Thomas Test (or as it called Hugh Owen Thomas well leg raising test) is used to measure the flexibility of the hip flexor muscles. It’s used to test for hip flexion contracture and psoas syndrome, which is more common in runners, dancers, and gymnasts with symptoms of hip “stiffness” and “clicking” feeling when flexing at the waist.
For more, See:
https://www.orthofixar.com/special-test/thomas-test/
