The control of human hand movements is investigated in a simple synchronization task. We propose and analyze a stochastic model based on nonlinear error corrections a mechanism which implies the existence of unstable Periodic orbits. This prediction is tested in an experiment with human subjects. We find that our experimental data are in good agreement with numerical simulations of our theoretical model. These results suggest that feedback control of the human motor systems shows nonlinear behavior. (C) 2001 Elsevier Science.