Coming to the Necessary Consideration of Complexity: Variability and Fractality in Rhythmic Motor behavior

 The variability of systems’ behavior has been for a long time considered as random fluctuation. A number of recent works have shown, however, that this variability had a characteristic temporal structure, and especially 1/f-type fractal correlations. This kind of fluctuation is considered the natural signature of complex systems, and questions the models classically adopted for taking into account the processes under study. We give some examples in the domain of rhythmical movement, and consider the theoretical consequences of the consideration of these fluctuations in performance.