In the natural history of idiopathic scoliosis, SGV and EMG ratio at the lower end vertebra are prominent risk factors of curve progression. The asymmetric muscle activity is associated with increased axial rotation, which in its turn is associated with increasing Cobb angle and diminishing kyphosis. The combination of these variables provides insight in the physiologic and 3-dimensional biomechanical evolution of the natural history of curve progression in idiopathic scoliosis.