the present invention relates to a light - weight , small and portable ambulatory sensor for measuring and monitoring a person &# 39 ; s physical activity . based on these measurements and computations , the invented system quantifies the subject &# 39 ; s physical activity , quantifies the subject &# 39 ; s gait , determines his or her risk of falling , and automatically detects falls . the invention combines the features of portability , high autonomy , and real - time computational capacity . high autonomy is achieved by using only accelerometers , which have low power consumption rates as compared with gyroscope - based systems . accelerometer measurements , however , contain significant amounts of noise , which must be removed before further analysis . the invention therefore uses novel time - frequency filters to denoise the measurements , and in conjunction with biomechanical models of human movement , perform the requisite computations , which may also be done in real time .