Effects of knee and hip flexion angle on region-specific and longitudinal compartmental muscle stiffness of the rectus femoris using shear wave elastography in male bodybuilders.

PURPOSE: To investigate the effects of different knee and hip flexion angles (KFA and HFA) on region-specific and longitudinal compartmental muscle stiffness of the rectus femoris (RF) using shear wave elastography (SWE). As muscle mechanical tension is a key indicator of activation, measurements of muscle stiffness offer new insights into muscle contractile dynamics.

METHODS

Isometric knee extension (KE) was performed by nine male bodybuilders at maximum voluntary contraction (MVC) using a dynamometer under combined conditions of KFA at 30°, 60°, and 90° and HFA at 0°, 40°, and 80°. Muscle stiffness during KE was measured in the RF, vastus lateralis (VL), and vastus medialis (VM) using SWE. The RF and VL were divided into proximal, middle, and distal regions, and the VL, VM, and RF were compared across joint angle conditions and regions. Additionally, muscle activity during the KE was assessed in the proximal, middle, and distal RF using multichannel electromyography (EMG).

RESULTS

Muscle stiffness in the proximal RF was higher at HFA 0° and 40° compared to HFA 80° under KFA 30° conditions (p < 0.05). The stiffness in the middle RF was higher than that in the middle VL at HFA 0° and 40° under a KFA 30° (p < 0.01). EMG revealed no differences across regions or joint angles.

CONCLUSION

Performing KE with the hip in an extended position enhanced the stiffness of the RF, particularly in the proximal region, compared to the VL. While region-specific muscle activity was observed using SWE, these differences were not detected using EMG.