When I bench press, I sometimes feel a muscle seize up slightly distal and lateral from my ASIS. I don't know what muscle this is, but I'm guessing it's involved with external rotation or abduction. This happens sometimes in the left hip, sometimes the right hip, sometimes both simultaneously.
It happens when I'm arching my back hard, with glutes pressed against the bench, legs open at about a 30° angle and feet flat against the ground.
I find it hard to maintain the arch of my back off the bench without these muscles occasionally seizing.
Am I tensing up an area that I should be leaving relaxed? Is this a sign of improper form?