DIABETES TECHNOLOGY & THERAPEUTICS Volume 5, Number 6, 2003 © Mary Ann Liebert, Inc. Actigraphy as Metabolic Ethography: Measuring Patterns of Physical Activity and Energy Expenditure COL. KARL E. FRIEDL, Ph.D. 1035 T O BETTER UNDERSTAND the relationship be- tween physical activity and metabolic reg- ulation, we need new measurement tools that are accurate, inexpensive, available, and un- tethered to a laboratory setting. Actigraphy, the continuous measurements of motion ampli- tude and frequency from body-worn ac- celerometers, has been actively investigated for applications in the measurement of patterns of voluntary activity. In this issue of Diabetes Tech- nology & Therapeutics, Chen et al. 1 have de- scribed the combined use of two accelerome- ters, mounted on the hip and the wrist, to accurately predict energy expenditure of phys- ical activity across a range of activity levels in a sample of overweight women. Not only does this approach provide the energy expenditure associated with activity (EE act ), it can also pro- vide EE act patterns throughout the day. David Marlowe, a former military social psychologist, used to say that life involves motion, and a technology such as actigraphy that quantifies human motion can be used to describe behav- ior, providing an important tool for ethogra- phers. We may finally have an accurate and practical tool for describing EE act patterns for individuals with respect to meal timing and medication, and for a variety of physical and emotional states, providing a tool for medical ethnographers interested in their patients’ be- haviors. Such a method for gathering accurate data could revolutionize our understanding of the role of physical activity and energy balance in health and disease, especially for weight management. The gold standard for total energy expendi- ture is still a calorimeter. This group at Van- derbilt is well known for the refined precision of their special metabolic chamber that has a force transducer instrumented in the floor. 2 This chamber was used as the reference method in this study. An alternative method that allows us to step outside of the laboratory measures the differential elimination of stable isotopes using deuterated 18 O water. This dou- bly labeled water method provides precise measurements of energy expenditure in free- ranging subjects, but requires careful attention to natural variations in isotope enrichment in the ambient environments, careful handling of specimens, and costly and time-consuming analyses. Nevertheless, this has been a break- through technology in military field studies, al- lowing us to understand energy balance for healthy individuals operating in extreme environments. 3 Unfortunately, the greatest problem in metabolic studies has been the worldwide shortage of 18 O enriched to the con- centrations needed for this research, and this method only provides energy expenditures in 24–48–h periods. A third method to obtaining estimates of EE act involves more pedestrian and practical methodology developed before doubly labeled water techniques were avail- able, such as continuous heart rate monitoring. Despite the large sacrifices in accuracy, esti- U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts. Analysis