Computational modeling for musculoskeletal system provides quantitative insights in analyzing human movement. One of the major challenges in neuromusculoskeletal modeling is to accurately estimate the musculotendon parameters on subject-specific basis. The ultrasound imaging technology presents a new approach to obtain the parameters in vivo. The pennation angle and fascicle length of brachialis (BRA) were measured in vivo with the use of ultrasonography to investigate the relationship between these muscle architecture parameters and elbow joint position. A generic interactive graphics-based geometrical model of the upper limb and BRA was developed to get the musculotendon length and moment arm of the muscle. The results indicate human brachialis architecture is significantly affected by changing of joint angle at passive horizontal movement. These in-vivo measurements provide subjects-specific information and these muscle parameters can be used to set up the neuromusculoskeletal model for the muscle force and torque estimation. ; Author name used in this publication: L. Li ; Author name used in this publication: K. Y. Tong ; Jockey Club Rehabilitation Engineering Centre