Is human Achilles tendon deformation greater in regions where cross-sectional area is smaller?

Abstract

The Achilles is a long tendon varying in cross-sectional area (CSA) considerably along its length. For the same force, a smaller CSA would experience higher tendon stress and we hypothesised that these areas would therefore undergo larger transverse deformations. A novel magnetic resonance imaging-based approach was implemented to quantify changes in tendon CSA from rest along the length of the Achilles tendon under load conditions corresponding to 10%, 20% and 30% of isometric plantar flexor maximum voluntary contraction (MVC). Reductions in tendon CSA occurring during contraction from the resting condition were assumed to be proportional to the longitudinal elongations within those regions (Poisson's ratio). Rather than tendon regions of smallest CSA undergoing the greatest deformations, the outcome was region specific, with the proximal (gastrocnemius) tendon portion showing larger transverse deformations upon loading compared with the distal portion of the Achilles (P<0.01). Transverse tendon deformation only occurred in selected regions of the distal Achilles tendon at 20% and 30% of MVC, but in contrast occurred throughout the proximal portion of the Achilles at all contraction levels (10%, 20% and 30% of MVC; P<0.01). Calculations showed that force on the proximal tendon portion was ∼60% lower, stress ∼70% lower, stiffness ∼30% lower and Poisson's ratio 6-fold higher compared with those for the distal portion of the Achilles tendon. These marked regional differences in mechanical properties may allow the proximal portion to function as a mechanical buffer to protect the stiffer, more highly stressed, distal portion of the Achilles tendon from injury.

The tendinopathic Achilles tendon does not remain iso-volumetric upon repeated loading: insights from 3D ultrasound

“…The mean resting contralateral free tendon CSA value in the present study (60 mm 2 ) was also in close agreement (56-75 mm 2 ) with prior studies using the same approach (Obst et al, 2015;Nuri et al, 2016), but was lower than that for several prior studies (80-90 mm 2 ) (Maganaris and Paul, 2002;Magnusson and Kjaer, 2003;Reeves and Cooper, 2017). We believe this discrepancy in normal tendon CSA between studies could be due to the differences in imaging methods [MRI (Magnusson and Kjaer, 2003;Reeves and Cooper, 2017); 2D US (Maganaris and Paul, 2002); 3D US ( present study)], tendon CSA segmentation methods [tendon core+tendon paratenon (Maganaris and Paul, 2002;Magnusson and Kjaer, 2003;Reeves and Cooper, 2017); only tendon core ( present study)] and tendon region from which the measurement of tendon CSA was made [free AT insertion (Maganaris and Paul, 2002); whole free AT length ( present study)]. Further, resting free AT volume in the tendinopathic and contralateral legs was 7.16 and 4.15 ml, respectively.…”

T1- and T2*-Mapping for Assessment of Tendon Tissue Biophysical Properties

“…However, to improve workflow and optimize a standardized treatment, tendon incubation in a water bath should be considered, to equally effect tendons, which might be more consistent compared with the needle injection applied in this study. Investigation of larger diameter tendons like the human Achilles (60 mm 2 at its smallest cross section, 120 mm 2 at its largest) 51 should be considered for future experiments. However, the authors assume that if biophysical properties are detectable in smaller tendons, they should be even more likely to be reliably quantified in larger diameter tendons.…”

Three‐dimensional imaging of the fibrous microstructure of Achilles tendon entheses in Mus musculus

“…Furthermore, fiber cross‐sectional area was found to decrease towards the insertion in the fibrous structure specimens. Tendon cross‐sectional area alone is not indicative of tendon strength (Riemersma & De Bruyn, ; Reeves & Cooper, ), but the decrease in fiber cross‐sectional area is a challenge to the idea that mechanical stress in the transition could be reduced due to a distribution of force over a greater area (Benjamin et al. ).…”