Fig. 1. Human gait and accelerometer data collected using body-worn devices.

a. Vector magnitude of raw accelerometer time series of walking strides measured at different body locations. Strides were extracted for five randomly selected subjects in each study and at each location available in that study. Vertical grid lines separate strides of different subjects, and horizontal grid lines mark stride acceleration equal to +1 g and −1 g above and below, respectively, of the acronym of the corresponding study. Colors indicate approximate locations of sensing devices. b. Walking activity is typically understood as a cyclic series of movements initiated the moment the foot contacts the ground, followed by the stance phase (i.e., when the foot is on the ground) and the swing phase (i.e., when the foot is in the air); the cycle is completed when the same foot makes contact with the ground again. c. Several examples of resting and walking acceleration signals collected simultaneously using smartphones at different body locations (thigh, waist, chest, arm) and a smartwatch worn on the wrist by two subjects. Corresponding time-frequency representation were computed with CWT.