Sedentary Physiology Part 1 – Not Just The Lack of Physical Activity

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Welcome to our 5-part series delving into the fascinating research being performed in the emerging field of sedentary physiology.  Today, we’ll start with an introduction.  For Part 2 in our series, click here.

As regular readers will know, sedentary physiology is one of our favourite topics here at Obesity Panacea.  Later this week we will be examining the relationship between sedentary behaviour (aka behavior) and health, as well as the mechanisms that are thought to mediate this association.  But before we delve into those deeper topics, I’d like to give a bit of background.

What is sedentary behaviour?

Sedentary behaviours are those characterized by very low energy expenditure – typically those requiring 1.5 METs or less.  MET stands for metabolic equivalent, and is equal to 3.5 ml/kg/minute of oxygen consumption.  All you really need to know is that by convention, 1 MET is considered to be the resting metabolic rate while sitting quietly.   In contrast, moderate physical activity is defined as an intensity between 3-6 METs, while anything above 6 METs is considered vigorous exercise.  Given that sedentary behaviours are those with an energy expenditure of 1.5 METs or less, these are activities that are burning roughly the same number of calories as sitting quietly – e.g. not much at all.  With few exceptions, this means that any time you are sitting or lying down (watching TV, playing traditional videogames, using the computer, reading, driving in a car, etc), you are likely engaging in sedentary behaviour.  This does not mean that riding an exercise bike (which involves sitting) is sedentary, since it also involves an energy expenditure above 1.5 METs.  Ditto for certain energy-intensive activities in the prone position.  The figure below is from a recent paper published by several colleagues and myself, which illustrates the entire “activity continuum”, ranging from completely sedentary to highly active.  The figure also differentiates the focus of traditional exercise physiology research (e.g. moderate and vigorous physical activity), from the focus of the emerging field of sedentary physiology.

Being “sedentary” is NOT the same thing as being insufficiently physically active.

Up until very recently, referring to someone as sedentary meant simply that they were not meeting current guidelines for physical activity.  In simple terms, if you were exercising for 60+ minutes/day, you were considered physically active.  If you were exercising 10 minutes/day, you were sedentary.  Case closed.  But as we will discuss later this week, accumulating evidence suggests that sedentary time is closely associated with health risk regardless of how much physical activity you perform on a daily basis. Further, it is entirely possible to meet current physical activity guidelines while still being incredibly sedentary.  Consider the following figure from the same paper as above:

As you can see, one individual (the white circles) performs a single bout of structured physical activity, but then remains completely sedentary throughout the rest of their waking hours.  In contrast, the individual represented by the black boxes accumulates a similar volume of structured physical activity, but dramatically less sedentary time.  And the evidence that I will present later this week suggests that the individual represented by the black boxes is likely to have significantly lower metabolic risk than the one represented by the white circles, even though they are both meeting physical activity guidelines.

All this to say that it is important that we recognize that sitting too much is not the same as exercising too little (a line which I am happy to borrow from Marc Hamilton, one of the leading researchers in this area).  As the above figure illustrates, an individual can be both highly active and highly sedentary at the same time, and it is thus important that we distinguish between the two whenever possible.

How much sedentary time do we accumulate on a daily basis?

The short answer?  A lot!

Several studies have aimed to quantify the amount of time that both adults and children spend sitting on a regular basis, and all have found it to be an incredibly large portion of the day.  For example, a recent nationally representative survey of more than 5700 Americans found that the average sedentary time was just over 8 hours per day.  That equates to roughly half of our waking hours.

The situation in children is, unfortunately, no different. There is evidence that children in both Canada and the USA accumulate more than 6 hours of screen-time (time spent in front of the TV, computer, or other screen-based device) on a daily basis.  Keep in mind that screen-time is almost exclusively sedentary (active video games excluded), and that all these hours of sedentary behaviour are in addition to the hours and hours (and hours) that kids spend sitting at school.  In fact, a recent study by our colleagues Lindsay Nettlefold and Ashlee McGuire reports that roughly 70% of class time, including physical education class, is completely sedentary (while slightly better than class time, children were also sedentary for the majority of lunch and recess).

Ok, so hopefully we are now on the same page with respect to what sedentary behaviour is, what it is not, and how it dominates most of our waking hours.  What is the health impact of this ubiquitous sedentary behaviour?  The short answer is that it’s certainly not good.  For the details, be sure to check back tomorrow for Part 2 (now online) in our series on sedentary physiology.

Travis

ResearchBlogging.orgTremblay, MS, Colley, RC, Saunders, TJ, Healy, G, & Owen, N (2010). Physiological and health implications of a sedentary lifestyle Applied Physiology, Nutrition and Metabolism

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32 Responses to Sedentary Physiology Part 1 – Not Just The Lack of Physical Activity

  1. nobody says:

    “Ok, so hopefully we are now on the same page with respect to what sedentary behaviour is, what it is not, and how it dominates most of our waking hours. ”

    No. If an exercise bike is non-sedentary and sitting perfectly still is sedentary,
    where’s the boundary between sedentary and non-sedentary sitting? Just how much fidgeting and moving is necessary to make sitting non-sedentary? Is sewing machine leg enough? or sitting on an exercise ball? Or idly lifting weights with one hand while computing with the other?

