1. A system for monitoring a cumulative physiological state and for predicting a target state under physical exercise, wherein the user's physiological state is defined by a EPOC (excess post-exercise oxygen consumption) value as a cumulative physiological variable indicating achieved training effect and the target state defines a respective value of the cumulative physiological variable after an exercise having a total duration, said system including: means for measuring time, means for measuring intensity of the exercise at each moment to obtain a measured intensity and for registering the measured intensity relative to time, and a control unit connected to said means for measuring time and said means for measuring and registering intensity, the control unit further comprising: a user-interface device, including an input device for selecting an output variable and for entering the values of initial variables before the exercise, and an output device for displaying the output variable, the output variable being one of the following three variables: said target state, intensity of future exercise, and duration of remaining exercise; the two other variables being entered by the user as said initial variables, first software means for calculating the current EPOC value during the exercise, with the aid of the measured intensity of exercise and the preceding current EPOC value, and second software means for calculating the output variable during the exercise using a mutual relationship between the current EPOC value and two variables of said three variables, wherein the system is arranged to calculate and display the value of the output variable using the output device, in real time and continuously.