System for effective training heart rate zone determination, the system comprising:—a training module adapted to determine target training heart rate zone based on at least one stored parameter;—at least one sensor for measuring core temperature of a user's body during training;—at least one sensor for measuring a user's heart rate during training;—a compensation module, which is arranged for compensating the target training heart rate zone based on said core temperature;—an indicating module adapted to compare the users heart rate to an effective training heart rate zone based on information from the compensation module and to inform the user when the heart rate exceeds or drops under the effective training heart rate zone. The invention further relates to use of such a system.