This invention relates to an activity monitoring system adapted to eliminate passive movement components caused by external forces from readout data produced by a first and a second motion sensor when attached to a subject during movement. The readout data include gravitational components, movement components caused by active movement of the subject or subject parts, and the passive movement components. A processor estimates first and second gravitational components produced by the at least first and a second motion sensors. It determines a rotation matrix based on the estimated gravitational components, the rotation matrix denoting rotation required for the first sensor to get aligned with the second sensor in orientation. It then multiplies the readout data produced by the first motion sensor with the rotation matrix when rotating the first sensor towards the second sensor. Finally, it subtracts the result of the multiplying from the readout data produced by the second motion sensor when rotating the first motion sensor towards the second motion sensor.