Background: Structural Context of In Vivo Measurements

In vivo, non-invasive biomechanical assessments of soft biological tissues, such as skeletal muscles and tendons, involve inherent challenges due to the layered structural composition of the human body.

Image 1. Cross-sectional view of layered tissue structure

Unlike in vitro examinations, where tissues can be isolated and studied in controlled environments, in vivo assessments must account for multiple biological layers surrounding the tissue of interest. These layers include the epidermis, dermis, subcutaneous fat, connective tissues, and the musculoskeletal system.

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Tissue of Interest

In the context of the MyotonPRO palpation device, the tissue of interest is a superficial soft tissue that serves as the intended target of the measurement, depending on the clinical or research objective. For measurements, the device’s probe is applied perpendicular to the skin surface and positioned directly above the tissue of interest.

Although the target tissue is the primary focus of the assessment, the nature of non-invasive measurement means that both overlying and underlying tissues also contribute to the physical oscillatory response.

For example, when evaluating a superficial skeletal muscle, the mechanical impulse generated by the device must pass through the skin and subcutaneous tissue before reaching the target tissue.

Consequently, the entire layered structure responds with a damped oscillation that reflects a weighted contribution of both the tissue of interest and the surrounding tissues, based on their interaction and individual mechanical properties.

As a result, the computed parameters represent a composite biomechanical response rather than the isolated behavior of the tissue of interest. Accordingly, measurement results should be interpreted as reflecting the integrated contribution of the entire structural unit involved in the oscillation.

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Influence of Overlying Adipose Tissues

In general, the skin and subcutaneous fat contribute to the dissipation of both the measurement impulse and the tissue’s response to it, thereby reducing the amplitude and propagation of the resulting oscillation. As a result, adipose tissue tends to affect the measurement outcome, primarily by slightly reducing the computed values of Tone and Stiffness, and increasing the values of Relaxation Time and Creep.

📌 Note:

There is typically an inverse relationship between Tone and Stiffness and the parameters Relaxation Time and Creep. When Tone and Stiffness decrease, Relaxation Time and Creep normally increase, and vice versa.

Therefore, greater subcutaneous fat thickness over the tissue of interest contributes to attenuated measurement results, which are expressed as apparently reduced values of the actual mechanical properties of the underlying skeletal muscle or tendon.

This occurs because subcutaneous fat is typically more compliant than muscle or tendon, and its dissipative nature reduces the transmission of the measurement impulse and the expression of underlying tissue properties through the resulting oscillatory response.

However, due to distinct structural differences, fibrous and structurally reinforced tissues such as skeletal muscle and tendon exert a dominant influence on measurement oscillations over more compliant adipose tissue.

📌 Note:

The Decrement parameter is not addressed in the paragraph because it is influenced by multiple factors, making it difficult to predict the direction of change with certainty. However, in general, subcutaneous fat tends to have a damping effect, typically leading to an increase in Decrement, which in turn reflects a decrease in the overall elasticity of the measured tissue structure.

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Influence of Underlying Rigid Structures

Underlying rigid structures such as bones or other skeletal support elements may influence the measured oscillatory response. Their presence can affect the transmission of the mechanical impulse and the resulting tissue oscillation, potentially leading to minor or even significant variations in the measurement outcomes.

A typical example of this effect is observed in superficial areas where the soft tissue layer above bone is particularly thin: the thinner the soft tissue layer over a rigid structure, the higher the measured stiffness.

In such cases, the measured values do not accurately reflect the intrinsic properties of the tissue itself. Instead, they are influenced by the compression of the soft tissue between the probe and the underlying bone, as well as by rebound effects caused by the mechanical impulse reflecting off the rigid surface.

A specific example of this effect can be observed when measuring the Orbicularis Oris muscle, which in this context is supported by rigid bony structures such as the maxilla or mandible. Due to the minimal soft tissue thickness in this area, the proximity of bone can cause artificially elevated stiffness values, primarily because of rebound effects from the underlying hard surface.

To reduce this influence, two key adjustments are recommended:

1. Increasing the standard probe surface area (D = 3mm, 7.1mm²). For this purpose, the addition of a flat disk adapter with a diameter of 10 mm is recommended. This disk adapter distributes the mechanical impulse energy over a wider surface (D = 10mm, 79mm²), thereby reducing the force applied per unit area.

 Image 2. Orbicularis oris measurement with disk adapter D = 10mm (79mm²)

2.Reducing the impulse duration from 15 milliseconds to 10 milliseconds. This shortens the duration of the mechanical stimulus by one-third, effectively lessening the displacement of the tissue.

While these modifications do not eliminate the influence of the underlying bone, they can significantly reduce its impact, thereby improving the reliability of the measurement.

Such interactions should be considered when interpreting results, especially in clinical and research applications where the accuracy and objectivity of tissue assessment are essential.

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✅ Key Takeaways: Interpreting MyotonPRO Measurements

1. Composite Response of Multiple Tissue Layers

MyotonPRO measurements reflect the combined mechanical behavior of the tissue of interest and the surrounding soft tissues. The recorded parameters do not represent the isolated properties of the target tissue, but rather a weighted contribution shaped by all layers involved in the oscillatory response.

2. Influence of Subcutaneous Adipose Tissue

Adipose tissue, being more compliant than muscle or tendon, dampens the mechanical impulse and attenuates the measured response. This typically results in lower recorded values for Tone and Stiffness, and higher values for Relaxation Time and Creep. Greater subcutaneous fat thickness can therefore mask the true mechanical properties of underlying tissues.

3. Influence of Underlying Rigid Structures

In regions with minimal soft tissue covering the bone, such as the Orbicularis Oris, the proximity of rigid structures can lead to artificially elevated Tone and Stiffness readings due to compression and rebound effects. These artifacts can be mitigated by using a flat disk adapter and reducing the impulse duration.

4. Importance of Contextual Interpretation

Accurate interpretation of MyotonPRO data requires consideration of anatomical, mechanical, and methodological factors. Surrounding tissue composition, probe placement, and skeletal proximity all influence results. In both clinical and research settings, such context is essential to ensure objective and meaningful assessment of tissue properties.