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A prospective study in which patients with idiopathic scoliosis were examined longitudinally by radiographic and electromyographic measurements according to a protocol.To measure the growth velocity of the spine and the electromyographic ratio of the paraspinal muscles to determine their relation to progression of the scoliotic curve.Several factors have been reported to be involved in the progression of idiopathic scoliosis. Possible factors may be growth disturbances and muscular abnormality.Thirty patients with idiopathic scoliosis were examined over periods of 4 to 5 months. The periods were scored for progression, defined as an increase in Cobb angle of >10°. Spinal growth velocity was measured as the length difference of the scoliotic spine between two consecutive radiographs. The electromyographic activity on both sides of the spine expressed as an electromyographic ratio was measured during relaxed upright standing using bipolar surface electrodes. Predictability of progression was evaluated with regression analysis and receiver operating characteristic analysis.There was an independent association between both spinal growth velocity and electromyographic ratio and progression of the scoliotic curve. An equal sensitivity and specificity of spinal growth velocity for progression of 79.1% was observed at a growth velocity cutoff point of 11 mm/year. Similarly, a cutoff point of 1.25 for the electromyographic ratio could be determined with a predictive value for progression of 68.9%. In the presented nomogram, a spinal growth velocity >15 mm/year combined with an electromyographic ratio >2 gave an 89% probability of progression of the scoliotic deformity. Growth velocities 38 mm/year never resulted in progression.The combined measurement of spinal growth velocity and electromyographic ratio has significant predictive potential and may be valuable in the evaluation and treatment of idiopathic scoliosis.