A vital signs monitoring system comprises a processing unit (300) for estimating an activity energy expenditure (AEE), a first activity energy expenditure determining unit (320) for determining a first activity energy expenditure (AEEHR) based on heart rate data (HR), a second activity energy expenditure determining unit (330) for determining a second activity energy expenditure (AEEAC) based on motion data (AC), and a weighting unit (340) for determining a first and second weighting factor (wHR, wAC) based on a first and second probability relating to a high exertion (hH) and relating to a low exertion (hL), and activity energy expenditure calculating unit (350) for computing an overall activity energy expenditure (AEEO) based on the first activity energy expenditure (AEEHR) weighted by the first weighting factor (wHR) and on the second activity energy expenditure (AEEAC) weighted by the second weighting factor (wAC).
