Non-uniform displacements within the Achilles tendon observed during passive and eccentric loading

Abstract

The goal of this study was to investigate Achilles tendon tissue displacement patterns under passive and eccentric loading conditions. Nine healthy young adults were positioned prone on an examination table with their foot secured to a rotating footplate aligned with the ankle. Subjects cyclically rotated their ankle over a 25 deg range of motion at 0.5 Hz. An inertial load geared to the footplate induced eccentric plantarflexor contractions with dorsiflexion. Passive cyclic ankle motion was also performed over the same angular range of motion. An ultrasound transducer positioned over the distal Achilles tendon was used to collect radiofrequency (RF) images at 70 frames/sec. Two-dimensional ultrasound elastographic analysis of the RF data was used to track tendon tissue displacements throughout the cyclic motion. Non-uniform tissue displacement patterns were observed in all trials, with the deeper portions of the Achilles tendon consistently exhibiting larger displacements than the superficial tendon (average of 0.9–2.6 mm larger). Relative to the passive condition, eccentric loading consistently induced smaller tissue displacements in all tendon regions, except for the superficial tendon in a flexed knee posture. Significantly greater overall tissue displacement was observed in a more extended knee posture (30 deg) relative to a flexed knee posture (90 deg). These spatial- and posture-dependent displacement patterns suggest that the tendon undergoes nonuniform deformation under in vivo loading conditions. Such behavior could reflect relative sliding between the distinct tendon fascicles that arise from the gastrocnemius and soleus muscles.

Longitudinal and transversal displacements between triceps surae muscles during locomotion of the rat

“…A recent study reported that the length of the merged distal portion (∼45% of the total Achilles tendon length) can change from 6.6 to 6.9 mm depending on knee extension and SO+GAS activation (Tijs et al, 2015b; male Wistar rat 315-340 g). Only a few studies have assessed the mechanical response of the SO and LG common tendon in physiological conditions (Finni et al, 1998;Franz et al, 2015;Slane and Thelen, 2014). Interestingly, the consistent finding that the Achilles tendon fascicles in series with the SO or LG undergo differential displacements was interpreted rather differently (Arndt et al, 1999;Bojsen-Møller and Magnusson, 2015).…”

Non-uniform in vivo deformations of the human Achilles tendon during walking

“…For example, varying knee flexion can elicit differential displacements of the soleus and gastrocnemius aponeuroses [19]. Similarly, our research group showed that knee flexion modulates the uniformity of AT tissue displacements [9]. In addition, Haraldsson et al [20] found negligible inter-fascicle force transmission within the AT and suggested that sliding may allow AT fascicles to act as functionally independent structures.…”

“…The free AT (i.e., calcaneus to soleus muscle-tendon junction) consists of distinct fascicle bundles arising from the triceps surae muscles; superficial fascicles arise from the medial gastrocnemius and deeper fascicles arise from the soleus and lateral gastrocnemius [7]. This unique anatomy gives rise to relative motion between fascicle bundles of the free AT [8, 9]. However, the relevance of such non-uniform AT deformations to biomechanical function remains unclear, particularly for tasks such as walking which involve complex coordination between these plantarflexor muscles [10-13].…”

“…Direct and indirect evidence for non-uniform deformations within the human free AT comes from cadaver experiments or from measurements made during isolated ankle exercises [8, 9, 15-19]. For example, varying knee flexion can elicit differential displacements of the soleus and gastrocnemius aponeuroses [19].…”

Three-dimensional morphology and strain of the Achilles free tendon immediately following eccentric heel drop exercise

“…These results could therefore represent evidence of altered biaxial strain of the free tendon, during muscle contraction, in response to acute exercise. These changes were not explained by post-exercise differences in muscle activation patterns or torque production, but could reflect non-uniform fatigue or creep of Achilles tendon fascicles, due to differences in mechanical properties and/or tensile loading during eccentric heel drop (Arndt et al, 2011(Arndt et al, , 1998Slane and Thelen, 2014). Irrespective of the cause, acute alterations in the normal biaxial strain could have implications for intra-tendinous force distribution (Haraldsson et al, 2008), fluid flow (Reese et al, 2010;Yin and Elliott, 2004) and tissue homeostasis (Smith et al, 2013) and are therefore relevant in the context of exercise-dependent regional adaptation of Achilles free tendon structure and function.…”