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Objective The biomechanical models of three common daily physical activities were built to calculate mechanical work and power.The energy efficiencies of the activities were examined by using a room-size human indirect calorimeter.Methods The angles of legs,feet and trunk relative to the ground were measured by a multiple sensor system,and Langrangian equations were developed for the activities:sit to stand movement,trunk moving back and forth while sitting,and leg crossing while sitting.The energy expenditures of the activities were measured by human room-size indirect calorimeters.Results The mechanical powers calculated by the multiple sensor system were highly correlated with the metabolic rates measured by the calorimeter(correlation index:0.975).The average mechanical efficiencies were 27.1% for sit to stand movement,21% for trunk moving back and forth,and 18% for leg crossing by using the joint power model.Conclusions Biomechanical model and joint power method are promising methods for the analysis of daily physical activity.By calculating the mechanical work and power,the intensity of physical activity can be quantified,which provides a method for the study of the relationship between energy expenditure of activities and health.