Quantifying mechanical loading and elastic strain energy of the human Achilles tendon during walking and running

The purpose of the current study was to assess Achilles tendon (AT) mechanical loading and strain energy during locomotion using a new in vivo approach for measuring AT length that considers the AT curve-path shape. Eleven participants walked at 1.4 m/s and ran at 2.5 m/s and 3.5 m/s on a treadmill. AT length, defined as the distance between its origin at the gastrocnemius medialis myotendinous junction (MTJ) and the calcaneal insertion, was determined experimentally by integrating kinematics and ultrasound analysis. Small foil markers were placed on the skin covering the AT path from the origin to the insertion, and the MTJ, tracked using ultrasonography, was projected to the reconstructed skin to account for their misalignment. Skin-to-bone displacements were assessed during a passive rotation (5 {degrees}/s) of the ankle joint and considered in the calculation of AT length. Force and strain energy of the AT during locomotion were calculated by fitting a quadratic function to the experimentally measured tendon force-length curve obtained from maximum voluntary isometric contractions. Maximum AT strain and force were affected by speed (p<0.05, ranging from 4.0 to 4.9% strain and 1.989 to 2.556 kN), yet insufficient in magnitude to be considered an effective stimulus for tendon adaptation. Further, we found a recoil of elastic strain energy at the beginning of the stance phase of running (70-77 ms after touch down) between 1.7 {+/-}0.6 and 1.9 {+/-}1.1 J, which might be functionally relevant for running efficiency. Summary statementA new accurate in vivo approach to assess Achilles tendon strain, force and strain energy during locomotion.