The Free Achilles Tendon Is Shorter, Stiffer, Has Larger Cross-Sectional Area and Longer T2* Relaxation Time in Trained Middle-Distance Runners Compared to Healthy Controls
Daniel Devaprakash, Steven J. Obst, David G. Lloyd, Rod Barrett, Ben Kennedy, Iain Ball, Kahlee L. Adams, Tyler Collings, Giorgio Davico, Adam Hunter, Nicole Vlahovich, David L. Pease, Claudio Pizzolato
Abstract
Tendon geometry and tissue properties are important determinants of tendon function and injury risk, and are altered in response to ageing, disease, and physical activity levels. The purpose of this study was to compare free Achilles tendon geometry and mechanical properties between sixteen trained elite/sub-elite middle-distance runners and a healthy control group. Magnetic resonance imaging was used to measure free Achilles tendon volume, length, average cross-sectional area, regional cross-sectional area, moment arm, and T2* relaxation time at rest, while freehand three-dimensional ultrasound was used to quantify free Achilles tendon mechanical stiffness, Young’s modulus, and length normalised mechanical stiffness. The free Achilles tendon in trained runners was significantly shorter and the average and regional cross-sectional area (distal end) were significantly larger compared to the control group. Mechanical stiffness of the free Achilles tendon was also significantly higher in trained runners compared to controls, which was explained by the group differences in tendon cross sectional area and length. T2* relaxation time was significantly longer in trained middle-distance runners when compared with healthy controls. There was no relationship between T2* relaxation time and Young’s modulus. The longer T2* relaxation time in trained runners may be indicative of a degree of accumulated damage, disorganised collagen, and increased water content in the free Achilles tendon. A shorter free Achilles tendon with larger cross-sectional area and higher mechanical stiffness may enable trained runners to rapidly transfer high muscle forces, as well as reduce the risk of tendon damage from mechanical fatigue.