Dear Bassam, the problem that you raise is very tricky and is potyentially answered in many different ways according to the particular experimental paradigm. In this respect some information is missing. Pleas edescribe more in detail what you want to do, especially: what movement is to be imagined? Do you have an event-related paradigm?

Thank you so much for the reply. The movement to be imagined is pinching, so that FDI MEPs parameter will be studied and we need to know what is the effect of motor imagery on the afferent inhibitory pathways like SAI and LAI

In this case the imagined movement is isometric and can be prolonged in time. In my opinion you do not need to be extra-precise in timing. If it was my experiment I would ask participants to imagine pinching on a GO signal, and stop at a STOP signal some 3-4 seconds later. I would deliver TMS 2 seconds after the GO-signal. A "validation condition" in which you give TMS with imagining a different movement could help in adding specificity to your data. I hope this is of any help. L.

Yes that is so helpful, Thank you so much again. But which other movement do you suggest to validate pinching?

I mean I would test motor imagery of a movement involving a different (possibly antagonistic) set of muscles (finger opening?) to show thaty indeed your subjects have corticospinal facilitation that is specific to the imagined movement, thus implicitly validating the fact that they are actually fulfilling the task of imagining rather than thinking about their holidays. An easier but slightly less powerful way to do this is to record more muscles, some of which are not involved in the task.