## 1 Introduction  
Maintaining balance and preventing postural instability are critical concerns for middle-aged adults, as these factors are closely linked to an increased risk of falls in older populations [1, 2]. Among middle-aged and older adults, degenerative knee arthritis (DKA)--the most common form of osteoarthritis (OA) affecting the knee joint--is a prevalent condition that leads to knee pain, altered gait mechanics, restricted joint motion, and increased joint loading, particularly during stair descent [3, 4, 5, 6]. This progressive condition is characterized by cartilage degeneration, pain, and stiffness, all of which contribute to impaired mobility and gait dysfunction [7]. The prevalence of OA in middle-aged and older adults is influenced by various factors, including occupational activities, sports participation, musculoskeletal injuries, obesity, and gender [6, 7, 8]. Evidence suggests that older women are at a higher risk of falls and fall-related injuries than men, likely due to biomechanical constraints such as reduced sagittal plane moments, increased frontal plane hip joint moments, and diminished joint power during stair descent [7; 8; 9; 10; 11]. Center of pressure (CoP) movement in the anterior-posterior (AP) and medial-lateral (ML) directions is commonly used to assess balance and postural stability, with increased CoP displacement indicating compensatory control strategies [12; 13]. Stair ambulation, a fundamental activity of daily living (ADL), imposes greater balance demands than level walking [14; 15], requiring higher knee joint loading and an increased range of motion (ROM) in the sagittal plane during both ascent and descent [16; 17]. Notably, stair descent presents even greater postural challenges, as it is associated with increased CoP sway, further elevating fall risk [18]. Several studies have established a strong link between DKA and fall risk, high-lighting the need for targeted interventions [8; 19; 20; 21; 22]. As DKA progresses, it leads to increasing mobility impairments, ultimately contributing to disability. Because knee OA is a progressive and irreversible condition, early-stage treatment typically focuses on conservative management strategies, including joint stress relief, rehabilitation, and physical therapy [5; 6]. Various exercise-based interventions have been explored for DKA, including aerobic exercise, gait retraining, resistance training, and balance training. [PERSON] et al. [23] stated that low-intensity aerobic exercise is beneficial for severe knee OA patients, while high-intensity aerobic exercise is more suitable for mild knee OA patients with chronic conditions, promoting cartilage recovery, enhancing immunity, and alleviating pain. [PERSON] et al. [24] emphasized the role of gait retraining in restoring normal gait mechanics and improving stability. Additionally, prior research has demonstrated that resistance training can partially restore functional deficits, balance impairments, and lower limb loading deficiencies in older women with knee OA [25; 26]. Incorporating balance training has also been shown to enhance knee stability, reduce joint loading, alleviate pain, and improve overall functional performance [20; 27]. Over the past decade, there has been growing interest in resistance training under unstable conditions, including the use of unstable surfaces or unstable load devices such as Swiss balls, BOSU balls, wobble boards, air cushions, slash pipes, and water- or sand-filled bags [28; 29]. While research has primarily focused on unstable surface training, highlighting its benefits for muscle activation, postural stability, and proprioception [30; 31], limited studies have explored the efficacy of unstable load training, particularly using water's inertial properties. Studies suggest that unstable load training enhances stabilizer and antagonist muscle activation, improving upper body strength and joint stability more effectively than stable load training [32; 33; 34; 35].