    The amount of sitting required in contemporary life isn’t going to go away. And maybe the research just isn’t there. It’s intensely frustrating as a layperson to be teased with bits of information without getting the full picture of what it would take to make our sitting active enough not to be dangerously sedentary.

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    • Travis says:

      Unfortunately no one has any idea at this point. There are studies going on in the USA and Australia that are hoping to clarify this (my PhD research is focused on this as well), but this is still a very new area of study with very few clear answers. All we really know is that doing a lot of sitting is bad, and so doing less should theoretically be good. What qualifies as “a lot” is really anyone’s guess at this point.

      As I will discuss later this week, simply introducing short breaks where you stand or walk at a casual pace may be enough to reduce the risk associated with prolonged sitting, but that research is cross-sectional, so it’s hard to know how much we can read into it. There are some options – have walking meetings whenever possible, sit rather than standing on the bus or during presentations, convert your desk to a standing workstation (admittedly this is a tough one). Personally, I am in the process of purchasing this small pedaling machine that can fit under my desk. You can also make changes to your personal life like minimizing TV and computer time outside of work, and simply removing some of the chairs in your house/apartment (again, this one is more difficult).

      It really comes down to engaging as many large postural muscles as possible – moving your legs is good, but standing (which engages lots of postural muscles in the back as well as muscles in the legs) is probably better. But again, whether or not this makes a difference, or how much sitting is too much, are questions that people are actively working on as we speak.

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      • Snarkyxanf says:

        Is standing sufficient to cross the boundary between sedentary and non sedentary behavior?

        I ask because there are plausible interventions that increase standing (standing desks at work/school, standing at the bar in the pub, etc). Does it matter if you are free standing or leaning (like you might against a standing desk)?

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        • Travis says:

          No one really knows. It is plausible, since it seems that when muscle is activated (whether by using the muscle to stand, or by stimulating it electronically) it prevents many of the negative changes that occur during prolonged sitting. By that logic it would also make sense that free standing would be better than leaning against a wall. But while plausible, this is all speculation as this point.

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  2. Larry Parnell says:

    Depending on where you want to take this series, which is examining a most pertinent topic, I’d encourage you to consider the genetic aspect to the equation of sedentary and obese. In this regard, you could have a look at the article from our group on HDL and physical inactivity or “screen time.” The reference is

    Smith CE, Arnett DK, Tsai MY, Lai CQ, Parnell LD, Shen J, Laclaustra M, Junyent M, Ordovás JM. (2009) Physical inactivity interacts with an endothelial lipase polymorphism to modulate high density lipoprotein cholesterol in the GOLDN study. Atherosclerosis. 206:500-4.

    Admittedly, HDL-cholesterol is more of a risk factor for cardiovascular disease (CVD) than obesity, but as obesity often leads to CVD just as obesity does; and because CVD is the USA’s number 1 killer (~25% of deaths) – this may be something to expand upon.

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  3. Travis says:

    Thanks for pointing out the paper, Larry. I’ve sent you an email, hopefully we can discuss it a bit more later this week or in the near future!

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  4. Jason Robertson says:

    Thanks for posting this summary; unfortunately, this was the part of the sedentary-guidelines talk I missed when I was at CSEP last month (it was the first day and I got my times crossed up), and it was the part I was most interested in. Looking forward to the rest of the series.

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    • Travis says:

      Glad to hear that! I couldn’t make it to CSEP this year as I was writing my comps, but I’ve heard the speakers on a number of occasions, and will be citing a lot of their work throughout this week. Did you catch any of the session?

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  5. Aggie says:

    And then there is this study to add to the confusion.

    http://www.prnewswire.com/news-releases/study-sheds-new-light-on-childhood-obesity-epidemic-111379384.html

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  7. Mary Saunders says:

    I can say from palpating while singing back-up in a band, that I can flex quads and hams and feel them quite engaged on the off-bits, so I feel that standing can be quite a lot better on a sedentary index. If you lean a bit this way, then that, you have to engage large leg muscles, core muscles, and proprioceptors, horizon with eyes open and inner ear with eyes closed. All this is going to be forcing brain to use glucose and triggering a higher use of O2.

    Yoga and some forms of tai chi have leaning-back moves, which trigger eccentric work by the front abs, with a possible trigger of side abs by taking head and torso diagonally.

    It is possible to do this sitting to some degree as well.

    I teach water exercise, where you can’t fall down fast, I like to say. I love hearing it back from my participants. I am always asking them how things work for them. They seem to be ok with that.

    Thanks for the opportunity to weigh in. I’m glad I found you.

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  14. Alan says:

    The fact that this line was in boldface, in a large font, made it almost physically PAINFUL to read:

    “Being “sedentary” is NOT the same thing as being insufficiently physically activity.”

    Please proofread. Thanks.

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