Single-trial estimation of quasi-static EMG-to-joint-mechanical-impedance relationship over a range of joint torques

Abstract Joint mechanical impedance is commonly measured by applying dynamic perturbations about a joint at a fixed operating point/ background torque, and quantifying torque change vs. angle change. Impedance characterization in functional tasks, therefore, requires multiple experimental trials over a range of operating points—a cumbersome, invasive, time-consuming and impractical task. As an alternative, studies have related EMG to impedance, after which EMG can estimate impedance without applying joint perturbations. But, the cumbersome calibration trials are still required. We describe a method of single contraction perturbations in which the background torque slowly ramps over the operating range, with EMG simultaneously acquired. Using one such “quasi-static” contraction for model training and another for testing, we show this method to be a reasonable surrogate for traditional second-order, linear impedance modeling. A simple, short-duration calibration results. We compared our single-trial ramp method to multiple constant background torque trials at 10, 20, 30, and 40% maximum effort (extension and flexion), finding only limited differences in traditional vs. EMG-based ramp impedance estimates (12–22%, most prominent at the two lower contraction levels). Such constant force and slowly-variable force contractions are relevant to many practical applications, including ergonomics assessment, prosthetic control and clinical biomechanics.