The invention determines an efficiency value (E) denoting preferably the relative period of muscle fibre activity during a recorded period of exercise, and a strength value (S) representing the number of muscle fibres recruited during a movement as part of the exercise or of a muscle contraction, and a temporal value (T) representing the frequency with which muscle fibres are activated repeatedly during exercise, and finally combines the efficiency value (E), the strength value (S) and the temporal value (T) by a linear combination to obtain an index value (ESTi) indicative of the fitness level of the individual. The obtained
