Passive dynamics of a hybrid neuromechanical joint incorporating living muscle.

We have developed a hybrid neuromechanical joint to investigate the nonlinear properties of muscle in a physiologically relevant context. We constructed the hybrid system by connecting a live frog gastrocnemius muscle to a joint which is simulated as an inverted pendulum. We first tested this hybrid system with a spring as the actuator to ensure that the perturbation-response trajectories using living muscle would accurately describe the mechanical properties of the muscle. This hybrid neuromechanical system, with muscle as the actuator, responded to impulse torque perturbations with stable damped oscillations. Joint settling times to perturbations were measured while the joint mass and the initial muscle length were varied. Passive muscle alone stabilized small perturbations. With heavier joint masses and shorter initial muscle lengths the system became increasingly unstable. We found that the hybrid neuromechanical joint is an accurate tool to study the neuromuscular system.