Methods and systems for relative inertial measurement may include a user device configured to couple with a user's body and measure motion of the user's body or a part of the user's body while the user rides in a vehicle. A second inertial measurement device may be configured to move with the vehicle but to not move with movements of the user's body within the vehicle. One or more processors may receive inertial measurements from the first and second inertial measurement devices and determine movement of the user's body or the part of the user's body relative to the vehicle by comparing the received inertial measurements.