Ancient Greek scholars realised long ago that physical activity was a requirement for good health. Hippocrates proposed that “eating alone will not keep a man well – he must also take exercise”, while Galen later noted that "the body is in need of motion, exercise is healthy and rest morbid.”
Roughly 2,000 years on, the empirical evidence for physical activity as an essential part of a healthy lifestyle is overwhelming. Across many different types of studies and for various health-related outcomes the message is clear and consistent. Physical activity has a demonstrably important effect on the risk of disease and mortality outcomes.
So you might expect that by now we would also be able to give people clear advice on exactly how much physical exercise is “enough”. Sadly, it is not that simple. Scientists recently claimed that World Health Organisation recommendations on the amount of exercise we should do are too low to beat chronic diseases.
Most countries have attempted to develop public health guidelines advocating a minimum amount of physical activity, usually along the lines of doing at least 150 minutes of moderate-to-vigorous exercise per week. So surely it should be easy for people to self-assess the effectiveness of their activity levels? And surely that task is made easier by using the latest wearable monitors?
Numerous devices available from dozens of different manufacturers mean that more than 100m of these instruments are predicted to be sold in 2016 alone.
But a recent study showed that feedback from physical activity monitors is incompatible with current physical activity guidance. The study showed that most people will erroneously form the view that they are exceeding recommendations several times over.
In practice, it is difficult to combine official guidance with these kinds of technologies. For example, based on a sophisticated assessment of their physical activity against these guidelines 90% of men would receive the confusing message that they are both “active” and “insufficiently active”. By doing the same amount of exercise, a person might be considered active according to the guidance from the British Department of Health, yet inactive in the view of the American Centers for Disease Control and Prevention. So even with advanced measurement tools we lack an unambiguous understanding of whether a certain level of physical activity will deliver the health benefits required.
One reason for this discrepancy is that the 150-minute target was originally proposed to be on top of “baseline” physical activity, or “normal lifestyle activities”. It is a prescription over and above background activity, a factor usually omitted from national guidelines that use the 150-minute target.
A guideline that specifies an amount of activity above “normal lifestyle activities” is fine until people start using devices that capture all physical activity. Current standards were not formulated with these kinds of sophisticated measurement technologies in mind. A monitor will measure the effort involved in everything from running up the stairs to walking to the kitchen to switching the kettle on, and will not differentiate between the two, giving us the sense that we are doing more effective exercise than we are.