1. Method for providing an index (BFI) depicting accumulated body fatigue (BFI) wherein one or more parameters from the measurement of one or more signals are obtained sequentially as input and these parameters being information on the intensity of physical activity, characterized in that the index (BFI) has a predetermined initial value, and next value of the index (BFI) value is always a sum of the value of the index (BFI) and a difference, and the difference is combination of upslope and optional downslope components of the index (BFI) determined with the said parameters, the upslope component and the optional downslope component are each determined with a function, which is scaled by a preset physiological character.