BOUTEN, C. V., K. R. WESTERTERP, M. VERDUIN, and J. D. JANSSEN. Assessment of energy expenditure for physical activity using a triaxial accelerometer. Med. Sci. Sports Exerc., Vol. 26, No. 12, pp. 1516–1523, 1994. A triaxial accelerometer was used to evaluate the relationship between energy expenditure due to physical activity (EEact) and body acceleration during different types of activity. In a laboratory experiment, 11 male subjects performed sedentary activities and walked on a motor driven treadmill (3–7 km·h-1). EEact was calculated from total energy expenditure (EEtot), as measured by indirect calorimetry, and sleeping metabolic rate (SMR): EEact = EEtot-SMR. Body accelerations were measured with a triaxial accelerometer at the low back. Special attention was paid to the analysis of unidirectional and three-directional accelerometer output. During sedentary activities a linear relationship between EEact and the sum of the integrals of the absolute value of accelerometer output from all three measurement directions (IAAtot) was found (r = 0.82, P < 0.001, Sy,x = 0.22 W·kg-1). During walking EEact was highly correlated with the integral of absolute accelerometer output in antero-posterior direction (IAAx; r = 0.96, P < 0.001, Sy,x = 0.53 W·kg-1). When all examined activities were included in a regression analysis, a strong linear relationship between EEact and IAAlot was found (r = 0.95, P < 0.001, Syx = 0.70 W·kg-1). Using this relationship, EEact during sedentary activities as well as EEact during walking could be estimated with an accuracy of about 15%. Although sedentary activities and walking represent a large part of normal daily physical activity, the validity and usefulness of the triaxial accelerometer—measuring IAAtot—to predict EEact in daily life must be studied under free-living conditions.