Role of Occupational Physical Activity in Chronic Disease Risk

Medical Q&A [Moderated]
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I want to run by you a dilemma regarding how to make sense of older data, and older stories we tell based on those data, when emerging data starting pointing to a different story.

My understanding is you have evolved your position on the role that PA plays in protecting against disease, specifically CVD, to largely exclude PA accumulated at work. The thing I struggle with over this newer perspective is the seminal work from the 50s that first identified the protective effect of PA came from observations on people who worked for the London busses. In short, those buses were staffed by a driver who sat and drove all day, and a conductor who walked the aisles, going up and down the stairs in the double decker busses, collecting the bus fares. The lower rate of CVD in the conductors pointed us as a field towards the role of PA, and from there an entire discipline emerged. This paper while difficult to track down the full text of these days is cited in pretty much every history of Ex Phys course/lecture as the paper that first proposed this relationship https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(53)90665-5/fulltext

So what is the dilemma? It is reasonable to accept that that lessons taken from an important paper from the 50s turn out to not be correct. But if that is the case with this paper, how serendipitous that it started us on the path of exploring the correct relationship between PA and CVD disease risk, but did so based on identifying a relationship that “isn’t real.” That does not seem believable to me and so I really struggle with this more modern interpretation [insert old guy refusing to change his opinion meme]

What it seems to me is that the better story is that there is a covariate of socioeconomic status. When we compare against Gen Pop, people with high levels of occupational physical activity tend to also exist on the lower end of the SES and so the protective effect, relative to Gen Pop, is masked. But in the case of the London bus study, where SES is held constant, you can still see the beneficial effect of even occupational PA.

Thoughts on that?

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Yes, the ‘physical activity paradox’, which suggests that occupational physical activity is not as beneficial to health as leisure time exercise (and may even be detrimental in some cases), does include elements of social and environmental factors that contribute to the observed outcomes.

I don’t think the paper provided is very supportive of OPA being as effective as LTPA for health, as the paper shows that the conductors had more angina and tracking both groups’ total activity levels are missing. I think the evidence showing volume-matched OPA being inferior to LTPA for health and performance outcomes is pretty strong. It also shows that those with high levels of OPA benefit from additional LTPA.

We definitely can’t ignore SES, as it certainly contributes. I also think a lot has changed within the last 70-80 years that make this data set less compelling in contrast to the more recent data comparing OPA and LTPA outcomes.

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