The Reliability and Validity of the Ankylosing Spondylitis Performance Index (ASPI) in Enthesitis-Related Arthritis

This study is testing a physical performance measurement tool called the ASPI (Ankylosing Spondylitis Performance Index) in children and teenagers with a type of juvenile arthritis called Enthesitis-Related Arthritis — a condition that causes joint inflammation, especially where tendons attach to bones. **You may be eligible if...** - You are between 7 and 18 years old - You have been diagnosed with Enthesitis-Related Arthritis by a rheumatologist - You are willing to participate voluntarily **You may NOT be eligible if...** - You have a serious heart, lung, liver, kidney, or neurological disease - You have had major bone or joint surgery - You have significant mental health difficulties that would affect participation Talk to your doctor to see if this trial is right for you.