conditions should be followed and monitored closely by their physician and an exercise professional, because of the higher intensity achieved with HIIT. It has been observed that for deconditioned individuals, the intensity of HIIT is comparable to what they may encounter during activities of daily living. The American College of Sports Medicine provides screening tools that can be used to identify risk factors using the HIIT format, to lower the risk of adverse events. [3] These checklists include medical conditions that are contraindicated to performing HIIT workouts (e.g., uncontrolled heart rate such as with arrhythmias, uncontrolled diabetes, retinopathy), and symptoms to watch for to end a HIIT workout early, such as a significant rise or drop in blood pressure during the workout. [2]

HIIT workouts should be tailored to the individual’s fitness level and medical conditions. Research has generally found HIIT to be a safe and enjoyable exercise for a range of ages and medical conditions. A meta-review of HIIT compared with control groups found HIIT to be safe (no acute injury reports or serious cardiovascular events) in controlled supervised settings, with mean compliance rates in completing the program reaching >80%. [4]

two people doing squats in a gym with a medicine ball

Example of a beginner HIIT workout

This workout can be performed at home using just an exercise mat and a timer or clock. [5] The speed of each exercise can be faster or slower, depending on one’s fitness level, but encourages the participant to work to their maximum ability. A 5-minute warm-up of walking or marching in place should be performed before the workout, and a 5-10-minute cool-down of slower movements allowing the heart rate to gradually decrease, along with stretches, should be included to end the workout.
  • 30 seconds of side lunges, alternating right to left
    • 15 seconds of slow marches in place
  • 30 seconds of squats (variation for higher intensity: jump squats)
    • 15 seconds of slow marches in place
  • 30 seconds of push-ups on the floor (modification: at a 45-degree angle on a sturdy chair, or against the wall)
    • 15 seconds of slow marches in place
  • 30 seconds of jumping jacks (modification: alternate right and left tapping toes to the sides while bringing arms overhead as you would a jumping jack)
    • 15 seconds of slow marches in place
  • 30 seconds of triceps dips using a sturdy chair or bed
    • 15 seconds of slow marches in place
  • 30 seconds of alternating high knees (variation for higher intensity: jogging high knees)
    • 15 seconds of slow marches in place
  • 30 seconds of sit-ups (modification: sit-ups on a stability ball, or abdominal crunches on the floor)

REST FOR 60 SECONDS AND REPEAT THE SESSION 2 MORE TIMES

HIIT and Health

HIIT is a well-researched exercise format, showing benefits for a range of medical conditions across a broad age range, from adolescents to older adults. [6] In research studies, HIIT is typically compared with moderate intensity continuous training (MICT), which incorporates lower intensity movements at a constant pace without interval breaks. Whereas HIIT causes individuals to reach 80-85% of their maximum heart rate, MICT reaches about 55-70% of their maximum heart rate.

When energy expenditure remains the same for HIIT versus MICT workouts, some studies show a greater benefit with HIIT because it achieves greater aerobic capacity (the body’s ability to use more oxygen). [5] Although initially applied to athletes to improve their performance, HIIT is now included as a potential exercise option for individuals with chronic diseases. It can help to improve their physical functioning, exercise tolerance, and quality of life.

Although the higher intensities reached with HIIT formats may appear too difficult for people with chronic diseases, research has shown that the intermittent rest intervals and overall shorter duration of the exercise make HIIT a potentially feasible and safe option for even more serious conditions like lung disease, heart disease, and chronic kidney disease under close supervision. [7] Modifications may be implemented, such as using specific exercises or incorporating longer warm-up and cool-down periods for people with heart disease or on blood pressure medications. [2] Some research in these populations found HIIT to be rated more enjoyable and with longer-term compliance than MICT. [2] However, larger and additional studies need to be performed with these at-risk groups before routinely prescribing HIIT. HIIT has also been found to improve depression more than MICT in people with major depression or bipolar disorders, without reported adverse events. [8] 

Bottom Line

HIIT is an effective exercise option to increase endurance and strength in those who have limited time to exercise. Because of the higher intensity format, it is advised to consult with a physician if you have any medical conditions before starting a HIIT program. All participants new to HIIT should choose a program that is facilitated by an exercise professional.

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