Stroke patients may present motor impairments that in many cases require an intensive rehabilitation process with experts helping the patient to recover the functionality of the affected limb. A target during this rehabilitation process is to induce neural plasticity in brain regions associated with the motor control of the affected limb. Electrical stimulation tightly synchronized with the intention to perform a movement has proven to be an effective way of enhancing cortical excitability in healthy subjects. The electroencephalogram can help to detect voluntary movements online. We propose here an Electroencephalography based system aimed to detect the instants at which stroke patients attempt to start voluntary movements with the affected upper limb. To accomplish this, the analysis of the cortical rhythms and their variations are used. In the preliminary results obtained with 3 chronic stroke patients, 63±14% of the movements were detected with a temporal precision in the detections of the onsets of the movements of -126±313 ms.