If there is pain, jerk, or a pinching sensation, it should mean that the Asana is not properly fixed. There is a prescription given by Patanjali to quicken fixity of posture. And that is attention on the infinite . Steadiness is nowhere to be found in the world. There is only oscillation and fleeting of things everywhere.
Fixity is unknown, as it is all motion in the world.
There is only one thing that is fixed, viz., the infinite. All finites move and change. If the student can concentrate his mind on the infinite, he would imbibe certain qualities from it, the first being fixity. Here concentration is to think nothing in particular but all things at once. Though no one can think of the infinite as it is, one can think everything in the sense of inclusion of everything that comes to the mind.