The aim of the current research is to study force decrease and energy distribution of seam region in sport apparel experimentally and theoretically. To do so, the major contributing factors were being considered including the seam type, the stitch type and density, the fabric material and weft knitting pattern. Thus, to reduce the number of tests, time and costs, Taguchi design of experiment was utilized in this study. According to this method, a total of eight weft knitted fabric samples varying in knitting structures (single jersey and double jersey) and material being used (polyester-lycra and viscose-lycra, both containing 4% lycra fibers) were produced. Furthermore, two types of seam (lapped and flat), two types of stitch (zigzag lockstitch and covering chain stitch) along with two stitch density (13 and 20 per inch) were considered as sewing parameters. In the experimental part, cyclic dynamic test was performed on the fabric samples and the force decrease diagram was extracted. In theoretical model, Vassiliadis geometrical equations were used to derive the geometrical shapes of loops through Python. Then, through using CATIA software, the knitting fabric structures with sewing yarns were designed. Afterwards, appropriate boundary conditions were applied on them to achieve the intended mechanical properties. Consequently, the elastic and viscoelastic properties of each meso-scale model were obtained and employed as inputs in subsequent macro-scale modeling. Since the viscoelastic properties of weft knitted fabrics are different in course and wale directions, therefore VUMAT subroutine was used to define these properties. As a final step in modeling, a similar cyclic loading to experimental one was applied on macroscopic models. Comparing the obtained results from both methods verified the suitability of the current modeling; however, the highest error was calculated 15% which is acceptable. Moreover, the viscoelastic properties of the stitched weft knitted fabric was obtained from macroscopic modeling to predict the force decrease after an hour. Then, using Tagouchi method, the effect of various parameters on the curve of force decrease-time behavior was evaluated. The optimum fabric sample which exhibited the lowest force decrease, based on the statistical results, was the 20 per inch stitched double jersey rib knitted fabric containing viscose-lycra yarn along with the seam and stitch type of lapped and zigzag lockstitch, respectively. In another section, the effect of absorbed energy in the seam area was investigated using the method of pulling the sewing yarn out from the fabric structure in both experimental and numerical modes. The results showed that most of the work performed was related to viscose-lycra fabric, with rib double jersey pattern and covering chain stitch. According to the theoretical and experimental findings of this study, a high-performance and durable sport apparel could be designed based on the final application.