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

Deep muscle tissue was consistently stiffer than superficial muscle tissue across almost all conditions and regions.

• DEEP was consistently stiffer than SUP muscle tissue in all conditions and regions (all p < 0.01), except for the relaxed proximal (REL PROX) condition.
• 20 healthy participants were assessed across two visits using shear wave elastography (SWE).
• Measurements were taken at three muscle regions (proximal, medial, distal) and three muscle lengths (relaxed, neutral, passively stretched).

Hip extension increased proximal tissue stiffness above medial and distal regions and above conditions of hip flexion.

• Hip extension increased proximal tissue stiffness above medial and distal regions (all p < 0.001).
• Hip extension also increased proximal stiffness above conditions of hip flexion (p < 0.001, p = 0.004 and p = 0.002, respectively).
• This effect was observed across fascial (FAS), superficial (SUP), and deep (DEEP) muscle tissue.

Knee flexion increased distal tissue stiffness above conditions of knee extension.

• Knee flexion increased distal tissue stiffness above all conditions of knee extension (all p < 0.001).
• This effect was observed in fascial, superficial, and deep muscle regions.
• The finding indicates that local strain at the distal end of the biarticular rectus femoris elevates stiffness in that region.

Passive stretching by combined hip extension and knee flexion removed regional differences in stiffness across fascial, superficial, and deep tissue and increased medial stiffness above relaxed and neutral conditions.

• The passively stretched (PAST) condition, achieved by combined hip extension and knee flexion, removed differences between proximal, medial, and distal regions in FAS, SUP, and DEEP tissues.
• PAST increased medial shear wave velocity (SWV) above both REL and NEU conditions (all p < 0.001).
• This suggests that when the entire muscle is under stretch, the medial region experiences increased strain compared to less stretched conditions.

Regional differences in shear wave velocity across all tissue types (fascial, superficial, and deep muscle) were dependent upon local strain conditions.

• SWV regional differences were observed in FAS, SUP, and DEEP tissues across varying muscle length conditions.
• The pattern of regional stiffness differences was consistent across all three tissue types assessed.
• The study used shear wave elastography (SWE), a technique with minimal prior application to deep muscle fascia in the literature.

The rectus femoris fascia was assessed by shear wave elastography alongside superficial and deep muscle regions, addressing a gap in the literature on mechanical properties of deep muscle fascia.

• Mechanical properties of the deep muscle fascia are described as important in myofascial force transmission and injury, yet investigation by SWE in the literature was minimal prior to this study.
• Three tissue layers were assessed: deep muscle fascia (FAS), superficial muscle (SUP), and deep muscle (DEEP).
• Measurements were taken at three anatomical regions (proximal, medial, distal) and three standardised muscle lengths across 20 healthy participants.