How Long Does It Take To Get To High Intensity? New Paper

General Discussion
1

Just published in the ACSM’s journal, Medicine & Science in Sports & Exercise, this paper looked at how much time it took to generate the physiological responses that are part and parcel with high or vigorous intensity.

For some background, the current physical activity guidelines use metabolic equivalents (METs) to categorize the intensity of exercise, with activities > 6 METs being deemed vigorous. Outside of this, there’s no consensus definition of “high-intensity" conditioning or HIIT despite its widespread use. A number of models for exercise intensity have been introduced over the years, e.g. the 3- and 5-zone models, where heart rate, blood lactate levels, breathing rate, and/or other physiological metrics are used to classify exercise intensity. Subjective metrics of exertion (RPE) and breathlessness have also been used.

Ratings of exercise intensity aside, one question that has been raised over the years is how long does a bout of exercise need to last for it to be effective? The original 2008 guidelines suggested a conditioning bout needed to last at least ~ 10-minutes to "count’, whereas the 2018 guidelines reduced that to 5-minutes. The latest guidelines also note that even shorter episodes of exercise may be effective. These so-called exercise snacks, eg. isolated 1 min bouts of vigorous activity exercise performed periodically throughout the day, can work to increase fitness and health in previously untrained individuals, though this is mostly studied in untrained, older individuals.

In any case, this new study found that it took an average of 65- to 95-seconds to reach the vigorous intensity exercise threshold using common activities like brisk walking, climbing stairs, etc.

Couple potentially interesting notes here:

  1. When using RPE, folks reached the vigorous intensity mark sooner than their HR would suggest.
  2. Some previous data showed that even very-short bouts of exercise lasting ~ 15-seconds can be beneficial. I wonder what sort of physiological response could be measured during those efforts.
  3. The idea of exercise snacks during activities of daily life seems like a reasonable suggestion to get folks more active, though I’d favor durations of ~ 2 minutes or longer given this data AND corroborating evidence from HIIT studies showing 2 min long intervals work better than shorter ones on average.

Just some morning musings while I have my coffee. Let me know what you think!

2

This is super interesting. My personal experience echo’s point number one. For me it seems like HR lags feelings of exertion.

I think this could be useful to get people more active, particularly among people that think a “workout” has to be a 30 min+ thing, all the time. You could stack 3 - 10 minute sessions instead. That might be more palatable for some

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I second this - My theoretical max HR is 171 BPM. When I track my HR during the prescribed HIIT sessions (20s hard, 100s easy, repeat 8x), my first couple of rounds don’t peak over 155 BPM, even though it feels much more intense.

Later rounds push past 165 BPM, yet my RPE feels roughly the same as the first couple of rounds. During the last rounds, I tend to back off a little due to fatigue, but the heart rate stays elevated.

Seems like there’s a bit of a flywheel effect there.

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To the palatability bit, I’m inclined to think you’re right! However, data on the uptake of exercise snacks or “minimum effective dose” is not great. I wonder if this type of exercise is most useful to counter the perceived barrier of not having enough time to exercise, and doesn’t address what really limits people. It could be that there’s just not enough study here and it would otherwise be effective in those interested in exercise. Hope we find out soon!

Either way, I do find that these findings corroborate my own experience. Nice to have a bit of explanation there.

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I suspect you are right. There are perceived barriers and more concrete barriers. People with families, careers, etc truly have a small amount of time to exercise. Those are probably more concrete.

However, watching how much time people spend on their phones seems to indicate there is time available, but its allocation leaves something to be desired :slight_smile:

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A recent paper I read suggested the average American adult has ~ 5 hours of free time per day, stating:

Lack of free time is not responsible for low levels of leisure time physical activity at the population level.

Big, if true!

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