Psychological grasping is your main source of suffering, not the physical sensations in your legs -- that is to say, how much you are tightening psychologically around those sensations is the main source of suffering.
You may be spending most of your time in habitual tightening or resisting of the sensations, but if you sit there long enough, every once in a while, just because of the impermanent nature of things, your resistance will lessen for a moment, just spontaneously. At that time you begin to make a correlation. Diminished "resistance" brings about diminished suffering. You literally train yourself out of the habit of suffering.
What you learn in this way with respect to pain of physical origin is immediately generalizable to pain of psychological origin. Suffering is a function of two variables: one's discomfort and one's habit of resisting that discomfort. […]
Therefore, there is nothing whatsoever to be said in favor of pain per se for meditators. It can just as much create new blockages as it can break up old ones. Everything depends on one's degree of skill in experiencing it. Very little depends on the intensity of the discomfort itself. A small discomfort greeted with a large amount of skill will break up old knots. A small discomfort greeted with a large lack of skill will create new knots. The same is true with respect to big discomforts. The trick is not so much to endure massive doses of pain, but to develop that skill which will allow you to get the maximum growth out of whatever happens to come up. […]
"Skill" with sensation means to be relatively more clearly aware of the sensation and relatively more accepting of the sensation than you would be otherwise. When a person greets a minor pain with great awareness and great acceptance, then it has a much more powerful growth effect than to greet a major pain with grudging endurance. This was nicely summarized by Thomas Merton. Merton was a Christian monk with a great appreciation of the Eastern meditative traditions -- not an uncommon combination nowadays. I'm paraphrasing, but somewhere I remember him saying something like "I did not become a monk to suffer more than other people, I became a monk to suffer more effectively."
--Shinzen Young, in an interview with Charles Tart