Systems and methods are disclosed for identifying users of portable user devices according to one or more accelerometer profiles created for a respective user. During a media session, the portable computing device collects media exposure data, while at the same time, collects data from the accelerometer and compares it to the user profile. The comparison authenticates the user and determines the physical activity the user is engaged in. Additional data may be collected from the portable computing device to determine one or more operational conditions of the device itself, including the detection of multiple devices being physically carried by one user. Gross motion strings may also be generated by devices and compared to see if strings match, thus suggesting multiple devices are being carried by one user.