Estimation of multijoint limb stiffness from EMG during reaching movements

Endpoint stiffness of the human arm was indirectly estimated using electromyographic activity (EMG) during reaching movements. Subjects performed point to point reaching movements in various dynamic environments while coupled to a robotic manipulator. Surface EMG from six muscles was recorded and direct measurements of endpoint stiffness were made. EMG was filtered using a pulse transfer function and a relationship between joint torque and EMG was obtained. From this a simple method was used to transform the muscle activity into joint stiffness and finally endpoint stiffness. The results were scaled based on a few selected values of measured endpoint stiffness. This technique successfully captured the main characteristics of the measured endpoint stiffness. This technique could be used to estimate endpoint stiffness during conditions when measuring stiffness would be too time consuming or difficult.