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

Mid-portion Achilles tendinopathy (MAT) alters the normal three-dimensional (3D) morphology of the Achilles tendon (AT) at rest and under a single tensile load. However, how MAT changes the 3D morphology of AT during repeated loading remains unclear. This study compared the AT longitudinal, transverse and volume strains during repeated loading in MAT with those of the contralateral tendon in people with unilateral MAT. Ten adults with unilateral MAT performed 10 successive 25 second submaximal (50%) voluntary isometric plantarflexion contractions with both legs. Freehand 3D ultrasound scans were recorded and used to measure whole AT, free AT, and proximal AT longitudinal strains and free AT cross-sectional area (CSA) and volume strains. The free AT experienced higher longitudinal and CSA strain and reached steady state following a greater number of contractions (5 contractions) in MAT compared to the contralateral tendon (3 contractions). Further, free tendon CSA and volume strained more in MAT than contralateral tendon from the first contraction, whereas free AT longitudinal strain was not greater than the contralateral tendon until the fourth contraction. Volume loss from the tendon core therefore preceded the greater longitudinal strain in MAT. Overall, these findings suggest that the tendinopathic free AT experiences an exaggerated longitudinal and transverse strain response under repeated loading that is underpinned by an altered interaction between solid and fluid tendon matrix components. These alterations are indicative of accentuated poroelasticity and an altered local stress-strain environment within the tendinopathic free tendon matrix, which could affect tendon remodelling via mechanobiological pathways.