What is your argument? Are you saying it's impossible to increase pain threshold since there are studies that show you can.You can do stretching and exercises to improve your flexibility.
It will help your muscles soreness and stiffness during the lengthening phase.
There is no such thing or exercises to increase the pain threshold.
It is all in your mind, you should learn to mediate for peaceful mind and strength during the difficult times.
Pain is always there, take painkillers as what the doctor told you to do.
Be positive and stop complaining cos noone force you to do LL.