NMClab, a model to assess the contributions of muscle visco-elasticity and afferent feedback to joint dynamics

The dynamic behavior of a neuromusculoskeletal system results from the complex mechanical interaction between muscle visco-elasticity resulting from (co-)contraction and afferent feedback from muscle spindles and Golgi tendon organs. As a result of the multiple interactions the individual effect of each of the structures to the overall dynamics is hard to recognize, if not impossible. Here a neuromuscular control (NMC) model is developed to analyze the functional contribution of the various physiological structures on the mechanical behavior of a limb. The dynamics of a joint are presented in admittances, i.e. the dynamic relation between input force (or torque) and the output displacement, which can be represented by either frequency or impulse response functions. With the model it can be shown that afferent feedback reduces, while muscle visco-elasticity increases, the stability margins. This implicates that there is a delicate balance between muscle co-contraction and afferent feedback, which depends on the joint specific physiological properties. The main application of the model is educational; it is implemented in a graphical user interface allowing users to explore the role of the various physiological structures on joint dynamics. Other applications of the model are more experimental, e.g. to elucidate experimentally measured admittances and to compare the quantified parameter values with the theoretically optimal ones. It is concluded that the NMC model is a useful and intuitive tool to investigate human motor control, in a theoretical as well as an experimental way.