Calibration and validation of the in situ achilles shear wave speed-stress relationship

We used a robotic gait simulator to calibrate and validate the in situ relationship between squared shear wave speed and axial stress in cadaveric Achilles tendons. We first performed isometric plantarflexion simulations at multiple ankle angles to assess linearity and posture-dependence, and to estimate calibration parameters. We then used calibrated shear wave tensiometers to estimate stress from wave speed during robotic gait simulations. The squared wave speed-stress relationship was highly linear at each fixed posture, but variable across postures. Calibration at a 10º dorsiflexed posture yielded the best estimates of tendon stress during simulated walking, likely because this best matched the posture where the Achilles tendon was maximally loaded during gait. Thus, it may be appropriate to tailor tensiometer calibration to postures that represent the movement of interest.