Apparatus and methods are described for use with a seat (82) of a vehicle, including a sensor unit (80) configured to be placed underneath the seat and configured to detect motion of a subject who is sitting on the seat during motion of the vehicle. The sensor unit includes a housing (90, 100), a first motion sensor (96, 106) disposed within the housing such as to generate a first sensor signal that is indicative of the motion of the vehicle, and a second motion sensor (98, 108) disposed within the housing, such as to generate a second sensor signal that is indicative of the motion of the subject and the motion of the vehicle. A computer processor at least partially distinguishes between motion of the subject and motion of the vehicle by analyzing the first and second sensor signals. Other applications are also described.
