The processes and neural bases used for motor imagery are also used for the actual execution of correspondent movements. Humans, however, can imagine movements they cannot perform. Here we explored whether plausibility of movements is mapped on the corticospinal motor system and whether the process is influenced by visuomotor vs. kinesthetic-motor first person imagery strategy. Healthy subjects imagined performing possible or biomechanically impossible right index finger movements during single pulse TMS of the left motor cortex. We found an increase of corticospinal excitability during motor imagery which was higher for impossible than possible movements and specific for the muscle involved in the actual execution of the imagined movement. We expand our previous action observation studies, suggesting that the plausibility of a movement is computed in regions upstream the primary motor cortex, and that motor imagery is a higher-order process not fully constrained by the rules that govern motor execution.