Supine testing assumes that minimal pelvic and femoral rotation (as seen with the green arrow) will occur will occur as the chair and body act as distal stabilizers of the thigh, however, the subject will often shift the pelvis (hip hitching) to gain a better leaver advantage( as seen with the red arrow).

The seat should extend to allow the appropriate amount of knee flexion. In most tests this would be somewhere towards the distal third of the thigh which would allow 75-90 degrees of flexion (the maximum knee flexion I have seen tested was 110 degrees whilst retaining reproducibility) You really need to test to 90 degrees in this position so you can obtain the peak torque measurement at the optimal angle of peak torque for the rectus femoris. This position then allows maximal extension (although debate rages over whether extension beyond -20 degrees should be permitted. Personally I would not test beyond 0 degrees extension as an absolute maximum, whilst subjects tend to find limitations beyond 5 degrees as irritating and tend to do large isometric contractions to try to complete the range).

Supine stabilization is normally accomplished using femoral and pelvic strapping and the subject is allowed to hold the chair or the handles provided.