Surface-Level Muscle Deformation as a Correlate for Joint Torque

Wearable technology excels in estimating kinematic and physiological data, but estimating biological torques remains an open challenge. Deformation of the skin above contracting muscles—surface-level muscle deformation—has emerged as a promising signal for joint torque estimation. However, a lack of ground-truth measures of surface-level muscle deformation has complicated the evaluation of wearable sensors designed to measure surface-level muscle deformation. A non-contact methodology is proposed for ground-truth measurement of surface-level muscle deformation using a 2D laser profilometer. It shows how three metrics of surface-level muscle deformation—peak radial displacement: r = 0.94 ± 0.05, surface curvature: r = 0.78 ± 0.10, surface strain: r = 0.83 ± 0.12—correlate strongly to changes in volitional elbow torque, further exploring the impact of measurement location or joint angle on these relationships. A nonlinear, lead-lag relationship between surface-level muscle deformation and torque is also found. The findings suggest that surface-level muscle deformation is a promising signal for non-invasive, real-time estimates of torque. By standardizing measurement, the methodology can help inform the design of future wearable sensors.