Achilles Tendon Mechanical But Not Morphological Properties Change with Rehabilitation in Patients with Chronic Midportion Achilles Tendinopathy.

Purpose: Achilles tendinopathy (AT) is associated with altered tendon's morphological and mechanical properties, yet it is unclear whether these properties are reversed upon mechanical loading to promote tendon healing. This study aims to determine the extent to which pathological tendon's morphological and mechanical properties adapt throughout a 12-week eccentric rehabilitation protocol.

Methods: Forty participants with midportion AT were recruited and participated in a 12-week eccentric rehabilitation program. Function and symptoms were recorded through the Victorian Institute of Sports Assessment-Achilles (VISA-A) and hopping. The tendon's morphological (i.e. volume, midportion cross-sectional area (CSA) and anteroposterior (AP) diameter) and mechanical (i.e. stiffness, Young's modulus, and tendon non-uniform displacement) properties were measured at baseline and at 6 and 12 weeks of the intervention.

Results: Significant functional improvements were observed, with VISA-A scores and hopping results showing marked improvements (p < 0.001). Morphologically, no significant changes in volume, midportion CSA, or AP diameter were found, although a trend toward decreased CSA between 30-70% of tendon length was noted. Mechanically, significant increases in Achilles tendon stiffness (p = 0.001) and Young's modulus (p < 0.001) were observed over the course of the rehabilitation program. No differences in tendon non-uniform displacement were found following a 12-week rehabilitation program.

Conclusions: These findings suggest that tendon adaptation in response to mechanical loading primarily involves changes in mechanical properties rather than morphology, highlighting the complexity and variability in tendon adaptation. Clinically, these mechanical properties could be considered in the load progression throughout rehabilitation as potentially higher strains will be induced when clinical improvements precede mechanical adaptations.