Most trousers (especially men's) are poorly suited to a flexible range of motion. I have pretty flexible hips and large thighs from squatting, and have split my trousers at the groin on more than one occasion doing something silly like a martial arts kick or demonstrating an olympic lift movement. I stick to corduroys now.

Apparently you used to get people wearing cut-off jeans shorts while squatting in the gym, because the tension provides an extra impetus out of the bottom of the movement. A bit like a weightlifting singlet I guess. I've never seen it in person though.