Reliability of A Vibration-Based Elastography Protocol For Assessing Achilles Tendon Stiffness Across Multiple Joint Angles In Elite Athletes
June 16th, 2026
This protocol describes a standardized, portable ultrasound-based method for quantifying the functional stiffness spectrum of the Achilles tendon across multiple ankle joint angles in elite athletes, enabling reliable and reproducible assessment of tendon mechanical behavior under different loading conditions.
We utilize a novel elastography protocol to evaluate Achilles tendon stiffness across multiple joint angles, aiming to characterize biomechanical adaptations in elite athletes from various sports. Unlike existing methods, our vibration-based elastography reliably assesses continuous tendon stiffness, changes across the full range of motion. To begin, recruit participants from national level sports teams, including professional male athletes aged 18 to 26 years across multiple sport disciplines.
Determine the dominant leg by asking the participant to kick a ball. Prepare the equipment, M5 soft tissue elastometer equipped with a 9L3 to 8.5 megahertz linear array probe, and a mechanical excitation module. Gather additional equipment, including an Achilles boot, ultrasound gel, and an examination bed.
Switch on the main power supply and confirm that the system status indicator illuminates. Then, power on the tablet interface, launch the ultrasound system software by selecting the designated application icon, and verify that the system enters the main ultrasound operating interface with real-time B mode imaging displayed. Set the B mode parameters to a frequency of 7.5 megahertz, four acquisition lines, a five millimeter depth range, and a 300 millisecond acquisition time.
Next, instruct the participant to remove shoes, then position the participant prone on the examination couch with the ankles fully extended over the edge by approximately five centimeters. Palpate to locate the superior apex of the calcaneal tuberosity. Mark a 0.5 centimeters proximal to this landmark using a skin marker to define the initial measurement site.
Apply a uniform layer of pre-warmed coupling gel to the transducer surface. Lightly place the probe against the measurement site and align the target point under the anterior side of the probe. Now switch the ultrasound system to elastography mode.
Position the reference line so that the acquisition depth range begins just below the superficial tendon fascia. Adjust the region of interest to cover the tendon thickness while avoiding the skin, subcutaneous tissue, and Kager fat pad. Before acquisition, confirm imaging quality by ensuring that the transducer plane is nearly perpendicular to the skin surface at greater than 75 degrees.
Confirm that the transducer to skin distance is approximately five millimeters. Verify that no visible air bubbles are present, and that the fascia and tendon fibers are clearly visualized. Once the excitation device is turned on, allow the system to automatically calculate the shear modulus, and display mean and standard deviation values of valid data.
Maintain constant participant and operator posture during acquisition to obtain at least 10 valid continuous data points. Stop data acquisition by pressing the freeze function once sufficient data points are collected. Review the data set for outliers and remove abnormal data points using the system editing function.
Conduct measurements sequentially on both Achilles tendons under relaxed, zero degrees neutral, 20 degrees plantar flexion, 40 degrees plantar flexion, 20 degrees dorsiflexion, and 40 degrees dorsiflexion conditions. Repeat measurements at least three times at each ankle angle. Validate each measurement only if the standard deviation of the continuous data points is less than 10%of the mean, as required by the device's internal validation criteria.
Save B mode images and mechanical imaging maps for documentation. After completing the relaxed state measurement, place the participant's foot into the adjustable ankle testing boot, ensuring the heel rests completely flush against the posterior heel cup of the boot base. Secure the forefoot, midfoot, and lower leg using the attached hook and loop straps to prevent heel lift or lateral shifting during testing.
Loosen the bilateral locking knobs on the boot hinge mechanism. Manually guide the ankle to the target angle by aligning structural markers with the goniometric scale. Tighten the locking knobs firmly to secure the ankle joint at the target angle.
Perform ultrasound measurement immediately after locking the angle to prevent viscoelastic tendon relaxation. After that, instruct the participant to remove the ankle boot. Clean and sanitize all instruments and ultrasound probes.
A successful measurement was visually characterized by a high quality B mode image with a clear continuous tendon fibrillar structure parallel to the skin surface and a stable homogeneous elastography color map within the predefined region of interest. The average coefficient of variation values ranged from 0.14 to 0.25 across different joint angles. Interclass correlation coefficient values ranged from 0.871 to 0.974 across all assessed joint angles.
The highest reliability was observed in the relaxed state, and the lowest reliability was observed at the neutral zero degree position. Achilles tendon stiffness increased non-linearly from plantar flexion to dorsiflexion in all participants. The generalized mixed models revealed a significant main effect of joint angle, whereas side and sport type showed no main effects.
The angle and sport interaction was significant, indicating sports specific stiffness differences at certain ankle angles. This protocol non-invasively measures the Achilles tendon's functional stiffness spectrum, capturing its non-linear mechanical behavior across dynamic angles. The key challenge during dynamic acquisition is maintaining constant contact pressure and the pro perpendicularity to avoid the motion artifacts.
Future studies can map tendon behavior to monitor tendonopsy rehabilitation, assess training adaptations, and predict dynamic sports performance.
