I never had back pain but an esi proved that my leg pain was coming from the back. How can this be?

Leg pain. The nerves from the lower back innervate the leg. The anterior portion of the lumbosacral nerves go the the leg. The posterior portion goes to the back. The nerves can be effected by toxins, viruses, etc. Does not have to be a mechanical compression of the nerve. There for you do not have to have back pain in order to have the nerve pain from the back to your leg.