The invention relates to a method for determining accumulated body fatigue index (BFI), wherein one or more parameters from the measurement of one or more signals are obtained sequentially as input and these parameters are information on the intensity of physical activity. BFI has a predetermined initial value, and next BFI value is always a sum of BFI-value and a difference. The difference is combination of upslope and optional downslope components of BFI determined with the said parameters and the upslope component and the optional downslope component are each determined with a function, which is scaled by a preset physiological character.