A fixed flexion deformity of the hip, for example due to osteoarthritis, may not be immediately apparent when the patient is laid on the examination couch, as arching the back will permit the leg to be placed flat.
Thomas' test is a reliable means of assessing such deformity even when both hips are affected. The patient is laid on their back and both hips flexed simultaneously to their limit. This position obliterates any excessive lordosis, and this should be confirmed by placing a hand behind the spine. With one hip held firmly in position - so keeping the pelvis still - the other limb is gently lowered. The knee will be unable to rest on the couch if any flexion deformity is present.
The range of flexion should be noted at the same time. Normally this is about 130 degrees.