We present research examining the function of complex muscles in the rat hindlimb. Two related sets of experiments are described. In the first, we examine the degree of specificity in spinal pattern generators, assessing whether the pattern generators at birth are capable of differentially activating intramuscular subdivisions in the complex hindlimb muscle biceps femoris. In the second, we describe a novel approach for creating a musculoskeletal model to capture the mechanical actions of individual muscles and evaluate its ability to capture the action of both simple and complex muscles in the rat hindlimb.