Electrical Impedance Myography quantifies holding and pushing during isometric muscle contractions

Introduction: To address muscle performance, a local Electrical Impedance Myography (EIM) framework is introduced that targets the quadriceps femoris muscle group, which is sensitive to age-related muscle loss. It can be used for force estimation during the seated leg extension exercise under isometric conditions, providing a novel tool for geriatric assessment and therapy tracking. Methods: In this pilot study involving ten healthy male adults, a quasi-static comparison of the measured torque with either EIM signals, parameters calculated from the Debye- or Cole model, and characteristic frequency was performed. Motivated by electromyography, a baseline-removal and normalization to approximate muscle activity was implemented based on an exponential function. Finally, the muscle torque was estimated by multiplying the estimated muscle activity by the maximal voluntary torque. Results: Our data suggests systematic variations of EIM signals caused by holding a joint angle, pushing against a immobile lever, and changes due to applying repetitive contraction scenarios. Conclusion: For frequencies below the intersection point of the Nyquist plots of the muscle at rest and under contraction f ≤ fx, the resistance and reactance are sensitive to the effects of muscle activity, with a strong Pearson correlation of nearly 0.9 with torque. During pushing and small extension angles, the averaged accuracy of the estimated torque is T = ±200 ± 5 Nm.