What causes the front muscle on both legs to feel numb?

Tight pants. What you're describing sounds like meralgia paresthetica. Also known as "calvin klein syndrome, " it is caused when tight pants compress the nerve going to the front of your thigh. The treatment is to wear looser-fitting clothing. If that doesn't work, you might have a neurologic problem, and you should see your doctor.
Many reasons. 1) pinched nerves 2) sleeping or postural problems is there weakness associated with the numbness?