A modular motor organization may be needed to solve the degrees of freedom problem in biological motor control. Reflex elements, kinematic primitives, muscle synergies, force-field primitives and/or pattern generators all have experimental support as modular elements. We discuss the possible relations of force-field primitives, spinal feedback systems, and pattern generation and shaping systems in detail, and review methods for examining underlying motor pattern structure in intact or semi-intact behaving animals. The divisions of systems into primitives, synergies, and rhythmic elements or oscillators suggest specific functions and methods of construction of movement. We briefly discuss the limitations and caveats needed in these interpretations given current knowledge, together with some of the hypotheses arising from these frameworks.