# Fatigue Effects on the Lower Leg Muscle Architecture Using Diffusion Tensor MRI
## 1 Introduction  
The architecture of skeletal muscle, which is defined as the arrangement of muscle fibers relative to the axis of force generation [1], is the main determinant of the mechanical behavior of skeletal muscles [2, 3]. Muscle fibers differ in their mechanical, histological, morphological, biochemical and biomechanical properties, allowing them to have different functionalities. During force production, there are several important muscle architectural parameters to consider. Muscle architectural parameters, including muscle fiber length, physiological cross-sectional area (PCSA) and pennation angle (the angle between muscle fibers and the tendon plate), are associated with force production [4, 5]. The anatomical cross-sectional area (ACSA) is simply the largest cross-section across the whole muscle, while the PCSA is the architectural parameter that is directly proportional to the maximum force generated by the muscle because it takes into account the pennation angle [6, 7, 8, 9]. It is also the parameter that is most difficult to measure because its value is based on the entire muscle volume and includes the estimate of fiber length to reach the value for the area across the entire muscle [9]; therefore, it is indirectly determined from the muscle volume and muscle fiber length. Computational models of the musculoskeletal system are widely used to study the mechanisms of musculoskeletal performance and to simulate specific conditions [10]. They were created based on anatomical data collected from cadaveric specimens. Models allow us to answer \"what if\" questions; isolate individual sources of input data; and estimate muscle parameters, such as muscle forces, that are difficult to measure experimentally [11; 12]. To estimate the muscle volume or PCSA, the data of muscle force-generating capacities derived from cadaveric studies are often used to scale models of the muscle-tendon unit [10; 13; 14]. The use of those databases allows for a better understanding of general principles of muscle function, but the comparability to individual muscle performance remains unclear [15]. It is likely that data from cadaveric specimens do not accurately reflect the absolute or relative sizes of muscles in young and healthy subjects [16]. Moreover, cadaveric studies are usually based on a small number of specimens [17; 18] and the fascicle length measured from each muscle are averaged in most cadaveric studies, and thus, muscle models usually assume that all fascicles within each muscle have the same length [19]. The same happens with the pennation angle: simulation models commonly assume muscle fibers to be one single structure with only one angle between the fiber's direction and the aponeurosis. Despite a valid and reliable 3D ultrasound technique for the assessment of the gastrocnemius medialis existing in the literature [20], no comprehensive ultrasound technique that allows for three-dimensionally measuring the architectural parameters of the anterior and posterior leg muscles exists. Therefore, this study used proton density and diffusion tensor imaging for 3D measurements of the leg muscles simultaneously [21]. Thus, the need to create accurate, individualized models of the musculoskeletal system is driving advances in imaging techniques, especially in magnetic resonance imaging (MRI) [22; 23; 24]. MRI is considered the most useful non-invasive imaging technique (although it is expensive and post-processing is laborious) that allows for the reconstruction of the whole muscle, enabling the quantification of detailed structure, function and metabolism from the muscle tissue [25]. MRI offers an abundance of novel methods for the acquisition of data from living subjects. One of those methods is diffusion tensor imaging (DTI) [26; 27; 28; 29]. This imaging technique relies on the correspondence between cell geometry and the anisotropic nature of water diffusion in the muscle [28].