Muscle hyperactivity can be a clinical feature on its own or, more commonly, an observation on electromyography (EMG) examinations. Whatever manifestation it takes, muscle hyperactivity always means enhanced excitability of muscle, axons or neurons. Clinical findings may be variable, ranging from fasciculations to muscle cramps. Even though clinical examination may lead in most instances to suggest the diagnosis of the underlying disease, EMG studies are necessary to identify the type of abnormal discharges and suggest the site of their suspected origin. Although in clinical studies, the action potential showing abnormal muscle hyperactivity is practically always recorded from muscle fibers, the site in which the impulse has arisen will determine its shape and firing patterns. In this review, we describe the EMG characteristics observed in syndromes featuring muscle hyperactivity and the pathophysiology underlying the abnormal firing of muscle fibers.