Measuring human energy expenditure: what have we learned from the flex-heart rate method?

The measurement of daily energy expenditure is an important aspect of research on human health and nutrition. Over the last 30 years, G.B. Spurr has been a leader in developing and implementing methods for more effectively assessing energy expenditure and physical activity in traditional and modernizing populations. One of his most notable contributions has been the development of the "Flex Heart Rate" (flex-HR) method. Since its inception in the late 1980s, the flex-HR method has become a standard tool for measuring daily energy expenditure in free-living human populations. This article reviews the initial development and validation of the flex-HR technique, and examines recent refinements of the method and its application to research in biomedicine and human population biology. The review and analyses highlight how the flex-HR technique has improved on earlier methods of assessing energy expenditure and greatly advanced our understanding of variability in human energy requirements.