The goal of this observational study is to determine whether there is a decrease in muscle mass and the relationship between lower extremity skeletal muscle mass, muscle strength and disease activity in Ankylosing spondylitis.

The main questions it aims to answer are:

• Is there a relationship between the muscle thickness and pennation angle of the quadriceps, Gastrocnemius medialis and lateralis, Vastus medialis and lateralis and tibialis anterior muscles with disease activity and muscle strength?
• Are there any differences in the results of morphological parameters of lower extremity muscles between Ankylosing spondylitis and voluntary participants? Researchers will compare voluntary participants to see if any difference in lower muscle morphological parameters.

Ankylosing spondylitis (AS) Ankylosing spondylitis (AS) is a systemic inflammatory rheumatic disease that can cause characteristic inflammatory low back pain and structural and functional disorders by affecting the axial and peripheral skeleton.

Functional impairment increases with age, disease duration, and severity of symptoms. It has been shown that there is a significant relationship between dysfunction and activity limitation in AS patients.

Recent studies have reported that both sarcopenia and decreased muscle strength occur in patients with chronic inflammatory diseases such as rheumatoid arthritis3. There are concerns that sarcopenia may affect exercise tolerance, activities of daily living, and ultimately have a negative impact on cardiovascular fitness and physical and emotional well-being. Given the risk of chronic inflammation and reduced physical activity, patients are also at risk of accelerated muscle wasting. In chronic diseases, loss of muscle mass, often referred to as cachexia, is a relevant systemic complication that leads to decreased muscle strength and endurance, reduced physical activity and fitness.

However, there is no study reporting variation in lower extremity information, muscle development, morphological effects measured by ultrasound, muscle strength and disease intensity in AS patients. Therefore, this study was conducted to determine whether there is a decrease in muscle according to the wide general spread in AS patients and to strengthen the detachable lower extremity structure with AS, muscle strength and possible presence. The relationship between strength, power and mobility of skeletal muscles was also evaluated. Although muscle loss was not morphologically evident in previous ultrasounds, it has not been investigated whether this leads to loss of muscle strength.

Inclusion Criteria:

• Being ≥18 years old,
• Being diagnosed with AS according to The Assesment of Spondyloarthritis international society (ASAS) criteria
• Ability to giving consent
• Ability to live independently
• Ability to walk without assistive devices.

Exclusion Criteria:

• History of surgery on the spine or lower extremities,
• History of inflammatory rheumatic disease,
• Severe cardiovascular disease,
• Neuromuscular disease affecting muscle strength,
• Individuals following a regular exercise program,
• Secondary osteoarthritis.

Patients who applied to the Bezmialem Vakıf University, Physical Medicine and Rehabilitation Outpatient Clinic appropriate for eligibility criteria will be enrolled.