Reliability of A Vibration-Based Elastography Protocol For Assessing Achilles Tendon Stiffness Across Multiple Joint...

The mechanical behavior of the Achilles tendon plays a critical role in athletic performance and injury risk; however, in vivo assessment of tendon stiffness remains challenging. Conventional approaches combining ultrasonography with dynamometry are expensive, laboratory-bound, and typically limited to single joint positions, while existing elastography-based techniques are often constrained by methodological assumptions or limited functional relevance. The purpose of this study was to present and validate a standardized, portable protocol for quantifying the functional stiffness spectrum of the Achilles tendon across multiple fixed ankle joint angles. This paradigm shifts the assessment from a single static stiffness value to a continuous mechanical profile, capturing the tendon's nonlinear response to loading. Using a force-ultrasound fusion system, mechanically induced low-frequency vibrations were applied to the tendon while ultrasound-based motion tracking was used to estimate the shear elastic modulus of superficial tendon tissue. Measurements were performed bilaterally in elite male athletes at predefined ankle joint positions ranging from relaxed and plantarflexed states to neutral and dorsiflexed positions. The protocol demonstrated good intra-trial repeatability and excellent intra-session reproducibility across all joint angles, with coefficients of variation remaining within acceptable limits for soft-tissue elastography and intraclass correlation coefficients indicating high reliability. Achilles tendon stiffness increased non-linearly with progressive dorsiflexion, indicating angle-dependent mechanical behavior. No significant main effect of side dominance was observed across the full functional range, while sport-specific differences emerged at selected joint angles. This protocol provides a practical and repeatable approach for characterizing Achilles tendon mechanical behavior under functionally relevant loading conditions. Its portability and standardized workflow make it suitable for laboratory, clinical, and field-based applications, offering a valuable tool for athlete monitoring, injury risk assessment, and longitudinal evaluation of tendon adaptation.