An exercise fitness system involves an apparatus comprising a rigid movable booster bar having two flexible elastic elements attached to the booster with an attachment. The attachments are spaced from each other and configured to allow winding the elastic element upon the booster bar by rotating the booster bar. Each flexible elastic element comprises at least two stages of flexible shock cords, with the stages disposed serially along the length of the flexible element, with adjacent stages having a different elastic stiffness.