Neurophysiological and neuroimaging studies in the human and the monkey brain indicate that links between action observation and execution are much tighter than previously believed. Indeed, the mere observation of movements performed by other individuals brings about a clear increase in activity in specific fronto-parietal neural networks (mirror system). Here, we report a series of four single-pulse transcranial magnetic stimulation studies of the motor system, which show that observation of index and little finger movements brings about a facilitation of potentials recorded from muscles that would be involved in the actual execution of the observed action. Remarkably, however, a clear representational selectivity was found also during observation of bio-mechanically impossible index or little finger movements. Thus, in movement observation tasks, the human cortico-spinal system reacts similarly to the vision of bio-mechanically possible and impossible movements but it is able to detect which muscle would be involved in the actual execution of the observed movement. Importantly, this system may be more related to coding body part movements than precisely simulating their execution.