Myoclonus and other jerky movements form a large heterogeneous group of disorders. Clinical neurophysiology studies can have an important contribution to support diagnosis but also to gain insight in the pathophysiology of different kind of jerks. This review focuses on myoclonus, tics, startle disorders, restless legs syndrome, and periodic leg movements during sleep. Myoclonus is defined as brief, shock-like movements, and subtypes can be classified based the anatomical origin. Both the clinical phenotype and the neurophysiological tests support this classification: cortical, cortical-subcortical, subcortical/non-segmental, segmental, peripheral, and functional jerks. The most important techniques used are polymyography and the combination of electromyography-electroencephalography focused on jerk-locked back-averaging, cortico-muscular coherence, and the Bereitschaftspotential. Clinically, the differential diagnosis of myoclonus includes tics, and this diagnosis is mainly based on the history with premonitory urges and the ability to suppress the tic. Electrophysiological tests are mainly applied in a research setting and include the Bereitschaftspotential, local field potentials, transcranial magnetic stimulation, and pre-pulse inhibition. Jerks due to a startling stimulus form the group of startle syndromes. This group includes disorders with an exaggerated startle reflex, such as hyperekplexia and stiff person syndrome, but also neuropsychiatric and stimulus-induced disorders. For these disorders polymyography combined with a startling stimulus can be useful to determine the pattern of muscle activation and thus the diagnosis. Assessment of symptoms in restless legs syndrome and periodic leg movements during sleep can be performed with different validated scoring criteria with the help of electromyography. Both the clinical phenotype and the neurophysiological tests support this classification: cortical, cortical-subcortical, subcortical/non-segmental, segmental, peripheral, and functional jerks. The most important techniques used are polymyography and the combination of electromyography-electroencephalography focused on jerk-locked back-averaging, cortico-muscular coherence, and the Bereitschaftspotential. Clinically, the differential diagnosis of myoclonus includes tics, and this diagnosis is mainly based on the history with premonitory urges and the ability to suppress the tic. A time-locked relationship between myoclonus electromyographic discharge and the sudden, brief movement, through signal averaging is shown. Fig. 3 Multichannel surface electromyographic (EMG) recording in upper extremities during postural activation from a patient with cortical myoclonus. There are myoclonus EMG discharges that occur with almost synchronous timing bilaterally. If the discharge trains are small and almost continuous, the term At times, the pre-myoclonus EEG transient can be observed grossly on the EEG recording. However, EEG back-averaging is a more sensitive and definitive assessment for cortical myoclonus for two reasons: a) the EEG transient is usually lost in the background of the gross EEG. EEG back-averaging increases the signal to noise ratio dramatically, thereby enables the detection of a direct cortical EEG transient that otherwise would not be detected, b) a back-averaged EEG transient also documents a consistent time-locked relationship between cortical discharge and the myoclonus, thereby providing strong evidence for the cortical genesis of the myoclonus ( Fig. 4 A and B). Jerk-lock method shows visual correlation with the myoclonus movement itself. The EEG-EMG back-averaging method ensures a specific correlation with the onset of the myoclonus if each EMG discharge trigger produces a myoclonus jerk movement. It is recommended that at least 100 artifact-free epochs are used in the averaging of the myoclonus triggers in order to ensure maximal sensitivity. If the purpose is to demonstrate the absence of a back-averaged transient, then 200 artifact-free triggers are recommended. Fig. 4 Cortical myoclonus. The presence of cortical myoclonus electrophysiology findings should, of course, be taken as evidence against those jerks being functional. High amplitude EMG discharges < 50 ms duration that correlate with a moderate amplitude jerk strongly favors involuntary myoclonus for that particular type of movement. Enlarged cortical SEP, C-reflexes, back-averaging of a short duration (<100 ms) EEG wave preceding the jerk suggest non-functional myoclonus. The Bereitschaftspotential (BP) is a back-averaged negative “slow” EEG cortical potential that occurs before self-paced, voluntary phasic movements ( Fig. 11 ). 11 Representative recording of an individual case with jerky movements in the arm of functional origin. Four seconds of raw electroencephalographic (EEG) and electromyographic (EMG) data of the extensor carpi radialis (ECR). Note the long duration EMG bursts (+/- 500 ms). After back-averaging of 70 epochs of jerks, a Bereitschaftspotential can be seen be on C3, Cz and C4, which starts approximately 1 s before jerk onset. 1.2.8 Peripheral myoclonus It is believed that peripheral nerve abnormality may produce myoclonus ( Valls-Solé, 2007 ). The most common example is the hemifacial spasm ( Jankovic and Pardo, 1986 ). Dreissen and colleagues hypothesized that the patients with functional abdominal jerks are constantly subconsciously ‘preparing’ (demonstrated with the presence of a Bereitschaftspotential) of their upcoming jerk, which are released by a startling stimulus. 3.3.2.2 Various culture-specific syndromes There are various culture-specific syndromes which can be described as non-habituating hyper startling of the second – the behavioral – response following the normal habituating early motor startle response. Peculiar behaviors have been observed evoked by loud noises or being poked forcefully in the side in different locations around the world ( Simons, 1996 ).