Author(s): LEY, C., MARTINEZ-VALDES, E., DRUST, B., Institution: UNIVERSITY OF BIRMINGHAM, Country: UNITED KINGDOM, Abstract-ID: 1728

INTRODUCTION:
Tissue biomechanical properties are important for both optimising musculoskeletal performance and reducing injury risk. The MyotonPRO was developed as a simple, portable, non-invasive, and highly reliable method for the estimation of muscle stiffness (Aird et al., 2012). It has not, however, been comprehensively validated against shear wave elastography (SWE), a validated ultrasound-based method for the assessment of tissue stiffness. For this purpose, we estimated the stiffness of the rectus femoris (RF) muscle and supra-adjacent tissue with both the MyotonPRO and SWE, in order to assess the validity of the MyotonPRO for the assessment of RF mechanical properties. The RF was chosen due to its high injury prevalence among elite soccer players as a result of its contributions to both hip flexion and knee extension as a biarticular muscle (Cross et al., 2004).
METHODS:
20 participants (10 male and 10 female; age: 26.3 ± 4.0 years; BMI: 23.2 ± 2.8kg/m2) completed 2 visits in which RF stiffness was measured twice with the MyotonPRO device and SWE at three distinct muscle regions (proximal, medial, and distal), three muscle lengths
(relaxed - REL, neutral - NEU, and passively stretched - PAST), and four depths (skin - SKIN, fascia - FAS, superficial muscle - SUP, and deep muscle - DEEP). Additionally, subcutaneous adipose tissue thickness (SAT) was recorded at each location under every condition using B-mode ultrasound imaging. To investigate the relationship between MyotonPRO measurements and shear wave velocity (SWV) at each depth, as well as SAT, simple linear regressions were conducted on the mean values obtained from the assessments.
RESULTS:
In REL, the MyotonPRO exhibited none to poor negative correlations with SWV. In NEU, stiffness reported by the MyotonPRO had a moderate positive relationship with SWV of the skin (r = .67***, R2 = .45), but smaller relationships with deeper tissue (FAS, r = .25*, R2 = .06; SUP, r = .26*, R2 = .07; DEEP, r = .33**, R2 = .11). Similarly, in PAST, stiffness reported by the MyotonPRO had a moderate positive relationship with SWV of the skin (r = .61***, R2 = .38), but smaller relationships with deeper tissue (FAS, r = .19, R2 = .04; SUP r = .29*, R2 = .08; DEEP, r = .46***, R2 = .22). Stiffness measured by the MyotonPRO was negatively correlated with SAT in every region under every condition. When comparing SAT with stiffness measured by the MyotonPRO, REL R2 values were .32, .14, .67, in NEU R2 values were .54, .31, .47, and in PAST R2 values were .53, .35, .61 at proximal, medial and distal portions of the muscle, respectively.
CONCLUSION:
These data indicate that the MyotonPRO measures different mechanical properties to SWE and/or is potentially influenced by the stiffness of overlying adipose and connective tissue. The results suggest that the two methods cannot be used interchangeably to estimate stiffness of the RF muscle of young, healthy adults (Bravo-Sánchez et al., 2021).