Transcranial magnetic stimulation (TMS) can be used to probe the excitability of central nervous system pathways before, during, and after a movement. In addition, it can be used to interfere with movement and give information about the role of different cortical areas in different aspects of a task. This article reviews the work that has been carried out using TMS measures to probe the excitability of central circuits before and after different types of contraction in healthy subjects. In some cases the results confirm previous work on animals, which means that the same measures can be used to investigate the pathophysiology in human neurological disease. However, many results reveal new information that had not previously been described in experiments on animals. Therefore, it is not wrong to say that TMS measures can be used to describe in humans what has already been described in animals; they can become drivers of new concepts as well.