The purpose of this investigation was to measure in vivo fascicle length (L_fas) and pennation angle (PA) of the long head of the biceps femoris muscle (BF_lh m.) at different hip and knee angles while the muscle was relaxed using ultrasonography (US). Data were collected from 18 healthy females (23.0 ± 1.8 years). To validate the measurements of the L_fas and PA, the US measurements on cadavers (n = 5), embalmed with the hip and knee in anatomical position, were compared to actual fiber lengths and PA from the dissected muscles. US images from volunteer subjects were recorded when the hip joint was positioned at 0°, 45° and 90° of hip flexion, and at each hip position, the knee joint was placed at 0°, 45° and 90° from full extension. The images were digitized to measure L_fas and PA. Results showed no significant differences between US and direct measurements of the L_fas and PA on the cadaver BF_lh m. (p > 0.05). L_fas and PA changed significantly between the different hip and knee positions in volunteer subjects (p < 0.05). Changes in the L_fas and PA are more sensitive to changes in hip position with the knee position constant than to changes in knee position with the hip position constant. This difference may be related to the larger muscle moment arm at the hip resulting in greater excursion of the muscle with changing hip position. Based on the changes in L_fas with changing joint positions, BF_lh sarcomere length was estimated to occupy a portion of the ascending limb, the plateau and descending limb of the length-tension relationship over the range of motion studied. US scanning is valid and reliable for measuring L_fas and PA of the BF_lh m. An understanding of the dynamic nature of muscle architecture will assist in determining effective and efficient clinical evaluation and rehabilitation techniques.

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