The neuromuscular junction connects the motor nervous system with the muscles. Thus, measuring the electric signals in the muscles provides information about the function of the motor cortex. We used quantitative analysis of surface electromyography (EMG) signals to characterize the pathophysiology of convulsive seizures. We found specific EMG features for convulsive seizures, and quantitative characteristics distinguishing generalized tonic and generalized tonic-clonic seizures (GTCS). Based on these EMG features we developed a neurophysiological biomarker for GTCS. This proved to be accurate in distinguishing between GTCS and convulsive psychogenic non-epileptic seizures. In consequence, an algorithm was created which detected GTCS with a high sensitivity and specificity. In this chapter we give a synopsis of our findings on quantitative analysis of surface EMG during epileptic seizures, with focus on clinical and biomedical applications.