HIIT Training - How is Intensity Measured?

You can control the pace

Boost your effort for short periods of time. Martin Poole / Getty Images

HIIT - or high-intensity interval training - has been shown to be beneficial for older adults who are short on time, even if they have age-related illnesses like heart disease and diabetes.

This style of training alternates brief bursts of fast cycling, walking, rowing or other exercises which use the large muscles of the legs, with slower movement or rest periods with no activity for recovery. 

There are two ways the intensity of the brief bouts are typically measured in HIIT research:

1.   Using a heart monitor:  In a 2012 study conducted on 4,846 older subjects recruited from cardiac rehabilitation centres in Norway, high-intensity shifts involved exercising at 85-95% of maximum heart rate for 4 minutes.

Maximum heart rate is usually calculated using this formula:

Maximum Heart Rate = 220 - Age

A 65-year old adult would therefore have an estimated maximum heart rate of 155 beats/minute
(220 - 65 = 155)

2.   Scale of perceived exertion:  A Borg Rating of Perceived Exertion (RPE) is a useful tool when you don't have a heart monitor at hand.  Based on a scale from 6 to 20 - with 6 requiring no effort at all, and 20 representing maximum exertion - the subjects in the Norwegian study were told to aim for an RPE of 15-17 during the intense bouts of activity. This target represents hard or very strenuous effort for the prescribed duration.

Be aware that HIIT can be a very tough way to train.

 The subjects in these and other HIIT studies were supervised by researchers throughout, which could well not be the case while you're on a stationary bike or elliptical at your own gym.  

Still, scientists like kinesiology professor Martin Gibala from Hamilton's McMaster University tell me that just varying the intensity of your nightly walk or run will soon bring greater benefits to your heart, lungs and sugar metabolism, compared with maintaining the same effort throughout.

An interval exercise expert, Gibala points to research such as a small study published in Diabetes Care in 2013, which showed intervals don't need to be all-out intense to be effective.

That trial assigned subjects with Type II diabetes to walking regimens of 5 hours per week: either an interval walking plan (based on HIIT principles), sessions of walking at a steady pace, or no walking at all.  The interval walkers were tested for their peak energy expenditure at the beginning of the study (measured as V02 Max), and were instructed to alternate between 3 minutes of fast walking above the target, and three minutes of slower walking below that target.  

Subjects in the continuous-walking group were told to aim above a target of 55% of their maximum energy expenditure.  All subjects completed 5 one-hour sessions each week, for 4 months.

​By the end of the study, the interval walkers - whose pace varied within each session - saw greater improvements in physical fitness, body composition, and glycemic control, compared with the steady-pace walkers.

What's more, the researchers note, few subjects dropped out even though past studies on diabetic subjects boasted only a 60% retention rate.

This doesn't surprise Martin Gibala.  He says that the rapid boost in fitness with interval training - along with the time it can safe - are both motivating.

"People are willing to put up with some transient discomfort, if they're able to get a bigger benefit spending the same time exercising, or to get the same benefit in less time," he notes.

"If you ask subjects how they like this style of exercise during their intervals, they'll say they hate it.  But if you ask them after  the interval, they'll rate it as more enjoyable."

Seems like just getting an exercise session over with can be a big part of the appeal.

Note: Check with your physician before embarking on, or changing, your exercise plan.


Martin J Gibala. Professor and Chair of the Kinesiology Department, McMaster University in Hamilton, Ontario. Interview conducted by phone January 14, 2015. 

Øivind Rognmo, PhD; Trine Moholdt, PhD; Hilde Bakken, BSc; Torstein Hole, MD, PhD; Per Mølstad, MD, PhD; Nils Erling Myhr, BSc; Jostein Grimsmo, MD, PhD; Ulrik Wisløff, PhD. "Cardiovascular Risk of High- Versus Moderate-Intensity Aerobic Exercise in Coronary Heart Disease Patients." Circulation. 2012 Sep 18;126(12):1436-40.

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