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Overview
This study introduces and validates a portable, standardized protocol for quantifying the functional stiffness spectrum of the Achilles tendon across multiple ankle joint angles. By employing a force–ultrasound fusion system, the method enables continuous mechanical profiling of the tendon, offering improved assessment over traditional single-value or laboratory-bound techniques. The protocol demonstrates high repeatability and reliability, making it suitable for diverse applications in athlete monitoring and injury risk assessment.
Key Study Components
Area of Science
- Sports medicine
- Biomechanics
- Musculoskeletal imaging
Background
- The Achilles tendon’s mechanical properties are crucial for athletic performance and injury prevention.
- In vivo assessment of tendon stiffness is challenging due to limitations of current methods.
- Conventional ultrasonography with dynamometry is expensive and limited to laboratory settings and single joint positions.
- Existing elastography techniques often lack functional relevance or are constrained by methodological assumptions.
Purpose of Study
- To present and validate a portable protocol for quantifying the Achilles tendon’s stiffness spectrum across multiple ankle joint angles.
- To shift assessment from a static stiffness value to a continuous mechanical profile reflecting nonlinear tendon behavior.
- To enable practical, repeatable tendon assessment in laboratory, clinical, and field settings.
Methods Used
- Application of mechanically induced low-frequency vibrations to the Achilles tendon.
- Ultrasound-based motion tracking to estimate the shear elastic modulus of superficial tendon tissue.
- Force–ultrasound fusion system for synchronized measurement.
- Measurements performed bilaterally in elite male athletes at predefined ankle joint positions (relaxed, plantarflexed, neutral, dorsiflexed).
Main Results
- The protocol showed good intra-trial repeatability and excellent intra-session reproducibility across all joint angles.
- Coefficients of variation were within acceptable limits for soft-tissue elastography.
- Intraclass correlation coefficients indicated high measurement reliability.
- Achilles tendon stiffness increased non-linearly with dorsiflexion, demonstrating angle-dependent mechanical behavior.
- No significant effect of side dominance was observed, but sport-specific differences appeared at selected joint angles.
Conclusions
- The protocol provides a practical and repeatable approach for characterizing Achilles tendon mechanical behavior under functionally relevant loading.
- Its portability and standardized workflow support use in laboratory, clinical, and field environments.
- The method is valuable for athlete monitoring, injury risk assessment, and longitudinal evaluation of tendon adaptation.
Frequently Asked Questions
Executive Industry Relevance
Quantitative assessment of tendon mechanical properties is critical for translational musculoskeletal research and preclinical model development. This portable vibration-based elastography protocol enables standardized, reproducible measurement of Achilles tendon stiffness across multiple joint angles, supporting predictive confidence in biomechanical target validation. Its reliability and functional relevance position it as a valuable tool for risk-adjusted advancement in sports medicine and musculoskeletal R&D portfolios.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Enables interrogation of tendon mechanical behavior under physiologically relevant loading conditions.
- Supports functional target validation by quantifying nonlinear stiffness profiles across joint angles.
- Facilitates biological de-risking for musculoskeletal intervention strategies.
Screening & Assay Development
- Provides a standardized, repeatable workflow for tendon stiffness quantification.
- Delivers quantitative outputs suitable for assay reproducibility and cross-study comparison.
- Prepares validated biomechanical systems for downstream screening or intervention studies.
Translational & Preclinical Research
- Aligns tendon mechanical profiling with disease-relevant and athlete-specific translational models.
- Enables longitudinal evaluation of tendon adaptation and intervention effects.
- Supports risk-adjusted decisions for advancing candidate therapies targeting tendon health.
Pipeline & Workflow Integration
This protocol integrates into the discovery-to-preclinical continuum for musculoskeletal research, bridging early biomechanical hypothesis testing with translational model validation.
- Discovery Biology: Quantifies tendon mechanical response to loading, clarifying functional pathways and de-risking targets.
- Screening: Standardizes measurement of tendon stiffness for reproducible, quantitative assay outputs.
- Analytics: Provides continuous mechanical profiles and statistical reliability metrics for robust condition comparison.
- Translational Research: Enables alignment of preclinical models with athlete-relevant mechanical endpoints.
- Enterprise Reuse: Offers a portable, standardized capability for repeated use across laboratory, clinical, and field settings.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence and reduces mechanistic ambiguity in tendon research.
- Operational Value: Delivers standardized, reproducible, and scalable tendon assessment workflows.
- Strategic Value: Improves go/no-go decisions and capital efficiency by enabling reliable mechanical phenotyping.
- Portfolio Impact: Supports risk-adjusted prioritization and advancement of musculoskeletal R&D assets.
Implementation Considerations
- Requires expertise in ultrasound imaging and biomechanical data interpretation.
- Needs access to force–ultrasound fusion instrumentation and vibration induction hardware.
- Demands cross-team standardization for protocol reproducibility and data comparability.
- Adaptable to various joint angles and subject populations within validated operational limits.
- Practical limitations include dependency on operator training and instrumentation availability.
