Surgical treatments for early onset scoliosis (EOS) typically require multiple operations and many complications. A more flexible growing rod construct might result in a more flexible spine with fewer complications. Polymer rods (polyetheretherketone, PEEK) are relatively flexible in bending, and so might allow for greater range of motion (ROM) during treatment. The purpose of this study was to determine changes in ROM of the spine after implantation of simulated growing rod constructs with a range of clinically relevant structural properties. The hypothesis was that ROM of spines instrumented with PEEK rods would be both much greater than metal rods and significantly lower than uninstrumented controls.