Characterizing Age- and Contraction-Dependent Mechanical Properties of Lateral Hip Soft Tissues Using Shear Wave Elastography

Abstract

Introduction: Hip fractures are a major health concern in Canada, with approximately 95% resulting from falls, and women being 2.9 times more likely than men to sustain one. Muscle contraction influences pelvic stiffness and load distribution, yet ethical constraints limit our understanding of age-related changes in muscle and adipose tissues during falls. Shear wave elastography (SWE) is a non-invasive ultrasound technique that quantifies tissue stiffness using Young's modulus (E), allowing muscle and adipose tissue mechanics to be characterized. Investigating age-related differences in tissue modulus will help bridge gaps in current knowledge, improving hip model biofidelity and supporting hip fracture prevention strategies. Objectives & Hypotheses: There were two main objectives of this thesis. The first objective was to quantify E of muscle and subcutaneous adipose tissue in the lateral hip region across multiple levels of muscle contraction (0, 20, 40, and 60% MVC) in young and older adult women. Based on previous literature, it was hypothesized that: (1) E would increase with increasing levels of muscle contraction in muscle but remain unchanged in adipose tissue; (2) E would be lower in older than younger women; and (3) the increase in E with muscle contraction would be smaller in older women. The second objective was to develop regression equations describing the relationship between muscle contraction level and E for each age group. As this objective was exploratory, no a priori hypotheses were proposed. Methods: Thirty healthy females were recruited and split into groups of younger (18-30) and older (60-70) adults, with inclusion limited to those with a body mass index ≤ 24.9 kg/m². Soft tissue properties were measured using SWE on a research-grade ultrasound system (GE LOGIQ E10) alongside a linear probe (L2-9VN-D). Measurements were taken at the region of peak pressure during lateral falls, posterior-distal to the greater trochanter, while participants performed isometric contractions using visual biofeedback. Two trials per tissue type at each contraction state (0, 20, 40, 60% maximum voluntary contraction) were performed. The ultrasound’s built-in software was used to output E using established acoustoelastic theory. To assess E by muscle contraction and age, tissue-specific two-way mixed measures analysis of variances (ANOVAs) were conducted. Additionally, tissue-specific linear-mixed effects models (LMEMs) were performed to generate exploratory predictive equations for E based on age group and muscle contraction. Results: As hypothesized, muscle E increased significantly with increasing contraction level (p < 0.001). Older adults exhibited lower values overall (p = 0.029), with a significant age × contraction interaction (p = 0.014). Specifically, from rest to 60% MVC, modulus increased by 96.2% in young adults compared to 71.7% in older adults. Age-related differences in muscle E emerged at higher contraction levels (40% and 60% MVC), where younger adults demonstrated greater E. No significant differences were observed in superficial adipose tissue, whereas deep adipose tissue showed a significant increase in E at 60% MVC (p < 0.001). Muscle was modelled using an age × contraction interaction (R²m = 0.56; R²c = 0.85), based on likelihood ratio testing, whereas adipose tissue was modelled using additive effects, as the interaction term did not significantly improve model fit (superficial: R²m = 0.06, R²c = 0.79; deep: R²m = 0.20, R²c = 0.56). Discussion & Conclusions: Lower muscle E in older adults may reflect age-related changes in actin–myosin cross-bridge formation and motor unit recruitment. The observed age-related divergence at higher contraction intensities, may reflect underlying differences in muscle composition or recruitment strategies that alter the relationship between contraction level and muscle stiffness with age. This highlights the importance of accounting for both age and contraction intensity when interpreting muscle mechanical behaviour. The corresponding increase in deep adipose tissue modulus likely results from its position between the contracting gluteus maximus and overlying superficial adipose layer, where muscle thickening at high contraction transmits compressive stress into the deep layer. Together, these findings suggest that age- and contraction-dependent changes in muscle propagate mechanically into adjacent soft tissue layers. The regression equations generated provide a framework for improving the biofidelity biomechanical hip models and enhancing the physiological representation of soft tissue behaviour.