You could endevour to make a useful and practical clinical measure such as having the subject ly in supine. then with a relaxed knee measure the distance from the inferpole of the patelly to a boney landmark such as the AC joint. As long as you maintain the alignment of one to the other then any change in flexion range sould be able to be measured easily. You would insist that the coccyx remain down on the table to standardise the pelvic postion. 8o Not sure about the extension but let me think about it.

Any other practical ideas?