The purpose of our study was to elucidate the dynamics of muscle activation during generalised tonic-clonic seizures (GTCS). We recorded surface electromyography (EMG) from the deltoid muscle during 26 GTCS from 13 patients and compared it with GTCS-like events acted by 10 control subjects. GTCS consisted of a sequence of phases best described quantitatively by dynamics of the low frequency (LF) wavelet component (2-8Hz). Contrary to the traditional view, the tonic phase started with a gradual increase in muscle activity. A longer clonic phase was associated with a shorter onset of the tonic phase and a higher seizure occurrence. Increase in LF occurred during the onset phase and during the transition from the tonic to the clonic phase, corresponding to the vibratory movements. The clonic phase consisted of EMG discharges of remarkably constant duration (0.2s) separated by silent periods (SP) of exponentially increasing duration - features that could not be reproduced voluntarily. The last SP was longer in seizures with higher EMG peak frequency whereas the energy of the last clonus was higher in seizures with a short clonic phase. We found specific features of muscle activation dynamics during GTCS. Our findings suggest that the same inhibitory mechanisms that contribute to GTCS termination counteract seizure initiation, accounting for the gradual onset. Both active inhibition and mechanisms related to metabolic depletion act synergistically to stop the seizure. Analysis of the ictal EMG dynamics is a valuable tool for monitoring the balance between pro-convulsive and anti-convulsive factors.