Lateral Epicondylitis; is a disease characterized by insidious onset pain in the lateral elbow of the forearm, which radiates to the distal part of the forearm and increases with grip and wrist extension. Pain originates from the origin of the wrist and finger extensors and is more felt during repetitive, forceful wrist extension or pronation and supination, during exercise or occupational use. It is a musculoskeletal lesion. Although it is so common, no consensus has yet been reached regarding its clinic, pathophysiology, and treatment. It is known that the primary etiological factor in the pathology of lateral epicondylitis is the overloading of the aponeurosis of the joint extensor muscles attachment site. Repetitive overuse causes tendon damage with macroscopic abnormalities of tendon collagen. The final stage of tendinopathy is characterized by abnormal tendon structure and degenerative features, including neovascularization. Primary pathological changes occur at the proximal musculotendinous insertion of the Extensor carpi radialis brevis. The currently accepted theory is that the process begins with overuse injuries that lead to small tears of the extensor carpi radialis brevis, sometimes the extensor digitorum communis muscle.

The prevalence of lateral epicondylitis peaks between the ages of 35 and 55, and lateral epicondylitis primarily affects the dominant side. There is no clear consensus on the involvement of men and women, and it appears independent of gender and ethnicity [6-8]. Due to the symptoms experienced, the people's daily life activities are affected and cause loss of workforce.

Conservative therapy is usually the first line of treatment for lateral epicondylitis. Conservative treatment typically includes rest, non-steroidal anti-inflammatory drugs, and physiotherapy and rehabilitation. Physiotherapy and rehabilitation applications include activity modification, orthosis use, cold-hot application, deep friction massage, stretching and strengthening exercises, electrical stimulation, ultrasound, laser, extracorporeal shock wave therapy, and manual therapy. In addition to FTR approaches, invasive procedures such as corticosteroid/botulinum toxin/glucosamine/autologous injections, prolotherapy, acupuncture, and topical nitric oxide application can be used. In cases where conservative treatment is insufficient, surgical applications are used. Although there are many different treatment methods known in the literature, the superiority of a particular approach for the treatment of lateral epicondylitis has not yet been proven and a consensus has not been reached.

It has been known for a long time that eccentric exercises based on the extension of the muscle length can cause damage to the muscle fibers due to stretching and late-onset muscle pain. However, when they are applied in a regular and controlled manner, they adaptively strengthen and protect the muscle tissue. In clinical and animal studies, it has been found that reaching muscle length at an angle greater than the optimum angle of the muscle causes eccentric exercise, which in turn reduces muscle damage and increases joint range of motion.

Although studies continue to understand the mechanisms of post-exercise muscle damage and the protective muscle response that develops after exercise, no study has been found in the literature on eccentric stretching applied to patients with lateral epicondylitis. Therefore, the aim of our study is to investigate the effect of eccentric stretching on pain, grip strength, and functional level in patients with lateral epicondylitis.

Inclusion Criteria:

• Being between the ages of 18 - 65
• Having been diagnosed with Lateral Epicondylitis,
• Pain localized in the elbow for more than 3 months

Exclusion Criteria:

• Those who have had major surgery or trauma related to the musculoskeletal system, especially the elbow region
• Steroid injection to the elbow area in the last 3 months,
• Those with neurological disease
• Those who received physiotherapeutic intervention in the elbow area in the last 3 months
• Having infection and local dermatological problems in the treatment area Those with rheumatic disease in the active period
• Those with systemic diseases (Diabetes, hypothyroidism, infection, malignancy...)
• Those with serious psychological problems (BDI score of 30 and above)
• Pregnancy
• Presence of malignancy