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tetanic contraction
A tetanic contraction (also called tetanized state, tetanus, or physiologic tetanus, the latter to differentiate from the disease called tetanus) is a normal type of muscle contraction in which a skeletal muscle stays full contracted, under voluntary control, until the mind decides to relax it. During this state, a motor unit has been maximally stimulated by its motor neuron and remains that way for some time. 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, the twitches will overlap, resulting in tetanic contraction. When tetanized, the contracting tension in the muscle remains constant in a steady state. This is the maximal possible contraction. It can exist in a variety of states, including isotonic and isometric forms—for example, lifting a heavy box off the floor is isotonic, but holding it at the elevated position is isometric.
Voluntary sustained contraction is a normal (physiologic) process, but involuntary sustained contraction exists on a spectrum from physiologic to disordered (pathologic). Muscle tone is a healthy form of involuntary sustained ''partial'' contraction. In comparison with tetanic contraction in an isometric state (such as holding up a heavy box for several minutes), it differs only in the percentage of motor units participating at any moment and the frequency of neural signals; but the low percentage and low frequency in healthy tone are the key factors defining it as healthy (and not tetanic). Involuntary sustained contraction of a hypertonic type, however, is a pathologic process. On the mild part of the spectrum, cramps, spasms, and even tetany are often temporary and nonsevere. On the moderate to severe parts of the spectrum are dystonia, trismus, pathologic tetanus, and other movement disorders featuring involuntary sustained strong contractions of skeletal muscle.
==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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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