Rectus femoris stiffness is associated with aerobic capacity in healthy young men: a comparative shear wave elastography study among lower-limb muscles.

BACKGROUND: This study aimed to comparatively investigate the relationships between the ultrasonographic architectural and viscoelastic properties of the soleus, medial gastrocnemius (MG), and rectus femoris (RF) muscles and objectively measured aerobic capacity in healthy young adult males. METHODS: This observational cross-sectional study included 59 healthy male medical students aged 18-30 years. The muscle thickness, pennation angle, fascicle length, and shear wave elastography (SWE) stiffness of the right lower-extremity RF, MG, and soleus were measured ultrasonographically. Aerobic capacity was evaluated using cardiopulmonary exercise testing (CPET) with the Bruce protocol, and VO₂max was recorded as the primary outcome. Participants were stratified into low- and high-aerobic capacity groups based on the median VO₂max value (29.4 mL·kg⁻¹·min⁻¹). Associations were examined using correlation analysis and multiple linear regression. RESULTS: No significant between-group differences were observed in any ultrasonographic or SWE parameter (low vs. high aerobic capacity groups). Conventional architectural parameters (muscle thickness, pennation angle, and fascicle length) showed no significant correlations with VO₂max across all assessed muscles. RF SWE velocity demonstrated a significant positive correlation with VO₂max (r = 0.364, p = 0.005), designated a priori as the primary comparison; RF Young's modulus showed a smaller, exploratory correlation (r = 0.274, p = 0.036) that did not survive multiplicity correction, and no soleus or MG SWE parameters correlated with aerobic capacity. In multiple linear regression analysis, RF SWE velocity (β = 0.280, p = 0.021) and BMI (β=-0.555, p < 0.001) were independently associated with VO₂max (R²=0.310). CONCLUSIONS: RF shear wave velocity is independently associated with aerobic capacity (VO₂max) in healthy young men, whereas no significant association was detected for the soleus or MG. These associations were not formally compared between muscles.