Abstract
The purpose of the current study was to assess in vivo Achilles tendon (AT) mechanical loading and strain energy during locomotion. We measured AT length considering its 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. The AT length was defined as the distance between its origin at the gastrocnemius medialis myotendinous junction (MTJ) and the calcaneal insertion. The MTJ was tracked using ultrasonography and projected to the reconstructed skin surface to account for its misalignment. Skin-to-bone displacements were assessed during a passive rotation (5°/s) of the ankle joint. 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. The 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 as an effective stimulus for tendon adaptation. Besides the important tendon energy recoil during the propulsion phase (7.8 to 11.3 J), 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.
Highlights
The purpose of the current study was to assess in vivo Achilles tendon (AT) mechanical loading and strain energy during locomotion
The original position of the gastrocnemius medialis (GM) myotendinous junction (MTJ) is not aligned with the reflective markers attached to the skin surface and, for an accurate AT length measurement, the identified position of the MTJ should be projected to the skin (Fig. 1a)
In order to assess the mechanical loading and strain energy of the AT during walking and running, we measured AT length using a new in vivo approach, which considers the tendon curve-path shape, taking into account the offset between MTJ and skin surface and the potential movements of the skin relative to the calcaneus bone at the AT insertion marker
Summary
The purpose of the current study was to assess in vivo Achilles tendon (AT) mechanical loading and strain energy during locomotion. Repetitive loading of the AT with a strain magnitude between 4.5 and 6.5% has been evidenced as an effective mechanical stimulus, improving AT mechanical properties[17,18,19] This tendon strain range is commonly reached at about 90% of a voluntary maximum isometric contraction of the adjacent muscle, which results in AT forces between 2.34 and 3.69 k N18–20. The instantaneous curved length of the AT can be obtained using a line of reflective markers attached to the skin from the tuber calcanei to the MTJ of the GM28,29 This method has been validated by comparing the outcomes with accurate AT length measurements from magnetic resonance imaging[30], it has not been applied to assess AT length during locomotion. The skin’s potential displacement relative to the bone underneath the calcaneus marker that defines the AT insertion can introduce errors in the AT length measurement[1]
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