Scoliosis is characterised by a more or less pronounced change in the balance of forces, starting already in the feet, legs and hips, with inequality of muscles in terms of length and size. The greater the deviations from the midline, the longer the affected muscles become and the more volume is lost. They become flaccid and finally inactive. They lose their supportive function. Shape changes are only possible because the muscles permit them to happen. They become longer or shorter depending on the direction in which the trunk is moved and rotated. In other words, deviations of the trunk to the side or backwards can only develop if the corresponding supportive muscles give way and become elongated.
Therefore, treatment must improve posture so that the body can regain its original vertical axis. This can only happen by developing and training the corresponding muscle groups responsible for upright posture. To restore muscular balance, those muscles that have grown longer must be shortened and those that have become shorter must be lengthened. In order for these to be able to hold the spinal column and ribcage in their normal vertical position again, they need to be strengthened -- and on both sides. It is absolutely essential that the inactive, shortened muscles perform strength work in the lengthened state. [pp. 42-43]