Author(s): KURZ, E., VALLI, G., MEYER, T., PRÖGER, S., SCHWESIG, R., BARTELS, T., DELANK, K.S., SACK, I., AGHAMIRY, H., Institution: MARTIN-LUTHER-UNIVERSITY HALLE-WITTENBERG, Country: GERMANY, Abstract-ID: 2531

INTRODUCTION:
MyotonPRO (MTP) and ultrasound elastography (USE) are increasingly used in clinical practice. Among USE techniques, time-harmonic elastography (THE) has gained attention due to its simplicity and affordable hardware. THE generates shear waves using external vibrators while a standard ultrasound device tracks wave propagation. Shear wave speed (SWS) is extracted using inversion algorithms as a proxy for tissue elasticity. MTP is a handheld device delivering a brief mechanical tap to the tissue surface. Assuming adequate coupling between skin and probe, the system is modeled as a damped mass-spring oscillator, reporting dynamic stiffness as a proxy for tissue elasticity. Although both technologies detect changes in tissue mechanics, their measurement principles and quantities differ fundamentally. Comparing their outcomes helps to understand these differences and enhance result interpretation.
METHODS:
Twenty-six healthy adults (M/F: 15/11, age: 25 [SD 4] years, BMI: 24 [3] kg/m²) participated. Vastus lateralis stiffness was measured at the right side sequentially via MTP and THE at rest and during isometric contractions at 15% and 30% maximal voluntary contraction (MVC). MTP stiffness (N/m) was automatically provided by the device. Shear modulus (kPa) was computed and the values were spatially averaged at MTP measurement points up to 15 mm depth. Skin layer and subcutaneous fat (SLSF) thickness was quantified from B-mode images. Effects of contraction intensities on tissue stiffness outcomes were assessed using univariate analyses of variance with repeated measures (contraction intensity) separately for each technology. The strength of the relationship between MTP and THE was determined using zero-order and partial correlation coefficients separately for each contraction intensity with adjustment of participants’ SLSF thickness.
RESULTS:
The MTP results showed that tissue stiffness was significantly affected by the contraction intensity (Rest: 337 [49] N/m, 15% MVC: 450 [116] N/m, 30% MVC: 529 [161] N/m), F = 49.25, p < 0.001. Tissue stiffness measured by THE revealed a significant effect of contraction intensity (Rest: 5.3 [1.2] kPa, 15% MVC: 10.8 [2.2] kPa, 30% MVC: 15.6 [6.1] kPa), F = 46.58, p < 0.001. Stiffness outcomes between MTP and THE showed intensity dependent associations (Rest: r = 0.255, p = 0.21, 15% MVC: r = 0.294, p = 0.14, 30% MVC: r = –0.547, p = 0.004). The relationships between both quantities changed (Rest: r = 0.358, p = 0.08, 15% MVC: r = –0.087, p = 0.68, 30% MVC: r = –0.373, p = 0.07) after controlling for participants’ SLSF thickness.
CONCLUSION:
Both technologies reflected rising contraction intensities. However, stiffness outcomes showed inconsistent weak-to-moderate relationships. Positive associations appeared only at rest, whereas active contractions showed inverted relationships. At 30% MVC, SLSF thickness reduced explained variance by 16%. Practitioners are warned against comparing results of both technologies during active muscle contractions.