The architectural feature of the rectus femoris (RF) has been scarcely investigated despite its substantial contribution to knee extension torque and large plasticity in the muscularity. This study aimed to examine the reproducibility and validity of ultrasound measurements of RF architecture and interrelationships among the architectural parameters. After confirming the measurement accuracy of an examiner (measurement errors <1%), in vivo and cadaveric measurements of fascicle lengths and pennation angles of RF were performed. Day-to-day reproducibility of measurements was investigated in vivo including muscle thickness measurement. Validity of measurements was investigated by comparing the values between direct and ultrasound measurements for the cadaver. The intraclass correlation coefficients were 0·984, 0·960 and 0·932, and root mean square difference between measured values on 2 days was 0·8 mm, 3·1 mm and 1·4° for muscle thickness, fascicle length and pennation angle, respectively. The validity of measurements was similar or better than those of previous reports on other muscles. We also confirmed a positive correlation between the muscle thickness and the pennation angle as has been shown in other muscles. The current results warrant the use of B-mode ultrasonography for examining the architectural characteristics of RF in vivo.
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