Many (most) patients have such a weak core, when they arch for a facet loading maneuver, they are co-contracting their extensors reproducing muscle pain. If I'm unsure if it's true positive in standing (in other words, if their site of pain doesn't localize to where I would expect after reviewing the MRI - or if no MRI available and I'm suspecting more muscle) then I have them arch their lumbar spine, palpate the paraspinals and have them relax their back and I support their weight - or if pt is too big, I'll perform the maneuver in sitting. If pain persists then, to me, that is a true facet loading. If it doesn't, then I squeeze the muscle and/or have them flex forward to give resistance to the paraspinals to confirm muscle.