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This brief review article was conducted to summarize the findings regarding correlation and agreement between different methods to assess muscle stiffness (shear wave elastography (SWE), myotonometry, and passive joint stiffness measurements). Muscle stiffness, an important biomechanical characteristic, influences joint flexibility, postural stability, injury risk, and athletic performance.

SWE provides insights into tissue elasticity by measuring the propagation speed of shear waves, while myotonometry assesses stiffness through induced muscle oscillations. Passive joint stiffness measurements offer a holistic perspective, capturing the resistance of the entire joint to movement. However, distinguishing the contributions of muscular and non-muscular tissues remains a challenge in this method.

The article highlights the variability in the correlation between these methodologies, influenced by factors such as muscle length, age, and examiner technique. While some studies report good agreement between SWE and myotonometry, others note discrepancies, underscoring the need for careful method selection based on the research or clinical context.

This review highlights the complexity of assessing muscle stiffness and the necessity of a nuanced approach in interpreting data from different measurement techniques, aiming to guide researchers and clinicians in their choice of method for a precise and accurate evaluation of muscle stiffness.

Keywords: elasticity, ultrasound, myotonometer, joint torque, joint stiffness

In summarizing this short review, it becomes evident that the measurement of muscle stiffness necessitates a careful approach. SWE, myotonometry, and passive joint stiffness measurements each offer unique insights, yet they are not without their distinct challenges and limitations. The key takeaways of the paper are as follows: (1) SWE provides a robust quantification of tissue elasticity, yet is susceptible to the anisotropic nature of muscle tissue and the influence of subcutaneous fat; (2) Myotonometry, though user-friendly and less expensive, primarily reflects the stiffness of superficial muscle regions and can be affected by skin and subcutaneous tissue properties; (3) Passive joint stiffness measurements, while offering a more comprehensive view of joint resistance, fail to isolate muscular contributions from other tissues; and (4) the discrepancies among these methods highlighted in this review emphasize the necessity for careful selection and interpretation of measurement techniques.

Future research should focus on refining these methodologies, exploring their applicability across diverse populations and settings, and striving for a standardized approach to enhance the reliability and validity of muscle stiffness assessment. Such efforts will not only expand the current body of knowledge but also pave the way for technological innovations in the biomedical engineering landscape. When selecting a method for clinical muscle stiffness assessment, consider the specific context and the muscle or joint of interest. SWE is recommended for a detailed analysis of muscle tissue properties, particularly in cases where deep muscle assessment is crucial. Myotonometry is suitable for quick assessments and monitoring superficial muscle changes, especially in rehabilitation settings. Passive joint stiffness measurements are ideal for a holistic assessment of joint health, particularly in conditions affecting multiple joint structures.