Effects of isometric exercise and stretching on rectus femoris muscle elasticity and shear wave elastography reliability

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Introduction: Ultrasound elastography, particularly shear wave elastography (SWE), has emerged as a practical and quantitative method for evaluating muscle elasticity in both clinical and research settings.Despite its growing use, the influence of measurement depth, exercise, and stretching on muscle elasticity remains inconsistently reported.Furthermore, comprehensive investigations that simultaneously evaluate intra-rater, inter-rater, and testretest reliability within the same study are limited.The rectus femoris (RF), a biarticular muscle of the quadriceps group, plays an essential role in lower-limb function, making it a relevant target for elasticity assessment.This study aimed to determine differences in RF elasticity across measurement depths, examine the effects of isometric exercise and stretching, and assess the reliability of SWE measurements.Methods: 45 healthy adults (21 men, 24 women; mean age 21.1 ± 1.45 years) with no history of orthopedic or neurological disorders of the lower limbs participated.Both lower limbs of all participants were examined, providing a total of 90 limbs.RF elasticity was measured along the longitudinal axis while participants were in a relaxed supine position.All participants performed straight leg raising (SLR) with isometric contraction of the RF and a self-administered stretch of the RF on one randomly assigned lower limb for 1 minute per set, for three sets each.The effects of these interventions on RF muscle elasticity were evaluated using SWE.Intra-rater reliability was evaluated using intraclass correlation coefficients (ICC [1,3]) on 70 limbs from 35 participants, while inter-rater and test-retest reliability were assessed on 20 limbs from 10 participants using ICC [2,2] and ICC [3,2], respectively.Results: Before the interventions, the upper region of the RF demonstrated significantly greater muscle elasticity than the middle and lower regions (p < 0.001).In addition, females demonstrated significantly greater pre-intervention RF muscle elasticity compared to males (p < 0.05).Neither the SLR exercise nor the stretching produced significant changes in RF muscle elasticity.The ICCs were 0.754 for intra-rater reliability, 0.709 for inter-rater reliability, and 0.616-0.692for test-retest reliability; all values were classified as "substantial."Conclusion: Although statistically significant differences in RF muscle elasticity were observed across measurement sites and between males and females, no statistically significant changes were found following isometric exercise or stretching in this study.These findings highlight the need for further research involving a wider range of age groups and patient populations, as well as investigations into the effects of different intervention intensities, frequencies, and durations.