A major limitation in understanding human energy balance is the ability to accurately quantify the amount of physical activity or work performed by human subjects. We describe the development and validation of a force platform (2.5 × 2.5 m) system for measuring external mechanical work performed by human subjects. The force platform system was constructed inside a whole-room indirect calorimeter, thus, allowing simultaneous measurement of the energy expenditure associated with physical activity. We demonstrated the accuracy of the system for measuring work performed using a solenoid (32 kg) to deliver controlled amounts of energy to the platform. In tests with human subjects we demonstrated that the system can accurately and reproducibly measure work performed, energy expenditure associated with work performed, and work efficiency (work performed divided by energy expenditure). Overall, we obtained high correlations (an average of 0.932 for 33 subjects), between mechanical work performed and energy expenditure. We believe this mechanical work-energy expenditure system will be useful in determining the importance of individual differences in amount and cost of physical activity in the regulation of body weight and in development of obesity. © 1993.