An apparatus for producing information indicative of metabolic state of a metabolic energy system comprises a processing device for receiving a signal that is indicative of a heat-flux generated by the metabolic energy system. The processing device is configured to maintain model data expressing relative contributions of the phosphagen system, the glycolytic system, and the aerobic system to muscular energy production as functions of time during physical loading of the metabolic energy system. The processing device is configured to form estimates for the energy production of the phosphagen system, the energy production of the glycolytic system, and the energy production of the aerobic system as functions of time and based on the model data and the signal indicative of the heat-flux. The estimates can be indicative of the instant metabolic state, and they can be utilized in physical training, weight control, and detection of metabolism-related health issues.
