Portable uniaxial accelerometer units have been widely used to quantify non-exercise activity, but these instruments are not sufficiently sensitive to quantify the physical activity of a given free-living individual. Significant improvements can he made over current systems by utilizing cutting-edge technologies, such as low-power microcontrollers and MEMS accelerometers, to develop power-efficient motion analysis systems with small form factors. A motion tracking system with a very compact form factor would experience greatly expanded use and allow a wider range of activities to be studied. We have built several small devices incorporating either 2-axis or 3-axis MEMS accelerometer, low-power microcontroller, and mini secure digital memory card. Our device has proven to be comparable to the existing motion analysis system at distinguishing subject posture and motion.