Measurement of tendon strain during muscle twitch contractions using ultrasound elastography

A 2-D strain estimation algorithm was used to estimate tendon strain from ultrasound data collected during muscle twitch contractions. We first used speckle tracking techniques to estimate frame-to-frame displacements of all pixels within a rectangular region of interest (ROI) positioned over a tendon. A weighted, least-squares approach was then solved for the displacements of the ROI endpoints that best fit the pixel displacements. We summed endpoint displacements across successive frames to determine the cumulative endpoint motion, which was then used to estimate the cumulative strain along the tendinous fibers. The algorithm was applied to ultrasound radiofrequency data, acquired at 74 frames per second over the tibialis anterior (TA) musculotendon junction (MTJ). The TA muscle was electrically stimulated with the subject holding voluntary preloads of 0%, 10%, 20%, 30%, 40%, and 50% of a maximum voluntary contraction (MVC). Peak tendon strains computed using elastography (0.06 to 0.80%) were slightly larger and occurred earlier (50-90 ms after stimulus) than calculations based on visual analysis of B-mode images. This difference likely reflected the more localized nature of the elastographic strain values. Estimates of the tangential elastic modulus (192 +/- 58 MPa) were consistent with literature values obtained using more direct approaches. It is concluded that automated elastographic approaches for computing in vivo tendon strains could provide new insights into musculotendon dynamics and function.