Too Much Work: Revisiting Ultrasound-based Estimates of Achilles Tendon Energy Storage and Return

INTRODUCTION Ultrasound imaging is increasingly used with motion and force data to quantify tendon dynamics and to understand the functional role of tendons during human and other animal movement. Frequently, tendon dynamics are estimated indirectly from measures of muscle kinematics (by subtracting muscle length from muscle-tendon unit length), but there is mounting evidence that this approach, which we term the Indirect method, yields implausible tendon work loops (tendon force vs. elongation) [e.g., 1-2]. Since tendons are passive, viscoelastic structures, they should exhibit negative work loops (i.e., net negative work over a loading-unloading cycle). However, prior studies using Indirect estimates of tendon kinematics report large positive work loops, estimating that tendons return 100400% more energy than they store [e.g., 1-2]. More direct ultrasound methods have emerged that estimate tendon elongation by tracking either the muscle-tendon junction (termed the Direct MTJ method) or localized tendon tissue stretch (termed the Direct Tendon method) [3]. However, it is unclear if these Direct estimates yield more plausible tendon work loops. Here, we estimated tendon work loops and hysteresis using these two Direct tendon kinematics estimates during human walking compared to previously reported values based on Indirect kinematics estimates.