Please test:
Does the pain occur when bending your left knee against some resistance
(possible way: lay down on a flat surface - belly facing the floor - have someone hold down your lower leg with a reasonable amount of strength (i.e. you are barely able to perform the full motion) - check if this causes your typical pain)
please test:
Do this again to make sure it is causing your typical pain "1. When bending over at the waist, I get a real aching pain down the back of my upper leg, hamstring area,"
if yes now redo the same test with your left knee bent (about90degree) without your muscles having to maintain the bent state
(possible way: do 1st part standing up bending forward - do 2nd part standing up with your left (bent!) knee and lower leg resting on a chair - bend over your waist/torso/hip in the same way in both cases)
do not repeat tests unnecessairly - stop the test once your typical pain occurs, do not try to see how much it can hurt if you continue doing it
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