## 1 Introduction  
Shoulder impingement syndrome causes supraspinatus tendon injury to throwing athletes such as baseball players [1; 2]. The syndrome is also present in the general public, including computer workers and the elderly, as well as elite athletes engaged in throwing activities [3; 4; 5]. Recently, studies are underway to identify the main causal factors in shoulder impingement syndrome and to develop appropriate prevention and treatment plans. The clinical cause of shoulder impingement syndrome has been known as supraspinatus tendon injury, caused by contact between musculoskeletal tissues inside the joint (subacromial space) [6]. However, the biomechanical mechanism still has not been clarified [2; 7; 8]. This study, which aims to identify the biomechanical mechanism of shoulder impingement syndrome, is essential in that supraspinatus tendon injury can lead to additional critical disorders such as rotator cuff [9; 10; 11; 12] or superior labrum anterior to posterior (SLAP) tears [13; 14; 15]. Typically, shoulder impingement syndrome consists of both external and internal impingement syndromes. External impingement syndrome involves a lesion or injury of the supraspinatus tendon caused by contact with the acromat subacromial space. External impingement syndrome cannot explain posterior shoulder pain during shoulder motion. Internal impingement syndrome analyses are required to explain posterior shoulder pain. Internal impingement syndrome involves a lesion or injury of the supraspinatus tendon caused by contact between the labrum and humeral head during shoulder motion. Internal impingement syndrome is thought to be fundamentally complex and multifactorial in its explanation of posterior shoulder pain [2]. Traditional studies for internal impingement syndrome estimate the mechanism of shoulder impingement syndrome through various methods based on arthroscopic findings, image diagnoses, and cadaveric experiments. The arthroscopic finding-based method [16] involves invasive observations of the interactions between musculoskeletal tissues in vivo. This method has the disadvantage that it cannot be used to experiment for injuries of soft tissues because of ethical issues. The image diagnosis-based method [3; 6] noninvasively analyzes soft tissues in vivo by using medical imaging information scanned from magnetic resonance imaging (MRI) results. However, this method has a low accuracy problem because it measures the three-dimensional (3D) shape of internal impingement syndrome on a two-dimensional (2D) image. The cadaveric experiment-based method [14; 17] measures the deformation of musculoskeletal tissues by using a mechanical device. This method has a low reliability problem because it is difficult to obtain sufficient samples [18]. In other words, these methods are not suitable for the analysis of shoulder impingement syndrome because the arthroscopic finding-, image diagnosis-, and cadaveric experiment-based methods cannot measure the contact stress that occurred in vivo in the shoulder joint. Conventional studies are underway to analyze the biomechanical mechanism of shoulder impingement syndrome through computer simulations by using the finite element method (i.e., FE simulations). The FE simulations allow us to analyze the injury of musculoskeletal tissue in vivo by using a 3D biometric model. This method measures the contact stress between musculoskeletal tissues in vivo. As above, most of the previous studies have focused on external impingement syndrome among the shoulder impingement syndromes. Specifically, there were no reports found of internal impingement syndrome among the shoulder impingement syndromes studied by using the finite element method. In this paper, we propose a new contact stress measurement method for analyzing internal impingement syndrome of the shoulder. The proposed method simulates internal impingement syndrome of the supraspinatus tendon between the humeral head and labrum based on the finite element method.