A tetanic contraction (or tetanized state) occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated at a sufficiently high frequency of multiple impulses. Each stimulus causes a twitch. If stimuli are delivered slow enough, the tension in the muscle will relax between successive twitches. If stimuli is delivered at high frequency, then the twitches will add up, resulting in tetanic contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal contraction.
Tetanic contraction 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. 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.
Tetanic contractions are a symptom of hypoparathyroidism, due to an underdeveloped parathyroid, or injury to the parathyroid.[1]