Tendinopathy alters the time course of free achilles tendon morphological recovery following eccentric exercise

The purpose of this study was to investigate the immediate and short-term (0-3 h) effects of a single bout of eccentric plantarflexion exercise on three-dimensional (3D) Achilles tendon (AT) morphology in healthy and tendinopathic tendons. The eccentric exercise protocol consisted of six sets of 15 unilateral heel drops performed on a step. Freehand 3D ultrasound scans of the AT were acquired immediately before exercise and at 30-minute intervals over a 3-hour post-exercise recovery period to quantify tendon volume, length, and cross-sectional area (CSA). An exponential function characterized the recovery time course and calculated recovery time constants. Following eccentric exercise, both tendinopathic and healthy tendons exhibited immediate reductions in volume and CSA and increased tendon length. Tendinopathic tendons demonstrated a faster initial recovery of morphological properties, as indicated by shorter recovery time constants, compared with healthy tendons. However, this early recovery plateaued after one hour, and morphological recovery remained significantly lower than that of healthy tendons during the 2-3 h post-exercise period. These findings suggest that tendinopathic tendons exhibit an early but incomplete morphological recovery characterized by faster initial fluid reabsorption followed by a reduced capacity for sustained recovery, likely reflecting degeneration-related alterations in tendon structure and poroelastic behavior.