Effect of tasks on intramuscular regional differences in rectus femoris elasticity during isometric contraction: An ultrasound shear wave elastography study.

Purpose: This study aimed to investigate intramuscular regional differences and task specificity of rectus femoris (RF) elasticity during isometric contraction.

Methods: Sixteen healthy males (aged 24.3 ± 4.1 years) participated in this study. The tasks included isometric hip flexion (HF) and knee extension (KE). The contractions were maintained at 0%, 30%, and 60% of their respective maximum voluntary isometric contraction (MVC) forces for 5 s each at 50˚ hip flexion and 90˚ knee flexion. RF elasticity was measured in two regions, proximal (33%) and distal (67%). The shear modulus (kPa), measured by shear wave elastography, was used to determine RF elasticity.

Results: A significant interaction effect was observed (intensity × region × task) (P = 0.01). Regarding regional differences during contraction, the proximal region exhibited a greater shear modulus than the distal region at 60% MVC in the hip flexion task (P = 0.001). Conversely, no regional differences were observed at either 30% MVC (P = 0.625) or 60% MVC (P = 0.568) in the knee extension task.

Discussion: Muscle shear modulus during contraction reflects active force, suggesting that mechanical stress can be applied primarily to the proximal region of the RF during the HF task.