The type of this study is an observational prospective study. It will be done to determine the oxygenation status of the intercostal muscles and quadriceps femoris muscle during exercise in patients with fibrosing lung and to examine its relationship with exercise capacity, respiratory functions and respiratory muscle strength. The main questions that the study aims to answer are:

• Question 1: Do changes in muscle oxygenation during exercise affect respiratory functions in patients with Fibrosing Lung?
• Question 2: Do changes in muscle oxygenation during exercise affect exercise capacity in patients with Fibrosing Lung?

Participants; demographic information such as age, height, weight will be questioned. Respiratory functions will be evaluated with a desktop spirometer, peripheral muscle strength measurement will be evaluated with a digital myometer, and functional capacity will be evaluated with a 6-minute walk test (6MWT). The Moxy device, which is a non-invasive near-infrared spectroscopy (NIRS), will be attached to the upper leg (the vastus lateralis of the quadriceps muscle) and the rib (intercostal muscles) with a silk patch, and the oxygenation of the muscles here will be measured during the 6-minute walking test. In addition, fatigue status will be evaluated with the Modified Borg Scale.

Interstitial Lung Disease (ILD); It constitutes a heterogeneous group of approximately 200 diseases with acute or chronic course that affect the lung diffusely and cause varying degrees of inflammation, fibrosis and structural deterioration in the lung parenchyma. Clinical features of ILD include exercise-induced dyspnea, exercise-induced hypoxemia, progressive skeletal muscle weakness, and impaired exercise tolerance. The main symptoms of fibrosis are progressive decreases in forced vital capacity (FVC) and diffusing capacity of the lung for carbon monoxide (DLCO). Restriction of pulmonary ventilation leads to respiratory failure, which is exacerbated by capillary destruction and decreased alveolar blood flow resulting from hypoxic vasoconstriction.

Exercise intolerance is a key feature of ILD and is usually associated with significant dyspnea on exertion. Exercise limitation in ILD is related to altered respiratory mechanics, impaired gas exchange, and circulatory limitation. Evidence suggests that pulmonary rehabilitation can provide significant benefits in patients with ILD. Daily use of corticosteroids for >1 year significantly reduces muscle function in patients with chronic respiratory disease. Muscle size, strength, and functional outcomes were evaluated in ILD patients. Decreased quadriceps strength and endurance were seen in patients with fibrotic idiopathic interstitial pneumonia compared to healthy controls. Compared to the upper extremity, more atrophy and weakness were found in the lower extremity muscles. Therefore, exercise training is a critical component of pulmonary rehabilitation. The effect of exercise on blood oxygenation has been little studied in ILD patients, and the few data available in the literature are conflicting.

In the studies conducted, there are studies stating that there is a positive improvement in blood biochemistry as a result of an acute exercise, as well as a change with chronic exercises. An important goal of hemodynamic monitoring is the early detection of inadequate tissue perfusion and oxygenation. The use of simple, non-invasive monitoring techniques has the advantage of facilitating earlier initiation of therapy. Near-infrared spectroscopy (NIRS) is a noninvasive measurement instrument that does not impair skin integrity, allowing the measurement of muscle hemoglobin-oxygen saturation (StO2) during exercise. There are few studies in the literature where muscle oxygenation was measured with the NIRS device. According to the results of our literature review, our study will be the first study to examine muscle oxygenation in fibrosing lung patients.

Inclusion Criteria:

• Being between the ages of 18-75.
• Have a diagnosis of fibrosing interstitial lung disease diagnosed according to the clinical diagnostic criteria of the American Thoracic and European Respiratory Societies (ATS-ERS).
• Presence of dyspnea on exertion Stable clinical state at the time of admission without infection or exacerbation in the previous 4 weeks.
• Ability to use a smart phone.

Exclusion Criteria:

• Patients with severe comorbid diseases, unstable coronary artery disease, collagen vascular diseases and needing high flow oxygen therapy (˃ 3-4 L \min).
• History of effort-related syncope or any comorbidity (such as severe orthopedic or neurological deficits or unstable heart disease) that precludes exercise training.
• Participating in a pulmonary rehabilitation program within the past 12 months.

The sample of the study will consist of male and female patients between the ages of 18-75 who have been diagnosed with Fibrosing Lung Disease (Interstitial Lung Disease) in Istanbul Yedikule Chest Diseases and Thoracic Surgery Training and Research Hospital.