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Should you ditch the desk chair? A study on the evolutionary biology of human inactivity

Background

Most scientists agree that humans are not well adapted to sedentary lifestyles, and a number of studies have linked increased sedentary time to increased risk of cardiovascular disease and mortality. During human evolutionary history, hunter-gatherers frequently needed to walk and/or run at moderate-to-high intensity levels, which could explain the present-day positive associations between moderate-to-vigorous physical activity levels and a reduced incidence of chronic diseases.

Human physiology seems to be adapted to higher activity levels. The health risks from more sitting (or being less active in general) seem to correlate with reduced levels of muscle contractions, which affect both the muscles themselves and whole-body energy metabolism. This is a bit of a paradox, however, because from an evolutionary standpoint, the increased activity levels demanded by a hunter-gatherer lifestyle likely coexisted with selective pressures to conserve and minimize energy expenditure.

The study

To test the hypothesis that inactivity in hunter-gatherers is associated with increased muscle activity — in contrast to the chair-sitting that is common in industrialized populations — the researchers conducted an 8-day study that examined inactivity in 28 people of the Hadza of Tanzania, a population of hunter-gatherers.

Activity levels were assessed using accelerometers, observational data, and measurement of muscle activity via electromyography.

The results

Hunter-gatherers have high levels of inactive time — around 10 hours per day, which is similar to the inactive time in industrialized populations. However, in Hadza adults, inactive time often occurred in a squatting position, which increased levels of muscle recruitment and could be considered “active rest”, in contrast to chair sitting. Based on these results, the authors proposed the “inactivity mismatch hypothesis”, which proposed that human physiology is better adapted to more consistently active muscles from a combination of physical activity and active rest.

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