Individual variation in Achilles tendon morphology and geometry changes susceptibility to injury

Abstract

The unique structure of the Achilles tendon, combining three smaller sub-tendons, enhances movement efficiency by allowing individual control from connected muscles. This requires compliant interfaces between sub-tendons, but compliance decreases with age and may account for increased injury frequency. Current understanding of sub-tendon sliding and its role in the whole Achilles tendon function is limited. Here we show changing the degree of sliding greatly affects the tendon mechanical behaviour. Our in vitro testing discovered distinct sub-tendon mechanical properties in keeping with their mechanical demands. In silico study based on measured properties, subject-specific tendon geometry, and modified sliding capacity demonstrated age-related displacement reduction similar to our in vivo ultrasonography measurements. Peak stress magnitude and distribution within the whole Achilles tendon are affected by individual tendon geometries, the sliding capacity between sub-tendons, and different muscle loading conditions. These results suggest clinical possibilities to identify patients at risk and design personalised rehabilitation protocols.

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

“…However, caution must be taken when interpreting these results, considering the complicated Achilles sub-tendon structure and the above-mentioned sliding mechanism. The inter-individual differences in triceps surae force distribution ( Crouzier et al, 2020 ) and the AT material properties ( Yin et al, 2021 ) need to be taken into account when targeting the “sweet spot.” All these parameters will inherently generate different internal tendon strain distribution. However, to our knowledge, there are no studies investigating strains yet, taking into account the inter-individual differences in Achilles tendinopathy patients.…”

“…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. To compare the effect of typical exercises from rehabilitation protocols for Achilles tendinopathy, one needs to use a FE model which includes all three sub-tendons, subject-specific geometry and muscle forces.…”

Structure-function specialisation of the interfascicular matrix in the human achilles tendon

“…While the sex of each donor was known, limited sample numbers means it was not possible to establish if any variability arose from sex-related differences at baseline or with ageing, as previously reported [38] . The presence of sub-tendons within the Achilles arising from the soleus and gastrocnemius muscle bellies further adds to the potential source of variation [39] . While it is possible to estimate IFM shear modulus as approximately 0.6kPa, based on average measurements from previous data, the inability to measure IFM contact area in a non-destructive manner, and subsequently accurately calculate IFM shear modulus for each sample likely increased the variability of these data.…”

Does different activation between the medial and the lateral gastrocnemius during walking translate into different fascicle behavior?

“…For example, the MG muscle has, on average, a higher passive shear modulus (Le Sant et al, 2017;Lindemann et al, 2020) and a larger volume (Crouzier et al, 2018) than the LG, likely providing more resistance to lengthening. Moreover, the subtendon stiffness seems to match the muscle passive stiffness with a stiffer MG subtendon compared with the LG subtendon (Yin et al, 2021). The nervous system may choose to activate the MG more than the LG to enhance the catapult-like mechanism of the stiffer MTU and larger muscle and thus to decrease the overall activation cost (Biewener and Roberts, 2000;Crouzier et al, 2018;Crowninshield and Brand, 1981).…”

“…Several factors could explain the absence of correlations, including different tendon and muscle mechanical properties across participants. For instance, a large variability in subtendon stiffness (Yin et al, 2021) and Achilles tendon twist (Edama et al, 2015;Knaus and Blemker, 2021) has been observed between individuals. It is therefore possible that activation varies between participants to account for known differences in mechanical and architectural properties of the muscle and tendon (Aeles et al, 2017a), and enable similar movement kinetics and kinematics during motor tasks.…”