# Fatigue Effects on the Lower Leg Muscle Architecture Using Diffusion Tensor MRI
## 1 Introduction
The theoretical basis for DTI states that the self-diffusion of water in tissue is restricted by membranes and other cellular constituents, resulting in an apparent diffusion coefficient, which is lower than the free diffusion coefficient and is orientation-dependent for elongated structures. Skeletal muscles are composed of fibers that have an elongated, generally cylindrical shape. The diffusion tensor can be described using a 3 \(\times\) 3 matrix, and at least six independent directions for the diffusion gradients must be assessed to calculate the diffusion tensor. A proton density (PD)-weighted scan has its only signal change coming from the differences in the number of available spins (hydrogen nuclei in water). It uses a spin echo or a gradient echo sequence, with a short echo time and a long repetition time. PD images are useful for determining the muscle boundaries due to the elevated detail in the image, enabling the determination of the muscle cross-sectional area and subsequently muscle volume using simple image processing software. Muscle volume is essential when evaluating muscle performance and functional consequences of changes in muscle size and strength due to training, aging or immobilization [15; 30]. Most of the studies regarding these effects in muscle architecture were obtained through ultrasonography, which is limited to planar measurements and can only be used in superficial muscles. Moreover, when dealing with long muscle fibers, the length is not directly measured but is instead estimated via regression. These studies reported increases in muscle thickness and pennation angles of the vastus lateralis muscle following a leg press exercise [31], a cycloergometer test until exhaustion [32] or strength training programs [33; 34]. Regarding the acute effects of exercise analyzed with MRI techniques, the following studies were reported: ankle plantar flexion exercises in the supine position with eccentric contractions showed changes in the gastrocnemius medialis (GM) values of FA, ADC and T2 after exercise [26]; during a walking exercise, followed by a 30 s dorsiflexion exercise, the ADC and T2 values of the tibialis anterior muscle (TA) increased [27]; another study showed that after isometric contraction of repeated plantar flexion and dorsal extension of the right ankle with a tiptoe position, the FA of soleus and MG muscles decreased but gradually recovered to the baseline after one week [35]; and another study showed that after a one-legged plantarflexion exercise, the CSA and T2 values of inactive muscles decreased, with no changes in ADC, and CSA, T2 and ADC values increased in the exercised leg [36]. Despite these results, the architectural muscle parameters that have a linear relationship with force production are still lacking in most of the research that uses this method. Regarding this, the aim of our study was to determine the in vivo 3D architectural parameters (volume, ACSA, pennation angle, fiber length and PCSA estimation) of the lower leg muscles using PD and DTI imaging data before and after an exhaustive unilateral jumping task. These methods were used in the in vivo 3D data acquisition of all the subjects' dominant lower legs, followed by a fiber-tracking algorithm with visualization and analysis tools. We hypothesized the following: (1) due to the fatigue effect and the amount of water content that would circulate in the body after the exercise bout, the muscle volume and cross-sectional areas would increase, as well as the pennation angle of the plantarflexor muscles, and (2) the shape of the analyzed muscles would not be different between subjects in both the pre- and post-exercise conditions.