Ultrasound Measurement of Local Deformation in the Human Free Achilles Tendon Produced by Dynamic Muscle-Induced Loading: A Systematic Review.

Achilles tendinopathy is the most prevalent lower limb tendinopathy, yet it remains poorly understood, with mismatches between observed structure and reported function. Recent studies have hypothesised that Achilles tendon (AT) healthy function is associated with variable deformation across the tendon width during use, focusing on quantifying sub-tendon deformation. Here, the aim of this work was to synthesise recent advances exploring human free AT tissue-level deformation during use. Following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, PubMed, Embase, Scopus and Web of Science were systematically searched. Study quality and risk of bias were assessed. Thirteen articles were retained, yielding data on free AT deformation patterns. Seven were categorised as high-quality and six as medium-quality studies. Evidence consistently reports that healthy and young tendons deform non-uniformly, with the deeper layer displacing 18%-80% more than the superficial layer. Non-uniformity decreased by 12%-85% with increasing age and by 42%-91% in the presence of injury. There is limited evidence of large effect that AT deformation patterns during dynamic loading are non-uniform and may act as a biomarker of tendon health, risk of injury and rehabilitation impact. Better considered participant recruitment and improved measurement procedures would particularly improve study quality, to explore links between tendon structure, function, aging and disease in distinct populations.