Most days when I sit in a five-point meditation posture on a cushion there isn't a problem when sitting for the duration of a meditation session, but for particularly long sessions or on some days, sporadically, my left hamstring will start to feel uncomfortable tight to the point where I will not be able to continue meditating through it. Usually when this happens can't even mindfully adjust to a position where it won't dominate the session from that point onward.
The pain is sharp and gradually builds until about all I can do is stretch it out and leave it that way. It appears to be muscular in nature. There are no medical conditions that I have that I am aware of that would lead to this problem, and there don't appear to be any circulation issues.
What are some of the elements I can ensure, adjust, do beforehand, or do regularly to try to keep this from happening or mitigate the problem?