Sciatica usually occurs because of a problem with the lumbar spine causing something to press against the sciatic nerve causing the pain.
The sciatic nerve, also known as the ischiatic nerve, is the longest, widest nerve in the body; providing energy to the lower part of the body relaying signals from the brain to the legs.
It actually forms at the level of the pelvis. In the lower spine where the sciatic nerve pain, referred to as sciatica is felt, there are five nerve roots passing down along the right and left hand side of the lower spine.
These five nerve roots pass through the pelvis and sacrum and form two nerves, the tibial and the peroneal. These two nerves eventually form one sheath and together make the sciatic nerve, which forms in front of the piriformis and extends all the way down the back of the leg.
At the back of the knee those two nerves, the tibial and peroneal, divide again. The peroneal nerve travels sideways (laterally) along the outside of the knee to the upper foot. The tibial nerve continues to travel downward to the feet, and it innervates the heel and sole of the foot.
Sciatica can manifest at any point along the lumbar spine where one of the nerve roots is exposed. Something—like a herniated disc—presses on one of these nerve roots sending a shockwave into the sciatic nerve, often running down the leg, sometimes all the way to the foot.
Sciatica symptoms manifest in all sorts of pain from a mild ache to a sharp electric shock like sensation running down your leg. When the problem is particularly bad the pain radiates down to the foot resulting in the foot going numb.
The way the pelvis is aligned when walking and standing has a profound effect on the space the sciatic nerve has to flow. The psoas muscle also plays a large role in the health of the sciatic nerve because of its relationship to the piriformis muscle.
Learning to walk and releasing the psoas are two excellent ways to combat the pain of sciatica and piriformis syndrome. Both of these techniques are non-invasive and easy to implement.