It is now well established that the elastic function of tendons has a profound influence on the mechanics, energetics, and control of locomotion. A model of a simple Hookean spring in series with a muscle actuator reveals mechanisms that reduce the energy cost of locomotion, amplify muscle power output for ballistic movements, and provide passive dynamic responses to perturbations. Yet recent work suggests that this simple model fails to capture some important interactions between muscles and elastic structures. For example, we find that elastic structures influence muscle shape changes during contraction, and these shape changes in turn influence muscle force and velocity. Shape changes in muscle likewise result in tendon loading along more than one axis, resulting in an effective stiffness of tendons that varies dynamically. These observations have implications for motor control that are not yet fully explored. On the one hand, changes in muscle force and velocity mediated by muscle-spring interactions can provide rapid adjustments in mechanical output independent of neural control. On the other hand, motor commands for prescribed motions must account for these complex dynamics.