Regional Differences in Muscle and Fascial Tissue Stiffness in the Rectus Femoris Are Dependent Upon Localised Stretching

Mechanical properties of the deep muscle fascia are important in myofascial force transmission and injury; however, its investigation by shear wave elastography (SWE) in the literature is minimal. Regional differences in biarticular muscle stiffness have implications in mechanism of injury. To determine region-specific differences in rectus femoris (RF) fascia and muscle stiffness, 20 healthy participants completed two visits in which RF fascia (FAS) and superficial (SUP) and deep (DEEP) muscle regions were assessed by SWE in three muscle regions (proximal - PROX, medial - MED and distal - DIST) and at three muscle lengths (relaxed - REL, neutral - NEU and passively stretched - PAST). DEEP was consistently stiffer than SUP muscle tissue (all p < 0.01) in all conditions and regions, except for REL PROX. Regional differences in SWV in all of FAS, SUP and DEEP were dependent upon local strain. Hip extension increased proximal tissue stiffness above medial and distal regions (all p < 0.001) and conditions of hip flexion (p < 0.001, p = 0.004 and p = 0.002, respectively). Similarly, knee flexion increased distal tissue stiffness above conditions of knee extension (all p < 0.001). Stretching the muscle by hip extension and knee flexion (PAST) removed differences between the regions in FAS, SUP and DEEP and increased medial SWV above REL and NEU (all p < 0.001). These results provide novel insight into regional differences in biarticular muscle and fascial tissue stiffness, implying that local strain increases stiffness in the adjacent region. These findings may have implications in force generation and region-specific mechanisms of injury.