Are we really HIITing it?

Manhattan is a 22.8 square mile island with over 500 fitness facilities.  Imagine my excitement when I moved here and realized all the possible classes labeled as my favorite type of training… HIIT!!!  The hottest trend is impossible to ignore!  You can now find HIIT in multiple formats from cycling to circuits to yoga. Is HIIT meditation next?!?!  Bahaha!

In 3 months I completed 25 group classes in different facilities that contained HIIT within the title or the description.  Reflecting on these experiences, I asked myself, are we really HIITing it?  No. Not even close.

The definition of HIIT is in the name: high intensity interval training.  Intensity can easily be monitored with heart rate (HR) and categorized into zones based upon a percentage of the maximum number of beats per minute.

Heart rate zones, relatable speeds, and ability to talk while moving.

The zones are also simply categorized as low (below 60%), moderate (60-80%), vigorous (80-90%), and high (90-100%).  Ideally, total exercise time per week would be around 21% low intensity, 45% moderate, 25% vigorous, and only 9% high. 

Out of the 25 HIIT classes that I completed, only one class elevated my heart rate above 90% max… this is a problem!

Training above 90% heart rate max is uncomfortable.  Not everyone embraces this feeling and therefore not everyone incorporates it into their regular routine.  This is FINE!  Exercising at this maximal intensity is NOT required to be healthy and fit.

Back to the definition of HIIT and the classes that utilize the term for marketing, interval training is the alternation of work and rest.  In order to make the workout high intensity there needs to be a strategy to lift the heart rate above a threshold for the majority of the session time.  There are no set rules for the duration of each segment.  Instead the goal is to maintain a heart rate above 85% max for two thirds of total time or above 90% max for half the time.  Programming this profile requires deliberate exercise sequencing and repeated trials.  It is not possible to be exercising at these intensities for over 30 minutes and therefore any workout over 30 minutes is almost certainly not high intensity training, but vigorous exercise.  Again, this is FINE – but do not classify the session as HIIT.

Heart rate profiles for a HIIT session and a vigorous session. Please note the shorter total duration of HIIT and the significant time above 90% heart rate max.

Defining HIIT through the name does not include an important fact about the type of training.  HIIT is a form of cardio.  HIIT is not strength training.  Building muscle cannot occur in this zone.

Weights can be utilized to increase intensity as can step benches or plyometric boxes.  But equipment is not required.  Sprint running is likely the most practiced form of HIIT.  And if space is a limitation, it is possible to create an entire workout with body weight exercises, such as burpees, tuck jumps, and high knee running.

So, why is it an issue to label a session as HIIT if it is not delivering the specific definition details from above?  Because it is misleading with respect to the potential benefits.  Multiple scientific reviews have demonstrated superior outcomes of HIIT for body composition, metabolic disease risk, and cardiorespiratory fitness compared to moderate intensity continuous training.

There are two options.  One, discontinue marketing workouts as HIIT unless they adhere to the definition.  Or two, modify the session programming with less time, lower weights, faster repetitions, and reduced recovery to push participants into the max zone for true HIIT!

Next week:  The benefits of HIIT.


12 Responses

  1. Outstanding article! I’ve been arguing this with fitness types for the past couple of years, and it’s really great to see an expert finally call out the inaccuracies of what is being incorrectly labeled and delivered. Well done.

    • Hi Meghan! 220-minus age is a fine initial estimate… but now you have a collection of Les Mills GRIT sessions. Could you browse some recent workouts and see if you are reaching heart rates above that estimate or if you are rarely getting above 90% max? Please feel free to email me at to discuss more!

      • I am so happy to find your blog! I am a LesMills grit instructor and I have a question about the grit strength format. I find that compared to cardio/athletic or sprint, my heart rate does not get nearly as high during grit strength compared to the other formats. Am I doing something wrong, or is the strength format designed differently for heart rate?

        • Hello Carolyn! That is an excellent question! GRIT and SPRINT are both cardio workouts… therefore in GRIT STRENGTH you will want to use less weight than BODYPUMP and perform the exercises faster with quality technique to lift the heart rate. Is there a specific release where you noticed a large difference? Thank you for reading my blog! Jinger

  2. I hadn’t realized that’s how that works! I’m going to have to start wearing a heart rate monitor on the regular.

    • YES!! It is totally life changing! I recommend a Polar watch and add the H10 chest transmitter!

      • Maybe vigorous workouts that don’t quite adhere to the definition of HIIT could be marketed instead as sub-HIIT (SHIIT)?

  3. Wondering if you’d be willing to follow up on the comment that “HIIT is not strength training.” Weight lifting is anaerobic, and if it pushes the Heart Rate to the prescribed levels, does that not qualify? Or does it not qualify because you just aren’t building muscle? Grit seems to paint a different picture in regard to this. Thanks for following up!

    • Hi Eric! Thank you for reading and asking a thoughtful question. Unfortunately GRIT STRENGTH is a misleading term because the 30 minute high intensity session is not strength training. It is high intensity cardiovascular training using weights. We are not building muscle at the intensity for such a long duration. Heavy weight lifting can elevate heart rate into the higher zones but the time is significantly less with full recovery between sets.

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