A three-dimensional approach to pennation angle estimation for human skeletal muscle

Pennation angle (PA) is an important property of human skeletal muscle that plays a significant role in determining the force contribution of fascicles to skeletal movement. Two-dimensional (2D) ultrasonography is the most common approach to measure PA. However, in principle, it is challenging to infer knowledge of three-dimensional (3D) architecture from 2D assessment. Furthermore, architectural complexity and variation impose more difficulties on reliable and consistent quantification of PA. Thus, the purpose of our study is to provide accurate insight into the correspondence between 2D assessment and the underlying 3D architecture. To this end, a 3D method was developed to directly quantify PA based on 3D architectural data that were acquired from cadaveric specimens through dissection and digitization. Those data were then assessed two-dimensionally by simulating ultrasound imaging. To achieve consistency over intermuscular variation, our proposed 3D method is based on the geometric analysis of fascicle attachment. Comparative results show a wide range of differences (1.1-47.1%) between 2D and 3D measurements. That is, ultrasound can under- or over-estimate PA, depending on the architecture.