## 1 Introduction  
Osteoarthritis (OA) is the most common joint disease worldwide, with age-associated increases in both incidence and prevalence [1]. Debilitating chronic pain and reduced mobility, a leading symptom of OA, severely decreases the quality of life in approximately 10% of men and 18% of women aged > 60 years worldwide [2]. Given its increasing prevalence and incidence, OA is now considered a major public health problem. The incidence of osteoarthritis increases with age, but it is not an inevitable consequence. Nevertheless, age is the strongest predictor [3]. Other notable risk factors include gender, race/ethnicity, bone density, postmenopausal estrogen decline, nutritional factors, and heredity. Biomechanical factors include obesity, quadriceps weakness, joint damage, joint trauma, and malformation [1]. Current treatments including pain relief, pharmaceutical drugs (e.g., non-steroidal anti-inflammatory drugs), lifestyle modifications (e.g., weight loss and exercise), intra-articular therapies, and surgical interventions are available to alleviate pain and improve joint function; however, their efficacy remains limited [2, 3, 4]. Thus, continuous efforts are needed to develop novel and effective therapeutic strategies. A growing body of evidence indicates that skeletal muscles, which appear irrelevant to the function or pathology of the attached joint, play important roles in the induction and progression of OA [5; 6]. A systematic review and meta-analysis showed that knee extensor muscle weakness is a risk factor for knee OA development [7]. Strategies to improve skeletal muscle health are increasingly recognized as important for improving symptomatic and functional outcomes in patients with knee OA. Lower limb muscle strengthening exercises are recommended by clinical practice guidelines as a core component of knee OA management [6; 8; 9]. Furthermore, a randomized clinical trial showed that quadricep strengthening exercises are effective in improving pain, function, and quality of life of patients with knee OA [10], and high-intensity resistance exercise demonstrated moderate quality of evidence for large and sustained improvements in knee muscle strength in patients with knee OA [11]. Although proper exercise is recommended by clinical guidelines as a key component for OA management [12; 13; 14], it is not always possible for patients with OA to perform such exercises to achieve muscle strengthening due to severe pain and weakened muscles. In this study, we tested whether a new MEST device, which delivers specially designed polydioxanone (PDO) filaments to major skeletal muscles associated with OA, affects the symptoms of OA. PDO has gained increasing interest in the medical and pharmaceutical fields owing to its degradation to low-toxicity monomers in vivo. Previous animal studies have shown that PDO threads have the advantages of high safety, ease of procedure, high efficiency, and lower risk of side effects [15; 16; 17]. Therefore, the PDO led to stress-shielding phenomena by unloading the autograft from tensional stimulation of fibroblasts in the tissue [18]. Paying attention to these points, in this study, we tried to confirm the improvement effect of OA symptoms using PDO filaments in OA-induced joint pain in terms of physical activity reduction and muscle weakness. For this purpose, flexible filaments equipped with special cogs were used. Cog PDO filaments may effectively hold and support surrounding muscles to increase joint stability and relieve pain. Cog filaments may also stimulate muscles for physiological changes such as muscle growth when directly inserted into muscles. Inserted filaments are designed to be securely anchored to muscle tissue using cogs, whereas quadriceps keep the contract and relax repeatedly in daily movements. Sustained and repeated mechanical stimuli of surrounding muscles are anticipated by inserted and securely anchored cog PDO filaments.