Determines the presense of a fixed flexion deformity at the hip (among other things..)
With one hip passively fully flexed if the CONTRALATERAL (opposite) hip rises off the bed it's a
POSITIVE for fixed hip flexion deformity (of that hip)
Look at angle of the femur (is it parallel to the floor or going up?)
This may be due to tightness/restriction in the joint capsule, ilopsoas or rec femoris
How to differentiate between the source of restriction you say!?
Passively extend patient's knee and then we'll see...
If this results in the patient's hip dropping down then it's REC FEM baby!
WHY? Because by extending the knee an element of the stretch has been taken off.
If the hip remains unaffected by this and remains in same degree of flexion then ilopsoas is your culprit.
Look at the angle of the knee, is it falling back loosely or staying held up in extension?