Correlation of Surface Acoustic Wave (SAW) force myography sensor output with elbow joint torque

Accurate assessment of skeletal muscle forces and net joint torque is essential for preventing fatigue-related injuries, optimizing physical training, and monitoring disease progression in neuromuscular conditions. However, existing joint torque evaluation techniques are hindered by limited portability and high operational costs, confining their use to controlled laboratory or clinical settings. Despite substantial advances in wearable joint torque estimation systems, ongoing challenges such as power constraints, bulky wired setups, and susceptibility to environmental or motion artifacts underscore the urgent need for truly batteryless, wireless solutions deployable in real-world settings. This paper proposes a novel surface acoustic wave (SAW)–based force myography (FMG) system for noninvasive measurement of joint torque, validated against a gold-standard electromechanical dynamometer. The approach uses a single SAW sensor embedded in an armband to detect volumetric biceps brachii changes, with a second-order polynomial mapping sensor output and elbow angle to torque. Seven participants were tested in both isometric (15°–90°) and isokinetic (10°/s and 20°/s) supinated elbow flexion tasks. Under isometric conditions, subject-specific calibration achieved a normalized root-mean-square error (NRMSE) of 13 . 6 % ± 6 . 0 % and R 2 = 0 . 834 ± 0 . 180 , while a group-level model yielded 14 . 4 % ± 6 . 8 % and 0 . 808 ± 0 . 208 , respectively. For isokinetic trials, the group model produced an NRMSE of 24 . 1 % ± 6 . 6 % at 10°/s and 24 . 9 % ± 08 . 9 % at 20°/s, highlighting the feasibility of using a single-sensor SAW-FMG setup across different speeds. Because SAW devices support wireless, battery-free operation, the proposed system offers a pathway to portable, real-time monitoring for sports medicine, rehabilitation, and clinical diagnostics. • First demonstration of a 128° YX-cut LiNbO 3 -based Surface Acoustic Wave (SAW) sensor for joint torque estimation through force myography. • Validation against a electromechanical dynamometer demonstrates reliable torque estimation under both isometric and isokinetic conditions, supporting applications in sports medicine, rehabilitation, and clinical diagnostics. • A second-order phase–angle model achieves 13.6% NRMSE (isometric) and ∼ 24% (isokinetic) with a single SAW sensor.