## 1 Introduction  
Sub-acromial Impingement Syndrome (SIS), also known as shoulder impingement syndrome, is a common musculoskeletal disorder that afflicts a significant portion of the population [1]. It is a prevalent shoulder pathology characterized by pain, muscle weakness, altered joint position sense, persistent shoulder pain, and functional limitations [2; 3; 4]. This condition is a substantial health concern with implications for quality of life, work productivity, and overall well-being [5]. The primary feature of SIS is the mechanical compression of structures within the sub-acromial space, including the rotator cuff tendons, sub-acromial bursa, and long head of the biceps tendon [6]. This compression results in pain, inflammation, and restricted shoulder movement [7]. Despite its prevalence, the exact etiology of SIS remains multifactorial, encompassing anatomical, biomechanical, and pathological factors [8]. Shoulder muscle strength and proprioception, while distinct in their functions, collectively play a pivotal role in the overall function and stability of the shoulder joint [9; 10]. Shoulder muscle strength is the foundation of shoulder stability and movement [11; 12]. The muscles surrounding the shoulder joint, including the rotator cuff muscles, deltoids, and scapular stabilizers, work together to control and execute precise movements while bearing the load of the upper limb [13]. Adequate strength in these muscles ensures that the shoulder functions optimally, preventing excessive joint stress and the potential for injury [14; 15]. Proprioception, often referred to as the \"sixth sense\", is the ability to sense the position, movement, and orientation of one's body parts without relying on visual cues [16; 17]. In the context of the shoulder, proprioception allows for fine-tuned control of joint movements and muscle contractions [18; 19]. It helps individuals maintain joint stability, execute accurate and coordinated movements, and adapt to changes in joint position or external forces [20]. Without proper proprioceptive feedback, the shoulder's ability to function effectively is compromised, leading to difficulties in tasks that require precision, such as reaching for objects or performing overhead sports activities [21]. An in-depth understanding of the intricate relationships among various contributing factors is paramount to advancing the diagnosis, treatment, and rehabilitation of SIS [22]. Two crucial components within this multifaceted puzzle are shoulder muscle strength and shoulder joint position sense (JPS) [23]. Shoulder muscle strength is a cornerstone of shoulder stability and function, and deficits in strength can disrupt normal shoulder mechanics and increase susceptibility to injury [24]. In contrast, JPS refers to the ability to perceive and replicate specific shoulder joint positions, a critical aspect of proprioception that underpins joint stability and precise motor control [25; 26]. Both muscle strength and JPS are integral to shoulder function and are frequently affected in individuals with SIS [27]. SIS is characterized by mechanical compression in the shoulder joint, which can lead to pain and restricted mobility [28; 29]. To better understand the impact of this condition, it is crucial to not only assess the individual components, such as muscle strength and proprioception, but also to establish a solid link between these elements and the clinical presentation of SIS. By delving beyond mere measurement, this objective centers on determining the extent of impairment in muscle strength and proprioception within the SIS population. To contextualize these findings, a comparison can be drawn with age-matched individuals without shoulder conditions. This comparative approach is pivotal because it illuminates the specific areas of dysfunction in SIS patients, helping to pinpoint which aspects of muscle strength and proprioception are most affected by this condition.