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

## TL;DR

This study shows that muscle and fascial stiffness in the rectus femoris varies by region and is influenced by localized stretching and joint angles.

## Contribution

The study reveals novel regional stiffness differences in biarticular muscle and fascia under localized strain using shear wave elastography.

## Key findings

- Deep muscle tissue is consistently stiffer than superficial tissue across all conditions and regions.
- Hip extension increases proximal stiffness, while knee flexion increases distal stiffness.
- Stretching the muscle removes regional stiffness differences and increases medial stiffness.

## Abstract

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.

Measured by shear wave elastography, deep rectus femoris muscle tissue is stiffer than superficial muscle tissue, regardless of measurement location.Regional differences in biarticular rectus femoris muscle and fascia stiffness are dependent on localised tension stress altered by manipulating joint angles. Hip extension increases proximal stiffness and knee flexion increases distal stiffness.Localised responsiveness to tension in biarticular muscles may have implications in region‐specific mechanisms of injury.

Measured by shear wave elastography, deep rectus femoris muscle tissue is stiffer than superficial muscle tissue, regardless of measurement location.

Regional differences in biarticular rectus femoris muscle and fascia stiffness are dependent on localised tension stress altered by manipulating joint angles. Hip extension increases proximal stiffness and knee flexion increases distal stiffness.

Localised responsiveness to tension in biarticular muscles may have implications in region‐specific mechanisms of injury.