A Real-time EMG-driven Musculoskeletal Model of the Ankle

The real-time estimation of muscle forces could be a very valuable tool for rehabilitation. By seeing how much muscle force is being produced during rehabilitation, therapists know whether they are working within safe limits in their therapies and patients know if they are producing enough force to expect improvement. This is especially true for rehabilitation of Achilles tendon ruptures where, out of fear of overloading and causing a re-rupture, minimal therapy is typically done for eight weeks post-surgery despite animal studies that show that low-level loading is beneficial. To address this need, we have developed a biomechanical model that allows for the real-time estimation of forces in the triceps surae muscle and Achilles tendon. Forces are estimated using a Hill-type muscle model. To account for differences in neuromuscular control of each subject, the model used EMGs as input. To make this clinically useful, joint angles were measured using electrogoniometers. A dynamometer was used to measure joint moments during the model calibration stage, but was not required during real-time studies. The model accounts for the force-length and force-velocity properties of muscles, and other parameters such as tendon slack length and optimal fiber length. Additional parameters, such as pennation angle and moment arm of each muscle in the model, vary as functions of joint angle. In this paper, the model is presented and it application is demonstrated in two subjects: one with a healthy Achilles tendon and a second six months post Achilles tendon rupture and repair.