Achilles Subtendon Structure and Behavior as Evidenced From Tendon Imaging and Computational Modeling

Abstract

The Achilles tendon is the largest and strongest tendon in the human body and is essential for storing elastic energy and positioning the foot for walking and running. Recent research into Achilles tendon anatomy and mechanics has revealed the importance of the Achilles subtendons, which are unique and semi-independent structures arising from each of the three muscular heads of the triceps surae. Of particular importance is the ability for the subtendons to slide, the role that this has in healthy tendons, and the alteration of this property in aging and disease. In this work, we discuss technical approaches that have led to the current understanding of Achilles subtendons, particularly imaging and computational modeling. We introduce a 3D geometrical model of the Achilles subtendons, built from dual-echo UTE MRI. We revisit and discuss computational models of Achilles subtendon twisting suggesting that optimal twist reduces both rupture loads and stress concentrations by distributing stresses. Second harmonic generation imaging shows collagenous subtendons within a rabbit Achilles tendon; a clear absence of signal between the subtendons indicates an inter-subtendon region on the order of 30 μm in our rabbit animal model. Entry of wheat germ agglutinin in both the inter-fascicular and the inter-subtendon regions suggests a glycoprotein-containing inter-subtendon matrix which may facilitate low friction sliding of the subtendons in healthy mammals. Lastly, we present a new computational model coupled with human exercise trials to demonstrate the magnitude of Achilles subtendon sliding which occurs during rehabilitation exercises for Achilles tendinopathy, and shows that specific exercise can maximize subtendon sliding and interface strains, without maximizing subtendon strains. This work demonstrates the value of imaging and computational modeling for probing tendon structure-function relationships and may serve to inform and develop treatments for Achilles tendinopathy.

Monitoring Muscle-Tendon Adaptation Over Several Years of Athletic Training and Competition in Elite Track and Field Jumpers

“…Hence, although a flexed knee joint could potentially influence the interactions between the individual AT subtendons, it seems not to cause a clear measurable alteration in the m. gastrocnemius medialis tendon force-elongation relationship during loading, particularly in the "linear" region were the tendon stiffness was determined. However, we cannot exclude that the observed higher tendon strain fluctuations in elite athletes may be influenced also by variations in the regional strain patterns and subtendon sliding (Handsfield et al, 2020;Maas et al, 2020). Incorporating regional and subtendon strain differentiation could contribute additional understanding into the complex tendon loading pattern during various contractions in elite athletes.…”

Foundational Principles and Adaptation of the Healthy and Pathological Achilles Tendon in Response to Resistance Exercise: A Narrative Review and Clinical Implications

“…Morphologically, tendon CSA increases in response to increased loading in vivo [ 29 , 32 , 102 ], though limited evidence suggests that transient fluid redistribution may mask this in the short-term [ 51 , 104 ]. Additionally, loading-induced changes may differ along the Achilles tendon as the regional variation in load management [ 98 , 105 , 106 ] may preferentially activate mechanotransductive pathways leading to region-specific tendon hypertrophy [ 43 , 44 ]. Though still an area of exploration, the opposite could also be the case in that the non-uniform stress distribution within the Achilles tendon could contribute to the location of abnormalities associated with AT [ 107 ].…”

Subject-Specific 3D Models to Investigate the Influence of Rehabilitation Exercises and the Twisted Structure on Achilles Tendon Strains

“…This was further confirmed by the fact that varying overall tendon twist in such models showed to impact tendon fascicle length, strain and energy storage ( Knaus and Blemker, 2021 ). The main limitation of these previous studies is that the three sub-tendons were not included in the AT models ( Hansen et al, 2017 ) or, if included, only two sub-tendons were considered (soleus and gastrocnemius) ( Shim et al, 2019 ; Handsfield et al, 2020 ). In studies that considered the three sub-tendons ( Handsfield et al, 2017 ; Knaus and Blemker, 2021 ; Yin et al, 2021 ), the focus was on the analysis of more static exercises.…”