The muscle strength of the trunk and electromyography (EMG) of paraspinal muscles were recorded during maximal isokinetic exercise in 17 girls with adolescent idiopathic scoliosis (AIS) and in 12 matched control subjects. Median power frequency (MdPF) and Integrated EMG (IEMG) were calculated. The muscle strength in the AIS group was significantly lower than in the control group. Positive correlation of Cobb angle with strength of trunk extensors, but not with flexors was observed, and negative correlation with the flexion-extension ratio (F/E value). There was a close relationship between the Cobb angle and the decrease of rotation muscle strength on the convex side to that on the concave side. MdPF's in AIS were lower than those in the control, and the difference between right and left sides tended to be large. Exercises to strengthen the muscles of the trunk at a high speed with an effort close to the maximum, with consideration given to flexion and rotation on the concave side of the trunk, are recommended for AIS patients.