Beyond mid-thigh: Ultrasound assessment of hamstring muscle architecture at multiple muscle lengths in relation to strength

OBJECTIVES: To investigate the relationship between ultrasound-based muscle architecture parameters and isometric knee flexor muscle strength in healthy individuals, and to evaluate whether distal (25%) and proximal (75%) measurement sites differ in their ability to reflect muscle strength compare to the commonly used mid-thigh (50%) region. METHODS: Isometric muscle strength was measured in 20 healthy (22.15 ± 2.41 y; 8 females, 12 males) participants using a hand-held dynamometer. Ultrasound parameters (cross-sectional area, muscle thickness, pennation angle) of the biceps femoris-long head, semitendinosus, and semimembranosus were measured at 25%, 50%, and 75% of thigh length. Associations between strength and ultrasound-based parameters were examined using multiple linear regression analysis. RESULTS: Moderate-to-high correlations (r = 0.615-0.815) were observed between muscle strength and cross-sectional area for all muscles at 50%. The regression model explained 89% of the variance in muscle strength (p < 0.001). Notably, the cross-sectional area of the semimembranosus and the total hamstring cross-sectional area at 25% length demonstrated strong correlations with muscle strength (r = 0.907 and 0.802, respectively). At 75%, the cross-sectional area of the semimembranosus showed a non-significant association with strength, while it could not be measured for the semitendinosus and biceps femoris-long head due to anatomical limitations. Muscle thickness showed significant correlations for the semitendinosus and semimembranosus (r = 0.702 and 0.698, respectively), whereas pennation angle demonstrated weaker and inconsistent relationships. CONCLUSIONS: The cross-sectional area of the semimembranosus at 50% of muscle length showed the strongest ultrasound-based predictor of isometric muscle strength. However, measurement at the distal site demonstrated robust associations, suggesting this region may serve as a practical alternative. In contrast, proximal measurements were limited and showed non-significant associations with strength. These findings can help expand the clinical and research applications of ultrasound imaging for assessing hamstring strength. Given the exploratory nature of this study, these findings should be interpreted cautiously.