A tetanic contraction (also called tetanized state or tetanus) occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency, then the twitches will run together, resulting in tetanic contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal possible contraction.
Toxins and drugs 
Tetanic contractions are characteristic of poisoning by strychnine. They may also occur as part of an extrapyramidal adverse drug reaction of some typical antipsychotic drugs, specifically the opisthotonos effect of acute dystonic reaction in which "tetanic" heightening of entire body, head and belly up occurs.
Electrical stimulation 
Tetanic contraction may be observed following the use of a device intended to apply transcutaneous electrical muscle stimulation (EMS) through skin contact electrodes for the purpose of improving abdominal muscle tone.
As well as being caused by tetanus (lockjaw), after which they are named, tetanic contractions are a symptom of hypoparathyroidism, due to an underdeveloped parathyroid, or injury to the parathyroid.
See also 
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